Large-scale modes of climate variability influence forest fire activity and may modulate the future patterns of natural disturbances. We studied the effects of long-term changes in climate upon the fire regime in the red pine forests of eastern North America using (a) a network of sites with dendrochronological reconstructions of fire histories over 1700–1900 A.D., (b) reconstructed chronologies of climate indices (1700–1900), and (c) 20th century observational records of climate indices, local surface climate and fire (1950s-2021). We hypothesized that (H1) there are states of atmospheric circulation that are consistently associated with increased fire activity, (H2) these states mark periods of increased climatological fire hazard, and (H3) the observed decline in fire activity in the 20th century is associated with a long-term decline in the frequency of fire-prone states. At the annual scale, years with significantly higher fire activity in the reconstructed and modern fire records were consistently associated with the positive phases of the Pacific North American pattern (PNA), either independently or in combination with the positive phase of the El Niño-Southern Oscillation index (ENSO). During years with both ENSO and PNA in their positive state, the region experienced positive mid-tropospheric heights and temperature anomalies resulting in drought conditions. The fire-prone climate states identified in the reconstructed records became less frequent in 1850 but re-emerged in the 20th century. While our study did not demonstrate a direct influence of climate on the observed decrease in fire activity in the 20th century, it does reveal a clear climate signal embedded within the fire history reconstruction of the region over the past centuries. This study underscores the importance of considering large-scale climatic patterns in understanding historical fire regimes and highlights their role for future fire dynamics in the region and shaping ecological effects of future fires.

Sugar maple (Acer saccharum Marshall) growth in the species’ southern range has been declining since the 1980s, putting at risk a variety of ecosystem services that the species provides. Heatwaves, drought, frosts, acidic deposition, and insect defoliation, all reducing photosynthetic activity, have been suggested to be behind the phenomenon. Because the geographic scope of previous studies on maple growth is limited to the southern temperate biome, it is not currently understood whether the same negative trends and factors affecting growth rates apply to the species in more northern regions of its distribution range. Here we used annual ring-width data of 1675 trees from a network of 21 sites in Quebec and Ontario between 45˚N and 48˚N to reconstruct maple growth and to analyze its trends and climatic drivers since 1950 C

Understanding factors driving fire activity helps reveal the degree and geographical variability in the resilience of boreal vegetation to large scale climate forces. We studied the association between sea ice cover in the Baffin Bay and the Labrador Sea and observational records of forest fires in two Nordic countries (Norway and Sweden) over 1913–2017. We found a positive correlation between ice proxies and regional fire activity records suggesting that the Arctic climate and the associated changes in North Atlantic circulation exercise an important control on the levels of fire activity in Scandinavia. Changes in the sea cover are likely correlated with the dynamic of the North Atlantic Current. These dynamics may favor the development of the drought conditions in Scandinavia through promoting persistent high-pressure systems over the Scandinavian boreal zone during the spring and summer. These periods are, in turn, associated with an increased water deficit in forest fuels, leading to a regionally increased fire hazard. The Arctic climate will likely be an important future control of the boreal fire activity in the Nordic region.

Warning: This article contains terms, descriptions, and opinions used for historical context that may be culturally sensitive for some readers.Background: Understanding drivers of boreal forest dynamics supports adaptation strategies in the context of climate change.Aims: We aimed to understand how burn rates varied since the early 1700s in North American boreal forests.Methods: We used 16 fire-history study sites distributed across such forests and investigated variation in burn rates for the historical period spanning 1700-1990. These were benchmarked against recent burn rates estimated for the modern period spanning 1980-2020 using various data sources.Key results: Burn rates during the historical period for most sites showed a declining trend, particularly during the early to mid 1900s. Compared to the historical period, the modern period showed less variable and lower burn rates across sites. Mean burn rates during the modern period presented divergent trends among eastern versus northwestern sites, with increasing trends in mean burn rates in most northwestern North American sites.Conclusions: The synchronicity of trends suggests that large spatial patterns of atmospheric conditions drove burn rates in addition to regional changes in land use like fire exclusion and suppression.Implications: Low burn rates in eastern Canadian boreal forests may continue unless climate change overrides the capacity to suppress fire.

Understanding long-term forest fire histories of boreal landscapes is instrumental for parameterizing climate–fire interactions and the role of humans affecting natural fire regimes. The eastern sections of the European boreal zone currently lack a network of annually resolved and centuries-long forest fire histories. To fill in this knowledge gap, we dendrochronologically reconstructed the 600-year fire history of a middle boreal pine-dominated landscape of the southern part of the Republic of Komi, Russia. We combined the reconstruction of fire cycle (FC) and fire occurrence with the data on the village establishment and climate proxies and discussed the relative contribution of climate versus human land use in shaping historic fire regimes. Over the 1340–1610 ce period, the territory had a FC of 66 years (with the 90% confidence envelope of 56.8 and 78.6 years). Fire activity increased during the 1620–1730 ce period, with the FC reaching 32 years (31.0–34.7 years). Between 1740–1950, the FC increased to 47 years (41.9–52.0). The most recent period, 1960–2010, marks FC's historic maximum, with the mean of 153 years (102.5–270.3). Establishment of the villages, often as small harbors on the Pechora River, was associated with a non-significant increase in fire occurrence in the sites nearest the villages (p = 0.07–0.20). We, however, observed a temporal association between village establishment and fire occurrence at the scale of the whole studied landscape. There was no positive association between the former and the FC. In fact, we documented a decline in the area burned, following the wave of village establishment during the second half of the 1600s and the first half of the 1700s. The lack of association between the dynamics of FC and the dates of village establishments, and the significant association between large fire years and the early and latewood pine chronologies, used as historic drought proxy, indirectly suggests that the climate was the primary control of the landscape-level FCs in the studied forests. Pine-dominated forests of the Komi Republic may hold a unique position as the ecosystem with the shortest history of human-related shifts in fire cycles across the European boreal region.

Abstract Fire regimes in North American forests are diverse and modern fire records are often too short to capture important patterns, trends, feedbacks, and drivers of variability. Tree-ring fire scars provide valuable perspectives on fire regimes, including centuries-long records of fire year, season, frequency, severity, and size. Here, we introduce the newly compiled North American tree-ring fire-scar network (NAFSN), which contains 2562 sites, >37,000 fire-scarred trees, and covers large parts of North America. We investigate the NAFSN in terms of geography, sample depth, vegetation, topography, climate, and human land use. Fire scars are found in most ecoregions, from boreal forests in northern Alaska and Canada to subtropical forests in southern Florida and Mexico. The network includes 91 tree species, but is dominated by gymnosperms in the genus Pinus. Fire scars are found from sea level to >4000-m elevation and across a range of topographic settings that vary by ecoregion. Multiple regions are densely sampled (e.g., >1000 fire-scarred trees), enabling new spatial analyses such as reconstructions of area burned. To demonstrate the potential of the network, we compared the climate space of the NAFSN to those of modern fires and forests; the NAFSN spans a climate space largely representative of the forested areas in North America, with notable gaps in warmer tropical climates. Modern fires are burning in similar climate spaces as historical fires, but disproportionately in warmer regions compared to the historical record, possibly related to under-sampling of warm subtropical forests or supporting observations of changing fire regimes. The historical influence of Indigenous and non-Indigenous human land use on fire regimes varies in space and time. A 20th century fire deficit associated with human activities is evident in many regions, yet fire regimes characterized by frequent surface fires are still active in some areas (e.g., Mexico and the southeastern United States). These analyses provide a foundation and framework for future studies using the hundreds of thousands of annually- to sub-annually-resolved tree-ring records of fire spanning centuries, which will further advance our understanding of the interactions among fire, climate, topography, vegetation, and humans across North America.

Purpose Mycorrhizal fungi are critical for the growth and survival of trees although the knowledge on the extent of their association with different tree species in the boreal forest remains limited. Methods We examined the vertical distribution and composition of the root mycorrhizal communities of black spruce (Picea mariana (Mill.) B.S.P) and trembling aspen (Populus tremuloides Michx) along three soil layers (organic, minerals top 0–15 cm and bottom 15–30 cm) in pure and mixed stands, using next generation sequencing. Results We found that spruce and aspen differ in the composition of their mycorrhizal communities in respective pure stands. The difference was present also in mixed stands, despite a shift in the composition of species-specific mycorrhizal communities between pure and mixed stands. In mixed stands, the relative abundance of spruce-specialist mycorrhizae in the organic layer was higher than that of aspen-specialists. The opposite pattern was observed in the mineral soil. The mixed stands exhibited lower richness and abundance of generalist mycorrhizae in the organic and in the mineral soil layers. Conclusion The results suggest that it is the soil chemistry that structure species-specific mycorrhizal communities between pure stands and along different soil depth within stands. However, in mixed stands, it is the identity of tree species that determines the structure of mycorrhizae communities within soil layers. We speculate that the differences in the richness and abundance of individual mycorrhizal communities of spruce and aspen along the soil profile would likely contribute to stronger partitioning of tree nutrient uptake between these two species in mixed stands.

Increasing air temperatures and changing precipitation patterns due to climate change can affect tree growth in boreal forests. Periodic insect outbreaks affect the growth trajectory of trees, making it difficult to quantify the climate signal in growth dynamics at scales longer than a year. We studied climate-driven growth trends and the influence of spruce budworm (Choristoneura fumiferana Clem.) outbreaks on these trends by analyzing the basal area increment (BAI) of 2058 trees of Abies balsamea (L.) Mill., Picea glauca (Moench) Voss, Thuja occidentalis L., Populus tremuloides Michx., and Betula papyrifera Marsh, which co-occurs in the boreal mixedwood forests of western Quebec. We used a generalized additive mixed model (GAMM) to analyze species-specific trends in BAI dynamics from 1967 to 1991. The model relied on tree size, cambial age, degree of spruce budworm defoliation, and seasonal climatic variables. Overall, we observed a decreasing growth rate of the spruce budworm host species, A. balsamea and P. glauca between 1967 and 1991, and an increasing growth rate for the non-host, P. tremuloides, B. papyrifera, and T. occidentalis. Our results suggest that insect outbreaks may offset growth increases resulting from a warmer climate. The observation warrants the inclusion of the spruce budworm defoliation into models predicting future forest productivity.

Dendrochronological reconstructions inform us about historical climate-fire-human interactions, providing a means to calibrate projections of future fire hazard. Most of these reconstructions, however, have been developed in landscapes with a considerable proportion of xeric sites that could potentially inflate our estimates of the historic levels of fire activity. We provide a 420-year long reconstruction of fires in a mire-dominated landscape of the Veps Nature Park, North-West Russia. The area has mostly escaped large-scale forestry operations in the past and is an example of pristine mid-boreal vegetation with a high (approximately 30% for the area studied) proportion of waterlogged areas with ombrotropic mires. The historical fire cycle was 91.4 years (90% confidence intervals, CI 66.2–137.6 years) over the 1580–1720 period, decreasing to 35.9 (CI 28.1–47.6 years) between 1730 and 1770, and then increasing again to 122.7 years (CI 91.0–178.0 years) over the 1780–2000 period. Early season fires dominated over late season fires during the reconstruction period. We documented a higher fire activity period between 1730 and 1780, resulting from the increase in early season fires. This period coincided with one of the largest multi-decadal declines in the reconstructed spring precipitation since 1600 CE, although we found no significant relationship between fire and precipitation over the whole reconstructed period. The nine largest fire years were associated with negative summer precipitation and positive summer temperature anomalies over the study region. Land-use history of the area did not appear to have an effect on historical fire dynamics. Modern (1996–2016) fire records indicate a regional fire cycle of ∼ 1300 years, featuring a pronounced pattern with early (April–May) and late (July–September) season fires. The uniform fire cycle in the area since 1780, occurrence of nine largest fire years during years with spring-summer droughts, and low ignition frequencies over the last 420 years (0.005 to 0.037 ignitions per year and km2) suggest that the fire regime of the Veps Highland remained largely natural until the onset of the 20th century.

Understanding factors driving fire activity helps reveal the degree and geographical variability in the resilience of boreal vegetation to large scale climate forces. We studied the association between sea ice cover in the Baffin Bay and the Labrador Sea and observational records of forest fires in two Nordic countries (Norway and Sweden) over 1913–2017. We found a positive correlation between ice proxies and regional fire activity records suggesting that the Arctic climate and the associated changes in North Atlantic circulation exercise an important control on the levels of fire activity in Scandinavia. Changes in the sea cover are likely correlated with the dynamic of the North Atlantic Current. These dynamics may favor the development of the drought conditions in Scandinavia through promoting persistent high-pressure systems over the Scandinavian boreal zone during the spring and summer. These periods are, in turn, associated with an increased water deficit in forest fuels, leading to a regionally increased fire hazard. The Arctic climate will likely be an important future control of the boreal fire activity in the Nordic region.

Fire is a major disturbance agent in the boreal forest, affecting the structure, dynamics and biogeochemical cycles in this biome. In the Asian section of boreal forest, the records of long-term fire history are few that limits our understanding of factors forcing regional fire dynamics. We presented an annually-resolved 352-year (1666–2017) fire chronology based on fire scars of Scots pine (Pinus sylvestris L.) and Siberian larch (Larix sibirica Ledeb) from the Transbaikal area in the southeastern Siberia. Fire activity showed an increasing trend from 1720 to 1929 (R2 = 0.80, P < 0.0001), and a significant decreasing trend from 1920 to 2010 (R2 = 0.62, P < 0.001). We assessed the potential relationships between drought (as represented by the Palmer Drought Severity Index, PDSI, and the Monthly Drought Code, MDC), ocean-atmosphere circulation and forest fire by Superposed epoch analyses, cross-wavelet analysis and Granger causality analysis. Increased fire activity was associated with stronger drought from previous winter to current summer of fire event years and positive Arctic Oscillation (AO) before and during major fire season (February and April to May), as revealed by superposed epoch analysis. Granger causality pointed to the significant role of drought in driving forest fires. Our findings provide insights into the climate drivers of forest fire activity and its prediction in the Transbaikal region.

Forest monitoring studies show contrasting trends in tree growth rates since the mid-twentieth century. However, due to their focus on annual and decadal dynamics, they provide limited insight into the effects of long-term climatic variability on tree growth. Here, we relied on a large tree-ring dataset (∼2,700 trees) of two common North American shade-intolerant tree species, trembling aspen (Populus tremuloides Michx.) and jack pine (Pinus banksiana Lambert), to assess their lifespan-long growth dynamics in the mixedwood forests of Québec. We also determined how the environmental conditions of the stands influenced tree growth. We observed a significant increase in the radial growth rate of trembling aspen during the study period, while the jack pine decline was not significant. Over the whole study region, the trees growing in sites with lower competition, and those at the lower sections of the terrain slope experienced more of the positive effects of temperature on growth rates. Our study suggests that the tree growth response to climate warming may be species-specific and will vary across the boreal mixedwoods.

Fire remains one of the main natural disturbance factors in the European boreal zone and understanding climatic forcing on fire activity is important for projecting effects of climate change on ecosystem services in this region. We analyzed records of annually burned areas in 16 administrative regions of the European boreal zone (countries or administrative units within countries) and fire weather variability to test for their spatio-temporal patterns over the 1901-2017 period.

Over the 1992-2017 period, the region exhibited large variability in forest fire activity with the fire cycles varying from ~1600 (St. Petersburg region) to ~37000 years (Finland). The clustering of administrative units in respect to their burned area, suggested the presence of sub-regions with synchronous annual variability in burned areas. Large fire years (LFYs) in each of the clusters were associated with the development of the high pressure cell over or in immediate proximity of the regions in question in July, indicating climatic forcing of LFYs. Contingency analysis indicated that there was no long-term trend in the synchrony of LFYs observed simultaneously in several administrative units. We documented a trend towards higher values of Monthly Drought Code (MDC) for the months of April and May in the western (April) and northern (April and May) sections. The significant positive correlation between biome-wide fire activity index and June SNAO (Summer North Atlantic Oscillation) (r = 0.53) pointed to the importance of large-scale atmospheric circulation, in particular the summer European blocking pattern, in controlling forest fires across EBZ. The forest fire activity of the European boreal zone remains strongly connected to the annual climate variability. Higher frequency of strongly positive SNAO states in the future will likely synchronize years with a large area burned across the European boreal zone.

The Sala fire in the Västmanland County of central Sweden that burned about 14,000 ha in 2014 has been the largest fire recorded in the modern history of Sweden. To understand the long-term fire history of this area, we dendrochronologically dated fire scars on Scots pine (Pinus sylvestris L.) trees (live and deadwood) to reconstruct the fire cycle and fire occurrence in the area affected by the 2014 fire. We identified 64 fire years, using a total of 378 pine samples. The earliest reconstructed fire dated back to 1113 AD. The spatial reconstruction extended over the period of 1480–2018 AD. Lower levels of fire activity (fire cycle, FC = 43 years, with the central 90% of the distribution limited by 35 to 57 years) dominated in the earlier period (1480–1690 AD) that was followed by a strong decrease in fire activity since 1700 (FC = 403 years, with 90% of the distribution being within 149 to 7308 years), with a fire-free period between 1756 and 2014. Sala area, therefore, features the earliest known onset of fire suppression in Scandinavia. The high demand for timber during the peak in mining activities in the study area around the 1700–1800s, accompanied by passive fire suppression policies, were possibly the main drivers of the decline in fire activity. Superposed epoch analysis (SEA) did not show significant departures in the drought proxy during the ten years with the largest area burned between 1480 and1690. It is unclear whether the result is due to the relatively small area sampled or an indication that human controls of fires dominated during that period. However, significant departures during the following period with low fire activity (1700–1756), which just preceded the last fire-free period, suggested that the climate became an increasingly important driver of fire during the onset of the suppression period. We speculate that the lack of major firebreaks, the homogenization of forests, and the lack of burned areas with low fuel loads might contribute to the occurrence of the exceptionally large 2014 fire in Sala.

We evaluated the skills of different palaeofire reconstruction techniques to reconstruct the fire history of a boreal landscape (Russian Karelia) affected by surface fires. The analysis of dated lacustrine sediments from two nearby lakes was compared with independent dendrochronological dating of fire scars, methods which have rarely been used in context of surface fires. We used two sediment sub-sampling volumes (1 and 3.5 cm3, wet volumes) and three methods of calculating the Charcoal Accumulation Rate to reconstruct fire histories: CHAR number, charcoal surface area and estimated charcoal volume. The results show that palaeofire reconstructions obtained with fossil charcoal data from lake sediments and dendrochronology are similar and complementary. Dendrochronological reconstruction of fire scars established 12 fire dates over the past 500 years, and paleo-data from lake sediments identified between 7 and 13 fire events. Several ‘false fire events’ were also recorded in the charcoal chronologies, likely because of errors associated with the estimation of the sediment accumulation rate in the unconsolidated part of the sediment. The number of replicates, that is, number of sub-samples and lakes analyzed, had an effect on the number of identified fire events, whereas no effect was seen in the variation in the analyzed sediment volume or the choice of the charcoal-based metric. Whenever possible, we suggest the use of the dendrochronological data as an independent control for the calibration of charcoal peak series, which helps provide more realistic millennia-long reconstruction of past fire activity. We also argue for the use of 1 cm3 sample volume, a sampling protocol involving sampling of more than one lake, and sufficient number of intra-sample replicates to achieve skilful reconstructions of past fire activity.

This study explored the underground interactions between black spruce and trembling aspen in pure and mixed stands to understand how their soil resource use help these species coexist in the boreal mixedwoods of Western Quebec. We analyzed species-specific fine root foraging strategies (root biomass and root tissue density) along three soil layers (organic, top 0–15 cm, and bottom 15–30 cm mineral soil), using 180 soil cores. We collected cores in three sites, each containing three 20 × 50 m2 plots of pure spruce, pure aspen, and mixed spruce and aspen stands. Spruce had a shallow rooting, whereas aspen had a deep rooting in both types of stands. Compared to pure spruce stands, spruce had a lower fine root biomass (FRB) and a higher root tissue density (RTD) in the organic layer of mixed stands. Both patterns were indicative of spruce’s more intensive resource use strategy and competitive advantage over aspen in that layer. Aspen FRB in the organic soil did not differ significantly between pure and mixed stands, but increased in the mineral soil of mixed stands. Since we did not observe a significant difference in the nutrient content of the mineral soil layer between pure aspen and mixed stands, we concluded that aspen may experience competitive exclusion in the organic layer by spruce. Aspen exhibited an extensive nutrient uptake strategy in the organic layer of mixed stands: higher FRB and lower RTD than spruce. In mixed stands, the differences in aspen rooting patterns between the organic and mineral layers suggested the use of contrasting nutrient uptake strategies along the soil profile. We speculate that the stronger spatial separation of the roots of spruce and aspen in mixed stands likely contribute to a higher partitioning of their nutrient uptake along the soil profile. These results indicate the competitive exclusion of aspen by spruce in boreal mixedwoods, which likely occurs in the soil organic layer.

Spatially explicit reconstructions of fire activity in European boreal forest are rare, which limits our understanding of factors driving vegetation dynamics in this part of the boreal domain. We have developed a spatially explicit dendrochronological reconstruction of a fire regime in a 25 × 50 km2 area within boreal biome located within the Kalevalsky National Park (Kalevalsky NP), over the 1400–2010 CE period. We dated 184 fire years using 212 fire-scarred living and dead Scots pine (Pinus sylvestris L.) trees collected on 38 sites.

The studied period revealed a pronounced century-long variability in forest fire cycles (FC). The early period (1400–1620 CE) had low fire activity (FC = 178 years), which increased during the 1630–1920 period (FC = 46 years) and then decreased over the 1930–2000 period (FC = 283 years). Dendrochronological results did not provide a conclusive answer on the origins of FC dynamics, although several lines of evidence suggest that climate drove the increase in fire activity in the early 1600s, while human-related factors were largely responsible for its decline in the early 1900s. The current FC in the Kalevalsky NP is close to the estimates reported for the pre-industrial colonisation period in Scandinavia, which suggests that the forests of the area currently maintain their close-to-natural fire regime. Fire has been the pivotal factor of forest dynamics in this biome and forest management should acknowledge that fact in developing conservation strategies in Karelia and other areas of European boreal forest. Introduction of prescribed burns of varying severity could be an important element of such strategies.

In many boreal regions of Russia the past natural variability of forest fire activity remains largely undocumented, preventing accurate assessment of the impact of current climate warming on forest ecosystem dynamics. This study aims to reconstruct the Holocene fire history of the northern Ural mountain foothills, in the Komi Republic, based on analyses of charcoal particles from peatland deposits and coupled with dendrochronological investigations. The results show that there was a gradual increase in forest fire activity during the past 11,000 years. Between 11,000 and 5100 cal. yr BP, the mean fire return interval (FRI) oscillated between 600 and 200 years. During this period, regional data showed that cold temperature, humid climatic conditions, combined with steppe vegetation between 11,000?9000 cal. BP, and then the development of spruce-dominated forest between 9300 and 4600 cal. yr BP, were less conducive to fires. After 5100 cal. yr BP, a gradual increase in drought conditions through reduced precipitations, associated with the establishment of a Scots pine forest favored fire frequency, with a mean FRI under 200 years (range, 200?40 years). Nowadays (since CE 1500), human activity induces an unprecedented fire activity with a mean FRI below 100 years (range, 100?40 years).

There is mounting evidence that species diversity increases the temporal stability of forest growth. This stabilising effect of diversity has mainly been attributed to species differences in their response to fluctuating environmental conditions. Interactions among individuals could also contribute to the stabilising effect of diversity by increasing the mean and reducing the variance of tree growth, however, this has never been directly demonstrated. We used tree?ring width chronologies from temperate and boreal mixed stands of Eastern Canada to identify the role of interactions among individuals in the stabilising effect of diversity on forest growth. Using neighbourhood competition index and a mixed model, we compared the effect of interspecific and intraspecific interactions on the mean and the variance of tree growth. We found that interspecific interactions are less detrimental to tree growth than intraspecific interactions. We also found that interspecific interactions buffer tree response to drought and thereby reduce the variance of tree growth. Our results indicate diversity may increase the mean and reduce the variance of tree growth through interactions among individuals. Thus, we demonstrate interactions among individuals play a role in the stabilising effect of diversity on forest growth, and in doing so, we bring to light other mechanisms of the insurance hypothesis. A plain language summary is available for this article.

Fire has been shown to shape successional pathways and dynamics of forest vegetation. However, its role in European hemiboreal forests remains poorly understood. Here we provide the first annually resolved reconstruction of fire history from the Eastern Baltic Sea region, developed in the pine-dominated landscape of Slitere National Park (SNP), northwestern Latvia, over the last 250?years. Our results suggest that forest fires have been a common disturbance factor in the studied landscape. In total, we dated 62 single fire years, with the mean-point scale fire return interval of 46?years and the length of the fire cycle ranging from 45 to 80?years. We identified periods of high (1750–1950) and low (1960–2000) fire activity, with the corresponding lengths of fire cycles being 45–68 and 58–80?years, respectively. Although both long-term (century and decade-long) and annual dynamics of fire activity in SNP was closely linked to socio-political changes in Latvia, fire activity in SNP was also affected by climate, as indicated by the close positive association of years with increased area burned and positive SST anomalies in the Baltic and North Seas. Future management of SNP should make fire an important element of natural forest dynamics and consider using prescribed fires of various spatial extent and severity.

Understanding intra-specific variation in climate sensitivity could improve the prediction of tree responses to climate change. We attempted to identify the degree of genetic control of tree phenology and growth of trembling aspen (Populus tremuloides Mchx.) in a natural stand of this species in northwestern Quebec. We mapped and genotyped 556 aspen trees growing within the plot, using seven nuclear microsatellite loci for clone identification. We selected 13 clones (n of trees per clone >5, in total 350 trees) and evaluated the explanatory power of clone identity in (a) variability of spring leaf phenology and (b) short- and long-term growth responses. The clone’s identity explained 43% of the variability in spring leaf phenology, between 18% and 20% of variability in response to monthly climate variables significantly affecting growth, between 8% and 26% of growth response to insect outbreaks, and 12% in the long-term growth rates. Strong clonal control of aspen phenology and moderate control of growth responses to monthly weather do not result in an equally large impact on long-term growth rates. The result suggests an important role of environmental extremes and within community interactions as factors averaging aspen growth performance at the stand level.

Due to anthropogenic emissions and changes in land use, trees are now exposed to atmospheric levels of [CO2] that are unprecedented for 650,000 y [Lüthi et al. (2008) Nature 453:379–382] (thousands of tree generations). Trees are expected to acclimate by modulating leaf–gas exchanges and alter water use efficiency which may result in forest productivity changes. Here, we present evidence of one of the strongest, nonlinear, and unequivocal postindustrial increases in intrinsic water use efficiency (iWUE) ever documented (+59%). A dual-isotope tree-ring analysis (?13C and ?18O) covering 715 y of growth of North America’s oldest boreal trees (Thuja occidentalis L.) revealed an unprecedented increase in iWUE that was directly linked to elevated assimilation rates of CO2 (A). However, limited nutrient availability, changes in carbon allocation strategies, and changes in stomatal density may have offset stem growth benefits awarded by the increased iWUE. Our results demonstrate that even in scenarios where a positive CO2 fertilization effect is observed, other mechanisms may prevent trees from assimilating and storing supplementary anthropogenic emissions as above-ground biomass. In such cases, the sink capacity of forests in response to changing atmospheric conditions might be overestimated.

Forest fires are a key disturbance in boreal forests, and characteristics of fire regimes are among the most important factors explaining the variation in forest structure and species composition. The occurrence of fire is connected with climate, but earlier, mostly local?scale studies in the northern European boreal forests have provided little insight into fire–climate relationship before the modern fire suppression period. Here, we compiled annually resolved fire history, temperature, and precipitation reconstructions from eastern Fennoscandia from the mid?16th century to the end of the 19th century, a period of strong human influence on fires. We used synchrony of fires over the network of 25 fire history reconstructions as a measure of climatic forcing on fires. We examined the relationship between fire occurrence and climate (summer temperature, precipitation, and a drought index summarizing the influence of variability in temperature and precipitation) across temporal scales, using a scale space multiresolution correlation approach and Bayesian inference that accounts for the annually varying uncertainties in climate reconstructions. At the annual scale, fires were synchronized during summers with low precipitation, and most clearly during drought summers. A scale?derivative analysis revealed that fire synchrony and climate varied at similar, roughly decadal scales. Climatic variables and fire synchrony showed varying correlation strength and credibility, depending on the climate variable and the time period. In particular, precipitation emerged as a credible determinant of fire synchrony also at these time scales, despite the large uncertainties in precipitation reconstruction. The findings explain why fire occurrence can be high during cold periods (such as from the mid?17th to early?18th century), and stresses the notion that future fire frequency will likely depend to a greater extent on changes in precipitation than temperature alone. We showed, for the first time, the importance of climate as a decadal?scale driver of forest fires in the European boreal forests, discernible even during a period of strong human influence on fire occurrence. The fire regime responded both to anomalously dry summers, but also to decadal?scale climate changes, demonstrating how climatic variability has shaped the disturbance regimes in the northern European boreal forests over various time scales.

In Northern Europe, human activities have caused a substantial decrease in the number of old deciduous trees over the last two centuries, leading to a decline in species populations associated with this habitat. One way to mitigate this trend is to increase the abundance of mature and old deciduous trees in commercial forests, such as by tree retention at final harvest. We analysed the biodiversity value of retained mature oaks in the production forests of Norway spruce in southern Sweden, using oaks in pastures as reference. The forest oaks were grown in two different levels of shade. We analysed two categories of saproxylic (i.e. dead wood-dependent) beetles: those utilizing oaks (Group I) and those utilizing oak but not spruce (Group II, which was, therefore, a subcategory of Group I). We found that forest oaks sustained high beetle diversity, in particular, Group I beetles, which were significantly more abundant in forest oaks in heavily thinned patches, as compared with pasture oaks and oaks in moderately thinned patches. For both beetle groups, the composition differed between the forest oaks and pasture oaks, indicating that the forest oaks can be a complementary habitat to that of pasture oaks. There was a positive relationship between oak dead branch diameter and beetle biodiversity, but only for older oaks (?200?years old). We conclude that retaining oaks in production spruce forests can increase the diversity of oak-associated beetles at the landscape scale. Since many oak associated species depend on relatively high levels of insolation, management of retained oaks in production forests should include periodic removal of encroaching trees.

The mid-20th century changes in North Atlantic Ocean dynamics, e.g. slow-down of the Atlantic meridional overturning thermohaline circulation (AMOC), have been considered as early signs of tipping points in the Earth climate system. We hypothesized that these changes have significantly altered boreal forest growth dynamics in northeastern North America (NA) and northern Europe (NE), two areas geographically adjacent to the North Atlantic Ocean. To test our hypothesis, we investigated tree growth responses to seasonal large-scale oceanic and atmospheric indices (the AMOC, North Atlantic Oscillation (NAO), and Arctic Oscillation (AO)) and climate (temperature and precipitation) from 1950 onwards, both at the regional and local levels. We developed a network of 6876 black spruce (NA) and 14437 Norway spruce (NE) tree-ring width series, extracted from forest inventory databases. Analyses revealed post-1980 shifts from insignificant to significant tree growth responses to summer oceanic and atmospheric dynamics both in NA (negative responses to NAO and AO indices) and NE (positive response to NAO and AMOC indices). The strength and sign of these responses varied, however, through space with stronger responses in western and central boreal Quebec and in central and northern boreal Sweden, and across scales with stronger responses at the regional level than at the local level. Emerging post-1980 associations with North Atlantic Ocean dynamics synchronized with stronger tree growth responses to local seasonal climate, particularly to winter temperatures. Our results suggest that ongoing and future anomalies in oceanic and atmospheric dynamics may impact forest growth and carbon sequestration to a greater extent than previously thought. Cross-scale differences in responses to North Atlantic Ocean dynamics highlight complex interplays in the effects of local climate and ocean-atmosphere dynamics on tree growth processes and advocate for the use of different spatial scales in climate-growth research to better understand factors controlling tree growth.

In the early 2000s, an extensive epidemic of the fungus Gremmeniella abietina (Lagerb.) Morelet occurred in Sweden and caused severe damages to coniferous species. This study aimed to evaluate the impacts of this outbreak on the stem growth of the surviving Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies L.) trees, using the country-wide data on pine and spruce from the Swedish National Forest Inventory and data from stem analyses of pine growth in four infected plots (ntrees = 12) in the Bergslagen area of middle Sweden. Analyses of volume dynamics in the four stands indicated a decade-long negative effect of pathogen infections on volume growth of pine trees. Over 2000–2012, the difference between projected (assuming no infection) and observed volume growth was 10–62%, depending on infection-related crown transparency (varying between less than 20% to above 80%). Height growth of pine in affected stands was reduced by 64–85%, although the reduction did not correlate with levels of crown transparency. The average reduction in basal area increment (BAI) in affected areas country-wide, accumulated over 2000–2006, was ?21% for pine and ?4% for spruce. The use of regular ring-width chronologies, as compared to volume increment chronologies, resulted in underestimation of volume losses by 25–30%.

• Mast seeding is a crucial population process in many tree species, but its spatio-temporal patterns and drivers at the continental scale remain unknown .
• Using a large dataset (8000 masting observations across Europe for years 1950–2014) we analysed the spatial pattern of masting across the entire geographical range of European beech, how it is influenced by precipitation, temperature and drought, and the temporal and spatial stability of masting–weather correlations.
• Beech masting exhibited a general distance-dependent synchronicity and a pattern structured in three broad geographical groups consistent with continental climate regimes. Spearman's correlations and logistic regression revealed a general pattern of beech masting correlating negatively with temperature in the summer 2 yr before masting, and positively with summer temperature 1 yr before masting (i.e. 2T model). The temperature difference between the two previous summers (DeltaT model) was also a good predictor. Moving correlation analysis applied to the longest eight chronologies (74–114 yr) revealed stable correlations between temperature and masting, confirming consistency in weather cues across space and time.
• These results confirm widespread dependency of masting on temperature and lend robustness to the attempts to reconstruct and predict mast years using temperature data.

Tree masting is one of the most intensively studied ecological processes. It affects nutrient fluxes of trees, regeneration dynamics in forests, animal population densities, and ultimately influences ecosystem services. Despite a large volume of research focused on masting, its evolutionary ecology, spatial and temporal variability, and environmental drivers are still matter of debate. Understanding the proximate and ultimate causes of masting at broad spatial and temporal scales will enable us to predict tree reproductive strategies and their response to changing environment. Here we provide broad spatial (distribution range-wide) and temporal (century) masting data for the two main masting tree species in Europe, European beech (Fagus sylvatica L.) and Norway spruce (Picea abies (L.) H. Karst.). We collected masting data from a total of 359 sources through an extensive literature review and from unpublished surveys. The data set has a total of 1,747 series and 18,348 yearly observations from 28 countries and covering a time span of years 1677–2016 and 1791–2016 for beech and spruce, respectively. For each record, the following information is available: identification code; species; year of observation; proxy of masting (flower, pollen, fruit, seed, dendrochronological reconstructions); statistical data type (ordinal, continuous); data value; unit of measurement (only in case of continuous data); geographical location (country, Nomenclature of Units for Territorial Statistics NUTS-1 level, municipality, coordinates); first and last record year and related length; type of data source (field survey, peer reviewed scientific literature, gray literature, personal observation); source identification code; date when data were added to the database; comments. To provide a ready-to-use masting index we harmonized ordinal data into five classes. Furthermore, we computed an additional field where continuous series with length >4 yr where converted into a five classes ordinal index. To our knowledge, this is the most comprehensive published database on species-specific masting behavior. It is useful to study spatial and temporal patterns of masting and its proximate and ultimate causes, to refine studies based on tree-ring chronologies, to understand dynamics of animal species and pests vectored by these animals affecting human health, and it may serve as calibration–validation data for dynamic forest models.

Summary

1. Differences between species in their response to environmental fluctuations cause asynchronized growth series, suggesting that species diversity may help communities buffer the effects of environmental fluctuations. However, within-species variability of responses may impact the stabilizing effect of growth asynchrony.
2. We used tree ring data to investigate the diversity–stability relationship and its underlying mechanisms within the temperate and boreal mixed woods of Eastern Canada. We worked at the individual tree level to take into account the intraspecific variability of responses to environmental fluctuations.
3. We found that species diversity stabilized growth in forest ecosystems. The asynchrony of species’ response to climatic fluctuations and to insect outbreaks explained this effect. We also found that the intraspecific variability of responses to environmental fluctuations was high, making the stabilizing effect of diversity highly variable.
4. Synthesis. Our results are consistent with previous studies suggesting that the asynchrony of species’ response to environmental fluctuations drives the stabilizing effect of diversity. The intraspecific variability of these responses modulates the stabilizing effect of species diversity. Interactions between individuals, variation in tree size and spatial heterogeneity of environmental conditions could play a critical role in the stabilizing effect of diversity.

The growth of high-latitude temperature-limited boreal forest ecosystems is projected to become more constrained by soil water availability with continued warming. The purpose of this study was to document ongoing shifts in tree growth sensitivity to the evolving local climate in unmanaged black spruce (Picea mariana (Miller) B.S.P.) forests of eastern boreal North America (49°N–52°N, 58°W–82°W) using a comparative study of field and modeled data. We investigated growth relationships to climate (gridded monthly data) from observed (50 site tree-ring width chronologies) and simulated growth data (stand-level forest growth model) over 1908–2013. No clear strengthening of moisture control over tree growth in recent decades was detected. Despite climate warming, photosynthesis (main driver of the forest growth model) and xylem production (main driver of radial growth) have remained temperature-limited. Analyses revealed, however, a weakening of the influence of growing season temperature on growth during the mid- to late twentieth century in the observed data, particularly in high-latitude (> 51.5°N) mountainous sites. This shift was absent from simulated data, which resulted in clear model-data desynchronization. Thorough investigations revealed that desynchronization was mostly linked to the quality of climate data, with precipitation data being of particular concern. The scarce network of weather stations over eastern boreal North America (> 51.5°N) affects the accuracy of estimated local climate variability and critically limits our ability to detect climate change effects on high-latitude ecosystems, especially at high altitudinal sites. Climate estimates from remote sensing could help address some of these issues in the future.

The length of the fire cycle is a critical factor affecting the vegetation cover in boreal and temperate regions. However, its responses to climate change remain poorly understood. We reanalyzed data from earlier studies of forest age structures at the landscape level, in order to map the evolution of regional fire cycles across Eastern North American boreal and temperate forests, following the termination of the Little Ice Age (LIA). We demonstrated a well-defined spatial pattern of post-LIA changes in the length of fire cycles toward lower fire activity during the 1800s and 1900s. The western section of Eastern North America (west of 77°W) experienced a decline in fire activity as early as the first half of the 1800s. By contrast, the eastern section showed these declines as late as the early 1900s. During a regionally fire-prone period of the 1910s–1920s, forests in the western section of Eastern boreal North America burned more than forests in the eastern section. The climate appeared to dominate over vegetation composition and human impacts in shaping the geographical pattern of the post-LIA change in fire activity. Changes in the atmospheric circulation patterns following the termination of the LIA, specifically changes in Arctic Oscillation and the strengthening of the Continental Polar Trough, were likely drivers of the regional fire dynamics.

Questions

We investigated whether the gap disturbance rate (percentage area disturbed by canopy gaps per year) differed at the northern range limit of sugar maple (Acer saccharum)–yellow birch (Betula alleghaniensis) forests compared to broad-leaf temperate deciduous forests located more to the south. As an ancillary question, we assessed the relationship between species composition and gap disturbance rate at the stand scale.

Location

Late successional sugar maple–yellow birch forests at their northern range limit in western Quebec, Canada.

Methods

To reconstruct past gap disturbances, we identified growth releases using the boundary line method applied to tree-ring chronologies obtained from 0.25-ha plots sampled within 11 late successional forest stands. We reconstructed past canopy gaps using release events, calculated historical gap disturbance rates and used redundancy analysis to evaluate the relationship between gap disturbance rate and species composition at the stand scale.

Results

The mean gap disturbance rate across the 11 late successional stands was 0.96 ± 0.51%·yr−1. Mean gap size was 39 ± 44 m2 and almost 85% of the gaps were <50 m2. Stands with smaller gaps and lower gap disturbance rates had high importance values for balsam fir (Abies balsamea) and/or yellow birch.

Conclusions

The gap disturbance rate at the northern limit of sugar maple–yellow birch forest was similar to that reported in broad-leaf temperate deciduous forests located ca. 575–1300 km to the south. However, gaps were more numerous and smaller, which could be related to latitudinal differences in allometric traits of the dominant tree species and climate.

Early successional competition among boreal forest tree and shrub species and its effects on growth of commercial tree species have been a major source of uncertainty in establishing efficient precommercial thinning and brushing prescriptions. We examined the effect of prethinning competitor density, postthinning competitor regrowth density, prethinning stem diameter, and the timing of thinning operations on the growth response of black spruce (Picea mariana [Mill.] BSP) and jack pine (Pinus banksiana Lamb.). In addition, we examined the mortality rate of hardwoods after thinning and the number of new shoots produced per surviving thinned stem. For jack pine, growth response was greatest when thinning occurred between 4 and 9 years after establishment, whereas for black spruce we observed no significant relationship between growth response and the timing of treatment. For jack pine, growth response was significantly affected by pretreatment competitor density, posttreatment competitor regrowth density, and pretreatment stem diameter. For black spruce, no significant relationship was observed between growth response and any variables. Mortality rates and production of new shoots in hardwoods varied significantly between species. Considering the high regrowth potential of willow (Salix spp.) and alder (Alnus spp.), we recommend that stands exhibiting low densities of these species should be left unthinned. Our results help foresters identify stands that require precommercial thinning and call for modification of currently used thinning strategies.

Root connections between trees can be an ecological advantage of clonal plant species in environments with unevenly distributed resources. We investigated the effects of root connectivity in stands of balsam poplar in Quebec (Canada). We evaluated differences in growth response between groups of trees with and without root connections through climate-growth analyses, comparison of the growth dynamics, and analysis of growth response to a severe forest tent caterpillar (FTC) outbreak. Current May temperature had a positive influence on radial growth of both connected and non-connected trees. Growth of non-connected trees was negatively affected by August temperatures (r = ?0.3) while connected trees did not reveal a significant relationship for that month. A mixed effect ANOVA showed a significant difference (F 1, 25 = 5.59, p = 0.02) in growth responses to FTC outbreak between connected and non-connected trees. Connected trees grew on average 16 % better than unconnected trees during the outbreak, with bootstrapped 95 % confidence range from 2.28 to 31.36 %. The study suggests a sharing of resources through root connections, affecting radial growth of connected balsam poplar trees under both average and extreme environmental conditions.

Nous avons étudié l’origine climatique des anomalies de croissance des forêts boréales en analysant 895 séries de croissance d’épinette noire (Picea mariana (Mill.) B.S.P.) provenant de 46 sites xériques repartis le long de trois transects latitudinaux dans l’Est Canadien. Nous avons identifié les anomalies de croissance interannuelles (comparaison à l’année précédente) et multi-décennales (comparaison à toutes les années) pour chaque site et transect de 1901 à 2001. Les anomalies de croissance apparaissent principalement à l’échelle du site mais rarement à de plus larges échelles géographiques. Les anomalies positives (interannuelles et multi-décennales) sont fortement associées à des températures basses et des précipitations fortes pendant la saison de croissance de l’année précédente. L’origine climatique des anomalies négatives (interannuelles et multi-décennales) est plus complexe et généralement associée à des anomalies climatiques de l’année en cours. Entre le début et la fin du XXe siècle, seules les anomalies multi-décennales négatives sont devenues plus fréquentes. Nos résultats révèlent l’importance du climat de la saison de croissance précédente dans l’apparition d’anomalies de croissance et suggèrent un lien positif entre le réchauffement climatique et l’augmentation de la fréquence des anomalies multi-décennales négatives. L’augmentation prévue des températures dans les prochaines décennies pourrait davantage accroitre la fréquence des anomalies.

Understanding the drivers of the boreal forest fire activity is challenging due to the complexity of the interactions driving fire regimes. We analyzed drivers of forest fire activity in Northern Scandinavia (above 60 N) by combining modern and proxy data over the Holocene. The results suggest that the cold climate in northern Scandinavia was generally characterized by dry conditions favourable to periods of regionally increased fire activity. We propose that the cold conditions over the northern North Atlantic, associated with low SSTs, expansion of sea ice cover, and the southward shift in the position of the subpolar gyre, redirect southward the precipitation over Scandinavia, associated with the westerlies. This dynamics strengthens high pressure systems over Scandinavia and results in increased regional fire activity. Our study reveals a previously undocumented teleconnection between large scale climate and ocean dynamics over the North Atlantic and regional boreal forest fire activity in Northern Scandinavia. Consistency of the pattern observed annually through millennium scales suggests that a strong link between Atlantic SST and fire activity on multiple temporal scales over the entire Holocene is relevant for understanding future fire activity across the European boreal zone.

Disturbance plays an important role in the distributional range of species by affecting their colonization potential and persistence. Short disturbance intervals have been linked to reduced seedbank sizes of some species, but the effects of long intervals are largely unknown. To explore the potential existence of seedbank sizes that may also be limited by long disturbance intervals, we studied an area in boreal eastern North America where time since fire (TSF) coincides with an increase in environmental stress (accumulating organic matter measured as depth of the soil organic layer (SOL)). Along a chronosequence dating back about 710 years, we counted the number of seeds cone?1 of black spruce (Picea mariana) and then estimated the number of seeds tree?1 and site?1 by upscaling. Younger sites [TSF 60–150 years] with mature first regeneration trees had average-sized seedbanks for black spruce [12.0–17.9 (105) seeds ha?1], whereas subsequent pulse trees that established in SOL depths greater than 35 cm showed highly reduced seed numbers. Sites with second- to fourth-regeneration pulse individuals [TSF c. 350–710 years] had exceptionally small seedbanks of 0.90 (105) and 0.46 (105) seeds ha?1, respectively. Radial tree growth rate showed a similarly negative response to SOL depth and could potentially be used as an indicator of seed output in plant species. Because the decline in seedbank size was possibly caused by more general environmental stress factors such as reduced nutrient availability, we suggest exploring whether other examples of ecosystems exist where long time since disturbance may lead to reduced seedbank sizes.

Years with climatically mediated increases in boreal forest fire activity, referred to as large fire years (LFYs), contribute to a disproportionally large portion of the burned area over centuries, and are important drivers of ecosystem processes by affecting forest structure, biodiversity, and carbon balance at regional and continental scales. We analysed changes in LFY return intervals in northern Sweden (the area above 60?°N) over 1273–1960 using a network of 29 sites with dendrochronologically reconstructed fires, complemented by documentary records of fires available from forestry statistics. We observed large variability in return intervals of LFYs, an increase in LFY frequency during the 1800?s, and consistent associations between LFY occurrence and 500-hPa pressure anomalies over the European sub-continent over 1800xps2#1960. An increase in LFY frequency during the 1800?s might be climatically driven, and would thus long precede the period of likely human-induced climatic changes of the 1900?s. Long-term variability in climatically driven LFYs may present a challenge in partitioning the effects of human-related and human-independent components of climatic forcing upon forest fire activity.

To improve our understanding of climate-driven long-term dynamics of eastern Asian mountain forests, we used field surveys and dendrochronological techniques to examine regeneration density, growth rate of mature trees, and growth sensitivity to climate of 3 common coniferous tree species at their respective altitudinal distribution limits on Changbai Mountain, northeastern China. The studied species were Manchurian fir (Abies nephrolepis, distributed between 780 and 1750 m asl), Korean pine (Pinus koraiensis, 780 and 1300 m asl), and Jezo spruce (Picea jezoensis var. komarovii, 1000 and 1750 m asl). Regeneration densities did not differ significantly among the elevations except for Jezo spruce, which showed a significantly lower regeneration density at 1000 m asl as compared to 1300 and 1750 m asl. All 3 species showed a significantly higher basal area increment (BAI) at the middle part of their distribution ranges than at their limits. The growth of Manchurian fir and Jezo spruce exhibited higher sensitivity to precipitation than to temperature at their lower distribution limits, and the inverse pattern was observed at the upper limit. In all cases the correlations between growth and the respective climate variable were positive, except for the correlation between Jezo spruce growth and precipitation. Growth of Korean pine was positively correlated with spring temperature and summer moisture at its lower distribution limit and with summer temperature at its upper limit. Our study suggests that elevational limits of forest vegetation were likely constrained by climate factors affecting growth of dominant species rather than those controlling regeneration density.

Afin d'améliorer notre compréhension de la dynamique à long terme de l'influence des facteurs climatiques sur les forêts de montagne de l'est de l'Asie, nous avons réalisé des relevés sur le terrain et utilisé des méthodes dendrochronologiques pour examiner la densité de régénération, le taux de croissance d'arbres matures et la sensibilité au climat de la croissance de trois espèces communes de conifères à leurs limites respectives de répartition altitudinale dans le massif Changbai, au nord-est de la Chine. Les espèces étudiées étaient le sapin de Mandchourie (Abies nephrolepis, retrouvé entre 780 et 1750 m asl), le pin de Corée (Pinus koraiensis, 780 à 1300 m asl) et l'épinette Jezo (Picea jezoensis, 1000 à 1750 m asl). La densité de régénération ne variait pas significativement entre les élévations, sauf pour l'épinette Jezo qui avait une densité de régénération significativement plus faible à 1000 m qu'à 1300 m et 1750 m asl. L’accroissement de la surface terrière pour les trois espèces était significativement plus élevée dans la partie centrale de leur aire de répartition qu'aux limites. La croissance du sapin de Mandchourie et de l'épinette Jezo montrait une plus grande sensibilité aux précipitations qu'aux températures à leurs limites inférieures de répartition alors que le patron inverse était observé à leurs limites supérieures. Dans tous les cas, les corrélations entre la croissance et une variable climatique en particulier étaient positives, sauf pour la corrélation entre la croissance de l'épinette Jezo et les précipitations. La croissance du pin de Corée était corrélée de façon positive avec les températures printanières et l'humidité estivale à sa limite inférieure de répartition et avec les températures estivales à sa limite supérieure. Notre étude suggère que les limites altitudinales de la forêt sont probablement contraintes par des facteurs climatiques affectant la croissance des espèces dominantes plutôt que par des facteurs régissant la densité de la régénération.

Aims We examined growth of red maple (Acer rubrum L.) to evaluate environmental controls of its northern distributional limit in Eastern North America and its potential response to future climate change.

Methods We collected growth data from nine sites located along a 300-km transect (47–49°N), which included frontier population of red maple and covered three bioclimatic domains in western Quebec. We analyzed three growth variables: growth rates during the first 30 years of maple lifespan, cumulative basal area increment (BAI) over the most recent decade (2000–09) and annual growth rate over the whole tree lifespan ranging from 58 to 112 years. We also examined growth sensitivity to climate by using response function analysis.

Important Findings Three different proxies of maple absolute growth (initial growth rate, BAI during 2000–09 and mean diameter growth rate) indicated a better growth with an increase in latitude. We speculate that stand history effectively overrode the direct effects of colder climate on maple growth along the S-N gradient. Regeneration of maple in the southern sites likely occurred in canopy gaps, whereas in the north it was contingent upon large disturbances such as stand-replacing fires, which apparently provided more favorable light environment for maple growth than canopy gaps. The annual growth variability, which reflects effects of annual weather on growth and is largely independent from the absolute growth rate, was significantly affected by monthly climate, suggesting a positive effect of higher summer temperature in the northern part of the transect (48–49°N) and a negative effect of summer drought in the south (47–48°N). In the future, the natural and human disturbance regimes will be dominant controls of the actual biomass productivity of red maple at the northern limit of its present distribution range. Direct effects of climate on maple growth would likely be less important in this context, and will likely entail negative effect of increased summer drought in the southern part of the study area and positive effects of increased temperatures in the north.

1. Forest fires are one of the main disturbance agents in boreal and temperate ecosystems. To decipher large-scale temporal and spatial patterns of past fire activity in Scandinavia, we analysed the synchronicity of dendrochoronologically reconstructed fire events in a large network of sites (n = 62; 3296 samples, 392 individual fire years) covering a wide geographical gradient (56.5–67.0° N and 9.3–20.5° E) over AD 1400–1900. We identified large fire years (LFY) as years with regionally increased forest fire activity and located the geographical centres of climatic anomalies associated with synchronous LFY occurrence across the region, termed LFY centroids.
2. The spatial pattern of LFY centroids indicated the presence of two regions with climatically mediated synchronicity of fire occurrence, located south and north from 60° N. The return intervals of LFYs in Scandinavia followed a Weibull distribution in both regions. Intervals, however, differed: a period of 40 years would carry a 0.93 probability of LFY occurrence in the southern region, but only a 0.48 probability of LFY occurrence in the northern region.
3. Over 1420–1759, the northern region was characterized by significantly higher temporal variability in LFY occurrence than the southern region. Temporal correlation of LFYs with reconstructed average summer temperature and total precipitation was evident mainly for the northern region. LFYs in this region were associated with positive temperature and negative precipitation anomalies over Scandinavia and with colder and wetter conditions in more southern parts of the European subcontinent.
4. Synthesis. Historical patterns of the occurrence of large fire years (LFY) in Scandinavia point towards the presence of two well-defined zones with characteristic fire activity, with the geographical division at approximately 60° N. The northern and mid-boreal forests, although exhibiting lower LFY frequencies, appeared to be more sensitive to past summer climate, as compared to the southern boreal forests. This would imply that fire regimes across Scandinavia may show an asynchronous response to future climate changes.

We used dendrochronological methods to study disturbance history of a mixed Korean pine (Pinus koraiensis Siebold et Zuccarini) dominated forest on the northern slope of Changbai Mountain, North Eastern China, over 1770–2000. Frequent small-scale canopy gaps and infrequent medium-scale canopy disturbances dominated natural disturbance regime in the forest, which did not experience stand-replacing disturbances over the studied period. Percentages of growth releases in subcanopy trees were below 6% in most decades, suggesting that disturbances initiating these releases were of low intensity. Strong winds were likely cause of moderate disturbance events. Two episodes with increased disturbance rates (19% and 13%) were dated to the 1920s and 1980s, timing of the 1980s event was consistent with a hurricane occurred in 1986 on the western slope of the Changbai Mountain. Age structure and growth release analyses revealed species-specific regeneration strategies of canopy dominants. Shade-intolerant Olga bay larch (Larix olgensis Henry) recruited mainly before the 1860s. Recruitment of moderately shade-tolerant P. koraiensis occurred as several regeneration waves (1820s, 1850s, 1870–1880s, 1930s, and 1990–2000s) of moderate intensity. Shade-tolerant Jezo spruce (Picea jezoensis Carr. var. komarovii (V. Vassil.) Cheng et L.K.Fu) and Manchurian fir (Abies nephrolepis (Trautv.) Maxim.) regenerated continuously over the last 220 and 130 years, respectively. Enhanced recruitment of P. koraiensis, P. jezoensis, and A. nephrolepis was observed during the 1930s and 1990s, coinciding with increased growth release frequency in the 1920s and 1980s, and suggesting disturbance events of moderate intensity. Our results indicate that the current disturbance regime of the mixed Korean pine dominated forest maintains coexistence of light-demanding and shade-tolerant species and that change in wind climate may be particularly important for future forest composition.

Afin d’évaluer les facteurs environnementaux expliquant la limite nordique de répartition du bouleau jaune (Betula alleghaniensis Britton) dans l’est du Canada, nous avons analysé l’abondance, la structure d’âge, le taux d’accumulation de biomasse et la sensibilité de la croissance au climat de cette espèce dans 14 sites répartis le long d’un gradient latitudinal de 200 km couvrant trois domaines bioclimatiques et atteignant les populations les plus nordiques dans l’ouest du Québec. Nous avons observé une grande variabilité de la densité de plantules entre les domaines, ainsi que de la présence de sites avec une régénération abondante du bouleau jaune dans chaque domaine bioclimatique. La densité de plantules était corrélée positivement à l’âge moyen et à l’abondance des bouleaux jaunes mâtures dans la canopée, alors que la densité de gaulis était positivement associée aux habitats plus secs. Les patrons de croissance des arbres mâtures n’ont pas montré d’effet négatif de la température le long du gradient sud-nord. Les facteurs contrôlant la limite nordique de répartition du bouleau jaune agissaient sur la régénération, mais pas sur la croissance des arbres mâtures. À l’échelle du peuplement, la densité de régénération était fortement contrôlée par les conditions locales de site, et non par les différences climatiques entre les sites. A l’échelle régionale, la variabilité climatique pourrait indirectement contrôler la répartition du bouleau jaune en affectant la fréquence des perturbations et, conséquemment, la disponibilité de sites propices à la régénération.

Synchronous production of large seed crops, or mast years (MYs), is a common feature of many Fagus species, which is closely linked to the dynamics of forest ecosystems, including regeneration of canopy trees and changes in animal population densities. To better understand its climatic controls and check for the presence of long-term temporal trends in MY frequencies, we reconstructed MY record of the European beech (Fagus sylvatica L.) for the southern Swedish province of Halland over 1753–2006. We used superimposed epoch analysis (SEA) to relate MY (a) to summer temperature fields over the European subcontinent and (b) to the patterns of 500 mb geopotential heights over the 35–75°N. For the MY reconstruction, we used newly developed regional beech ring-width chronology (1753–2006), an available summer temperature reconstruction, and a discontinuous historical MY record. A Monte Carlo experiment allowed identification of the thresholds in both growth and summer temperature anomalies, indicative of historical MYs, which were verified by dividing data into temporally independent calibration and verification sub-periods.

MYs were strongly associated with both the 500 mb height anomalies and average summer temperatures during two years preceding a MY: a mast year (t) followed a cold summer two years (t-2) prior to the mast year and a warm summer one year prior (t-1) to the mast year. During t-2 years, the geographical pattern of 500 mb height anomalies exhibited a strong height depression in the region centered in the Northern Sea and extending toward eastern North America and statistically significant (p < 0.05) temperature anomalies covering predominantly southern Scandinavia (area below 60 N) and British Isles. A year immediately preceding a mast year (t-1) was characterized by a strong regional high pressure anomaly centered in southern Scandinavia with significant temperature anomalies extended mostly over southern Scandinavia and Germany.

The long-term mean MY return interval was 6.3 years, with 50 and 90% probabilities of MY occurrence corresponding to 6 and 15 years, respectively. Periods with intervals significantly shorter than the long-term mean were observed around 1820–1860 and 1990–2006 (means – 3.9 and 3.2 years, respectively). However, the difference in return intervals between two sub-periods themselves was not significant.

Geographically large and temporally rapid changes in atmospheric circulation among years, responsible for summer temperature conditions in the Northern Europe, are likely primary environmental drivers of masting phenomenon. However, decadal and centurial variability in MY intervals is difficult to relate directly to temperature variability, suggesting the presence of conditions “canceling” would-be MYs. Long-term MY reconstruction demonstrates high variability of reproductive behavior in European beech and indicates that a period with shorter MY intervals at the end of 20th may be not unique in a multi-century perspective.

Nous avons étudié les effets de l’épaississement de la couche organique au sol (COS) sur la croissance et la distribution de l’épinette noire (Picea mariana (Mill.) Britton, Sterns, Poggenb.) et du peuplier faux-tremble (Populus tremuloides Michx.) dans la ceinture d’argile du Québec. À l’échelle du paysage, l’épinette couvrait un plus large gradient d’épaisseur de la COS (1 à 100 cm) que celui du peuplier (1 à 30 cm). Pour les arbres âgés de 60 à 100 ans, l’épaisseur de la COS n’avait pas d’effet sur l’accroissement en surface terrière (AST) de l’épinette, mais montrait une forte corrélation négative avec l’AST du peuplier. La croissance radiale de l’épinette noire était favorisée par de fortes précipitations en juin de l’année précédente, des températures élevées au début de l’hiver et au printemps, et par des températures froides en été. Bien que statistiquement significative, l’épaisseur de la COS avait des effets modérés sur la relation entre le climat et la croissance de l’épinette, en ayant apparemment un impact sur l’isolation des racines durant la période de dormance et sur la disponibilité en eau durant la période de croissance. Dans le cas du peuplier, la température en juin de l’année courante était le plus important facteur corrélé positivement à la croissance. L’épaisseur de la COS influençait la relation entre la croissance du peuplier et (i) la température de janvier et (ii) l’indice de sécheresse mensuel de juin à août. Nous prévoyons que la réaction de l’épinette noire aux changements climatiques devrait être assez uniforme dans la région étudiée, alors que celle du peuplier sera probablement fortement influencée par l’épaisseur de la COS.

• To understand how the future climate will affect the boreal forest, we studied growth responses to climate variability in black spruce (Picea mariana [Mill.] B.S.P.) and trembling aspen (Populus tremuloides Michx.), two major co-occurring boreal tree species of the eastern Canadian boreal forest.
• We analysed climate–growth interaction during (i) periods of non-anomalous growth and (ii) in years with strong growth anomalies. We utilized paired tree-level data for both growth and soil variables, which helped ensure that the studied growth variability was a function of species-specific biology, and not of within stand variation in soil conditions.
• Redundancy analysis conducted on spruce and aspen tree ring chronologies showed that their growth was affected differently by climate. During non-anomalous years, growth of spruce was favoured by cooler temperatures and wetter conditions, while aspen growth was favoured by higher temperatures and drier conditions.
• Black spruce and trembling aspen also showed an inverse pattern in respect to expression of growth anomalies (pointer years). A negative growth anomaly in spruce tended to be associated with positive ones in aspen and vice versa. This suggested that spruce and aspen had largely contrasting species-specific responses to both ‘average’ weather conditions and extreme weather events.
• Synthesis. Species-specific responses to environmental variability imply that tree responses to future climate will likely be not synchronized among species, which may translate into changes in structure and composition of future forest communities. In particular, we speculate that outcome of climate change in respect to relative abundance of black spruce and trembling aspen at the regional levels will be highly dependent on the balance between increasing temperatures and precipitation. Further, species-specific responses of trees to annual climate variability may enhance the resilience of mixed forests by constraining variability in their annual biomass accumulation, as compared with pure stands, under periods with high frequency of climatically extreme conditions.

The study of forests dominated by red pine (Pinus resinosa Ait.), one of the few fire-resistant tree species of eastern North America, provides an opportunity to reconstruct long-term fire histories and examine the temporal dynamics of climate forcing upon forest fire regimes. We used a 300-year long spatially explicit dendrochronological reconstruction of the fire regime for Seney National Wildlife Refuge (SNWR, 38,531 ha), eastern Upper Michigan to: (1) identify fire size thresholds with strong vs. weak climate controls, (2) evaluate effect of landform type (outwash channel vs. sand ridges) in modifying climate–fire associations, and (3) check for the presence of temporal changes in the climate control of large fire events over the time period 1700–1983. We used a summer drought sensitive red pine chronology (ITRDB code can037) as a proxy of past fire-related climate variability. Results indicated that fires >60 ha in sand-ridge-dominated portions of SNWR and >100 ha in outwash channels were likely climatically driven events. Climate–fire associations varied over time with significant climate–fire linkages observed for the periods 1700–1800 (pre-EuroAmerican), 1800–1900 (EuroAmerican settlement) and 1900–1983 (modern era). Although an increase in fire activity at the turn of 20th century is commonly associated with human sources of ignitions, our results suggest that such an increase was also likely a climatically driven episode.

We used tree-ring data from a major North American boreal tree species, Jack pine (Pinus banksiana Lamb.), to decipher spatial and temporal tree-growth responses to climate variability within the area of northwestern Quebec and northeastern Ontario. Fifteen sites with clay soils were selected and grouped into North and South sub-regions at approximately 49°N and 50°N, respectively. Tree-ring chronologies were analyzed through a response function for the years 1951–2000 to identify growth-limiting climate factors. Increased precipitation in June in the previous year and a warm month of April this year favored radial growth whereas higher temperature in September and increased precipitation in October, both of the previous year, and current June precipitation were negatively related to growth. There was a clear difference in climatic response between the southern and northern sub-regions: southern sites were more responsive to temperature dynamics while on northern sites Jack pine growth appeared negatively influenced by an excess of precipitation. Soil conditions, with larger areas covered by less water permeable clay deposits in the northern sub-region, explain this result.If recently observed trends towards warmer springs continue, Jack pine may increase its radial growth in the study area. However, increases in fall precipitation, also predicted under the future climate, may offset the positive effect of previous years weather on clay sites.

Variation in natural disturbance regime within a landscape is important for species population dynamics, because it controls spatial arrangement of sites providing regeneration and survival opportunities. In this study, we examine the differences in fire regime and evaluate possible sources of its variation between the surrounding mainland and the islands of Lake Duparquet (44.5 km²), a typical boreal lake in north-western Quebec, Canada. Dendrochronological reconstructions suggest that fires were frequent and of variable intensity on the islands, whereas fires were less frequent on the adjacent mainland, but were usually large and intense. Islands were significantly drier and warmer than the mainland, and maximum values of Fire Weather Index were significantly higher on the islands during both the early part of the fire season (May–June) and the whole fire season (May–September). The lightning density within the lake perimeter was significantly higher than in the surrounding mainland (0.63 v. 0.48 year^–1 km^–2 respectively). This pattern was a result of the differences in lightning density during the first half of the lightning season. The study suggests that more fire-prone local weather and higher frequency of lightning strikes could cause a higher frequency of low-intensity fires on the islands, compared with the mainland.

Island-lake ecosystems are suitable for testing scale dependence in forests disturbance theories thanks to differences in the potential for fire spread on islands and the mainland. We investigated past fire regime on the mainland and on islands in a large lake in south-east Sweden. We used dendrochronological methods to reconstruct fire disturbances on 18 small islands (0.04–24.1 ha) and in 43 sites in the surrounding 75-km² landscape over the last 400 years. In the past, fires were frequent on both islands and mainland but not synchronised on an annual scale. Significant temporal changes occurred around the middle of the 18th century. Before 1750, fires were less frequent on islands than on the mainland (median fire return interval 58 v. 25 years respectively). However, an inversion of this pattern was observed during 1750–1860: islands showed even shorter fire intervals than mainland locations, suggesting additional and likely human-related source of ignitions (median fire return interval 15 v. 29 years respectively). A substantial decrease in fire activity in both islands and mainland was apparent in 1860–1890. We suggest that the present fire regime (the last 100 years) on the small islands is largely natural as fire suppression is not present there. The dynamic nature of the fire regime on islands still requires further studies: islands may, at times, attract lightning, humans with fire, or both.

The observed long-term decrease in the regional fire activity of Eastern Canada results in excessive accumulation of organic layer on the forest floor of coniferous forests, which may affect climate–growth relationships in canopy trees. To test this hypothesis, we related tree-ring chronologies of black spruce (Picea mariana (Mill.) B.S.P.) to soil organic layer (SOL) depth at the stand scale in the lowland forests of Quebec’s Clay Belt. Late-winter and early-spring temperatures and temperature at the end of the previous year’s growing season were the major monthly level environmental controls of spruce growth. The effect of SOL on climate–growth relationships was moderate and reversed the association between tree growth and summer aridity from a negative to a positive relationship: trees growing on thin organic layers were thus negatively affected by drought, whereas it was the opposite for sites with deep (>20–30 cm) organic layers. This indicates the development of wetter conditions on sites with thicker SOL. Deep SOL were also associated with an increased frequency of negative growth anomalies (pointer years) in tree-ring chronologies. Our results emphasize the presence of nonlinear growth responses to SOL accumulation, suggesting 20–30 cm as a provisional threshold with respect to the effects of SOL on the climate–growth relationship. Given the current climatic conditions characterized by generally low-fire activity and a trend toward accumulation of SOL, the importance of SOL effects in the black spruce ecosystem is expected to increase in the future. Content Type Journal Article

We investigated changes in wildfire risk over the 1901−2002 (ad) period with an analysis of broad-scale patterns of July monthly drought code (MDC) variability on 28 forested ecoregions of the North American and Eurasian continents. The MDC is an estimate of the net effect of changes in evapotranspiration and precipitation on cumulative moisture depletion in soils, and is well correlated with annual fire statistics across the circumboreal (explaining 25–61% of the variance in regional area burned). We used linear trend and regime shift analyses to investigate (multi-) decadal changes in MDC and percentage area affected by drought, and kernel function for analysis of temporal changes in the occurrence rates of extreme drought years. Our analyses did not reveal widespread patterns of linear increases in dryness through time as a response to rising Northern Hemisphere land temperatures. Instead, we found heterogeneous patterns of drought severity changes that were inherent to the nonuniformly distributed impacts of climate change on dryness. Notably, significant trends toward increasing summer moisture in southeastern and southwestern boreal Canada were detected. The diminishing wildfire risk in these regions is coherent with widely reported decreases in area burned since about 1850, as reconstructed by dendrochronological dating of forest stands. Conversely, we found evidence for increasing percentage area affected by extreme droughts in Eurasia (+0.57% per decade; P<0.05) and occurrence rates of extreme drought years in Eurasian taiga (centered principally on the Okhotsk–Manchurian taiga, P=0.07). Although not statistically significant, temporal changes in occurrence rates are sufficiently important spatially to be paid further attention. The absence of a linear trend in MDC severity, in conjunction with the presence of an increase in the occurrence rate of extreme drought years, suggest that fire disturbance regimes in the Eurasian taiga could be shifting toward being increasingly pulse dependent.

We investigated changes in wildfire risk over the 19012002 (AD) period with an analysis of broad-scale patterns of July monthly drought code (MDC) variability on 28 forested ecoregions of the North American and Eurasian continents. The MDC is an estimate of the net effect of changes in evapotranspiration and precipitation on cumulative moisture depletion in soils, and is well correlated with annual fire statistics across the circumboreal (explaining 25–61% of the variance in regional area burned).We used linear trend and regime shift analyses to investigate (multi-) decadal changes in MDC and percentage area affected by drought, and kernel function for analysis of temporal changes in the occurrence rates of extreme drought years. Our analyses did not reveal widespread patterns of linear increases in dryness through time as a response to rising Northern Hemisphere land temperatures. Instead, we found heterogeneous patterns of drought severity changes that were inherent to the nonuniformly distributed impacts of climate change on dryness. Notably, significant trends toward increasing summer moisture in southeastern and southwestern boreal Canada were detected. The diminishing wildfire risk in these regions is coherent with widely reported decreases in area burned since about 1850, as reconstructed by dendrochronological dating of forest stands. Conversely, we found evidence for increasing percentage area affected by extreme droughts in Eurasia (10.57% per decade; Po0.05) and occurrence rates of extreme drought years in Eurasian taiga (centered principally on the Okhotsk–Manchurian taiga, P50.07). Although not statistically significant, temporal changes in occurrence rates are sufficiently important spatially to be paid further attention. The absence of a linear trend in MDC severity, in conjunction with the presence of an increase in the occurrence rate of extreme drought years, suggest that fire disturbance regimes in the Eurasian taiga could be shifting toward being increasingly pulse dependent.