Under climate change, modifications on plants’ growth are expected to be the strongest at species margins. Therein, tree acclimation could play a key role as migration is predicted to be too slow to track shifts of bioclimatic envelops. A requirement is, however, that intra-population genetic diversity be high enough for allowing such adaptation of tree populations to climate change. In this study, we tested for the existence of relationships between genetic diversity, site environmental conditions, and the response of annual tree growth to climate of Pinus cembra at its southern limit in the Alps. Site-specific climatic and environmental factors predominantly determined the response of trees along the precipitation gradient. The growth-climate interactions were chiefly linked to mean annual precipitation and temperature, slope and tree-size, and less to genetic diversity. We show that genetic background of Pinus cembra has exclusively indirect modulating power with limited effects on tree-ring formation, and within the southern limit in the Alps, genetic variability is not necessarily well expressed in the patterns of annual tree growth. Our results may imply little adaptive capacity of these populations to future changes in the water balance.

Although lacustrine sedimentary charcoal has long been used to infer paleofires, their quantitative reconstructions require improvements of the calibration of their links with fire regimes (i.e. occurrence, area, and severity) and the taphonomic processes that affect charcoal particles between the production and the deposition in lake sediments. Charcoal particles >150?µm were monitored yearly from 2011 to 2016 using traps submerged in seven head lakes situated in flat-to-rolling boreal forest landscapes in eastern Canada. The burned area was measured, and the above-ground fire severity was assessed using the differentiated normalized burn ratio (dNBR) index, derived from LANDSAT images, and measurements taken within zones radiating 3, 15, and 30?km from the lakes. In order to evaluate potential lag effects in the charcoal record, fire metrics were assessed for the year of recorded charcoal recording (lag 0) and up to 5?years before charcoal deposition (lag 5). A total of 92 variables were generated and sorted using a Random Forest-based methodology. The most explanatory variables for annual charcoal particle presence, expressed as the median surface area, were selected. Results show that, temporally, sedimentary charcoal accurately recorded fire events without a temporal lag; spatially, fires were recorded up to 30?km from the lakes. Selected variables highlighted the importance of burned area and fire severity in explaining lacustrine charcoal. The charcoal influx was thus driven by fire area and severity during the production process. The dispersion process of particles resulted mostly of wind transportation within the regional (<30?km) source area. Overall, charcoal median surface area represents a reliable proxy for reconstructing past burned areas and fire severities.

We used bioproxies from paleosoils buried within two aeolian dunes to test hypotheses concerning the origin of dry sandy boreal forests in Canada. These forests are dominated today by Pinus banksiana Lamb. One hypothesis is that too frequent Holocene stand-replacing fires would have transformed the original vegetation through extirpation of susceptible species to fire in water stress habitat. Alternatively, the ecosystem would have not changed since the dunes stabilized enough to support forest establishment. The vegetation composition and richness were determined by identification of charcoal and macroremains and radiocarbon dating for the chronology. Both sites revealed a similar history covering 6400 years. Half of the charcoal layers were less than 2500 years old in both sites, indicating that they had been subjected to the same fire history. Data indicated a stable plant composition and richness, although the percentage of Pinus decreased slightly over 4000 years (decreasing rate 1% per century). The fungus Cenococcum geophilum was consistently present, with a stochastic abundance. The vegetation grew under natural fire conditions and soil dryness since 6000 years. The ecosystem was probably not stressed by late-Holocene fires or climate changes, as the multi-millennial steady state reveals a resistant and resilient ecosystem.

Genetic processes shape the modern-day distribution of genetic variation within and between populations and can provide important insights into the underlying mechanisms of evolution. The resulting genetic variation is often unequally partitioned within species’ distribution range and especially large differences can manifest at the range limit, where population fragmentation and isolation play a crucial role in species survival. Despite several molecular studies investigating the genetic diversity and differentiation of European Alpine mountain forests, the climatic and demographic constrains which influence the genetic processes are often unknown. Here, we apply non-coding microsatellite markers to evaluate the sporadic peripheral and continuous populations of cembra pine (Pinus cembra L.), a long-lived conifer species that inhabits the subalpine treeline ecotone in the western Alps to investigate how the genetic processes contribute to the modern-day spatial distribution. Moreover, we corroborate our findings with paleoecological records, micro and macro-remains, to infer the species’ possible glacial refugia and expansion scenarios.

In the boreal forest of eastern North America, the distribution of eastern white cedar (Thuja occidentalis) is characterized by a latitudinal fragmentation gradient from south to north. Marginal populations could be outposts allowing cedar to expand its presence in the north in response to climate change. This study aimed to characterize the spatio-temporal regeneration dynamics of 20 marginal cedar stands in order to evaluate their expansion capacity into adjacent black spruce stands. Cedar recruitment within marginal cedar stands was mainly by layering, which allowed the species to maintain for a long time in the landscape. However, the rate of expansion of marginal stands into adjacent black spruce stands through seed dispersal was very low (0.28 m.year?1) and it was negatively influenced by distance to seed trees. Substrate had no significant effect. Global warming could lead to increased seed production by cedar, which could favour the species’ expansion at its northern distribution limit. However, global warming could also increase the frequency and severity of wildfires, which would have a negative effect on cedar expansion capacity.

Tree species responses to climate change will be greatly influenced by their evolutionary potential and their phenotypic plasticity. Investigating tree-rings responses to climate and population genetics at the regional scale is crucial in assessing the tree behavior to climate change. This study combined in situ dendroclimatology and population genetics over a latitudinal gradient and compared the variations between the two at the intra- and inter-population levels. This approach was applied on the northern marginal populations of Thuja occidentalis (eastern white-cedar) in the Canadian boreal forest. We aimed first to assess the radial growth variability (response functional trait) within populations across the gradient and to compare it with the genetic diversity (microsatellites). Second, we investigated the variability in the growth response to climate at the regional scale through the radial growth-climate relationships, and tested its correlation with environmental variables and population genetic structure. Model selection based on the Akaike Information Criteria revealed that the growth synchronicity between pairs of trees of a population covariates with both the genetic diversity of this population and the amount of precipitation (inverse correlations), although these variables only explained a small fraction of the observed variance. At the regional scale, variance partitioning and partial redundancy analysis indicate that the growth response to climate was greatly modulated by stand environmental variables, suggesting predominant plastic variations in growth-response to climate. Combining in situ dendroclimatology and population genetics is a promising way to investigate species' response capacity to climate change in natural stands. We stress the need to control for local climate and site conditions effects on dendroclimatic response to climate to avoid misleading conclusions regarding the associations with genetic variables.

Aim

Towards the cold margins of the Northern Hemisphere boreal zone, continuing warming should theoretically provide a longer vegetative season, favouring growth and a northward shift in tree species distribution. The northern distribution of Thuja occidentalis L. (eastern white cedar) is marked by the presence of isolated marginal populations distant from the continuous distribution. If those populations proved to be well adapted to their future local climatic conditions, their expansion could accelerate cedar poleward migration. We tested the hypotheses that (1) there will be a growth increase in cedar northern marginal populations as a result of global warming, and (2) the edaphic conditions and regional precipitation regimes will modulate their response to warming.

Location

Canadian boreal forest, western Québec (47–50° N, 74–80° W).

Methods

We investigated radial growth using tree-ring measurements from dominant and co-dominant eastern white cedar trees (n = 723) distributed along a latitudinal gradient from the species' northern margin to the centre of its natural range. First, low-frequency growth variations were analysed on whole chronologies (ad 1720–2010). Second, inter-annual growth variations were tested against ad 1953–2010 monthly temperature and precipitation time series with a bootstrapped correlation function. Finally, the impact of environmental variables on the growth–climate relationships was assessed.

Results

Unexpectedly, a growth decline was observed starting in 1980 in marginal sites. Dendroclimatic analyses revealed that radial growth was not only limited by short growing seasons but also by summer droughts in the marginal zone. This response was exacerbated in sites that received less summer precipitation. Counterintuitively, autumn and spring precipitation negatively impacted on growth, especially in wet soil stands.

Main conclusions

Northern marginal populations of cedar may have already reached their optimum temperature threshold for radial growth. Our results suggest that they will probably be facing increasing hydric stress selection pressure under the assumptions of climate change. Their responses to future warming will be highly dependent on the seasonality and magnitude of variation in precipitation regimes.

Paleofire events obtained from the statistical treatment of sedimentary charcoal records rely on a number of assumptions and user's choices, increasing the uncertainty of reconstructio\ns. Among the assumptions made when analyzing charcoal series is the choice of a filtering method for raw Charcoal Accumulation Rate (CHARraw). As there is no ultimate CHARraw filtering method, we propose an ensemble-member approach to reconstruct fire events. We modified the commonly used procedure by including a routine replicating the analysis of a charcoal record using custom smoothing parameters. Dates of robust fire events, uncertainties in fire-return intervals and fire frequencies are derived from members' distributions. An application of the method is used to quantify uncertainties due to data treatment in two CHARraw sequences from two different biomes, subalpine and boreal.

• Strategic introduction of less flammable broadleaf vegetation into landscapes was suggested as a management strategy for decreasing the risk of boreal wildfires projected under climatic change. However, the realization and strength of this offsetting effect in an actual environment remain to be demonstrated.
• Here we combined paleoecological data, global climate models and wildfire modelling to assess regional fire frequency (RegFF, i.e. the number of fires through time) in boreal forests as it relates to tree species composition and climate over millennial time-scales.
• Lacustrine charcoals from northern landscapes of eastern boreal Canada indicate that RegFF during the mid-Holocene (6000–3000 yr ago) was significantly higher than pre-industrial RegFF (ad c. 1750). In southern landscapes, RegFF was not significantly higher than the pre-industrial RegFF in spite of the declining drought severity. The modelling experiment indicates that the high fire risk brought about by a warmer and drier climate in the south during the mid-Holocene was offset by a higher broadleaf component.
• Our data highlight an important function for broadleaf vegetation in determining boreal RegFF in a warmer climate. We estimate that its feedback may be large enough to offset the projected climate change impacts on drought conditions.

In this era of climate change, understanding past and predicting future fire activity are scientific challenges that are central to the development of sustainable forest management practices and policies. Such objectives, however, are difficult to achieve for several reasons. Uncertainties about future fire activity can be superimposed on the short time period covered by existing meteorological data and fire statistics, from which a historical range of variability can be determined. Regional fire activity is also tremendously variable over time, such that contemporary fire records cannot provide information on the full range of fire activity variability a given forest experienced and adapted to. This factor is increasingly important when it comes to determining the resilience of boreal forests to changes in climate and disturbance regimes. In this paper, we present a synthesis of past, present and future trends in seasonal fire danger and fire activity based on data gathered in eastern Canadian boreal forests over the last 20 years, and we provide a critical assessment of the ability to conduct sustainable forest management over the 21st century. The data synthesis provides compelling evidence of a synchronous pattern of decreasing fire-conducive climatic conditions and activity of large fire seasons over the last 2000 years in the eastern coniferous boreal forest. Model simulations suggest that the climate will become drier in upcoming decades, driving future fire activity close to the upper bound of the pre-industrial range of variability. The effects of increasing fire incidence cumulated with forest harvesting may thus pose a risk to forest resilience in the future. This ecological knowledge should help us to define forest management strategies and practices considering future fire activity changes forecasted under climate change. Development of alternative silvicultural interventions that would emulate secondary disturbances (e.g. wind, insects) rather than fire would be necessary to maintain pre-industrial forest characteristics (e.g. composition and age class distribution), and associated forest resilience.