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.

Permanent landscape attributes such as topography (elevation [m]) and microtopography (local variation in elevation [cm]) can increase the risk of cold air drainage down-slopes and in microtopographic depressions, causing important temperature gradients that generate localized growing-season frosts. Since most studies on growing-season frosts are restricted to the northern parts of the boreal forest or to mountainous areas, their negative consequences on tree productivity at the boreal-temperate forest ecotone is often ignored. We quantified the intensity and probability of growing-season frosts at the boreal-temperate forest ecotone in regard to topographic and microtopographic landscape attributes, which were extracted from airborne LiDAR surveys. In situ air temperature was measured for two summers (2016–2017) with 252 temperature loggers installed in two 18-years-old spruce plantations established in both temperate and boreal mixedwood forests. Growing-season frosts were more intense and probable at the boreal mixedwood forest site compared to the temperate forest site. Still, at both sites, when growing-season frost occurred, air temperature could vary by 4 °C along the elevation gradient of 15 m, often reaching sub-zero values at low elevation while reaching above-freezing values at high elevation. The importance of microtopography on the risk of sub-zero temperatures increased where frost events were less likely to occur such as at the temperate forest and at high elevation. Considering that growing-season frosts can considerably reduce tree productivity, the effects of both topography and planting microsite should be considered when determining where to establish plantations in the landscape and to determine suitable frost-free planting microsites within a plantation.

Reforestation in the boreal forest is challenging; trees must survive to large daily temperature variations and to the cold environment. Even if local tree species are adapted to withstand these harsh environmental conditions, spruce plantation failure after artificial regeneration occurs frequently, with important impacts on sustainable forest management. We hypothesized that this regeneration problem is caused by recurrent frost events occurring during the growing season. These events would freeze the terminal bud and the newly formed needles of the planted trees, thus limiting photosynthesis capacity and height growth. Our goal was to identify key permanent physical attributes of the landscape (elevation, slope shape and angular slope) and of microsite conditions (hole vs. mound) that best predict tree height and frost damage to foliage. In summer 2016 and 2017, we sampled tree height of 2,943 white spruce (Picea glauca [Moench] Voss) and black spruce (Picea mariana [Mill.] B.S.P.) trees in 66 monoculture plantations aged between 6 and 13 years distributed in the Clay Belt region of Quebec (Canada), and environment prone to frequent growing-season frosts. Using linear and binomial mixed regression models, we analyzed the effects of the physical attributes of the landscape and of microsite conditions on tree height (linear) and on frost damage (binomial). Tree height increased with increasing elevation and when seedlings were planted on mounds compared to planted in holes. The impact of microsite conditions on tree height increased as plantations aged, but the importance of elevation on tree height decreased with age. The probability of frost damage to foliage decreased for trees planted on mounds compared to trees planted in holes and from concave to convex slopes. These relations were most important in young plantations, but trees showing growth problems were still shorter by 2 m, even 13 years after planting. We also observed differences between species: white spruce was significantly more damaged by frost and was smaller compared to black spruce. Therefore, growing-season frosts can cause growth suppression problems in white spruce plantations established in the boreal mixedwood region. Since microsite conditions also play a key role in driving plantation success, mechanical site preparation techniques should not only focus on reducing the competition between the planted trees and the competing vegetation but should also focus on limiting frost damage by planting trees on elevated microsites. Our results will support forestry practices limiting plantation failure in boreal mixedwoods.

Aim

Non?climatic constraints on species northern range boundaries are often overlooked in attempts to predict climate?induced range shifts. Here, we examined the effects of habitat availability and fire disturbance on the distribution of a species that transitions from being common to being found only in marginal populations at the northern boundary of its range.

Location

North?western Quebec, Canada (46–51° N and 74–79° W).

Taxon : Eastern white cedar (Thuja occidentalis L.)

Methods

We used forest inventory data (n = 4,987) to characterize white?cedar habitat based on edaphic and topographic conditions at sampled sites along a 600?km latitudinal gradient. Non?metric multidimensional scaling was used to assess habitat similarity of sites in the south, where white?cedar stands are abundant, and sites in the north, where white?cedar stands are rare. We constructed ensemble white cedar distribution models based on habitat variables in the south and compared ensemble forecast projections of white cedar in the north with observed occurrences to determine if habitat availability was limiting. We independently estimated the age of white?cedar stands and adjacent stands without white cedar along the gradient. ANOVA was performed to test the age difference between white?cedar and adjacent stands to determine whether the location of white?cedar stands was influenced by disturbance, primarily stand?replacing fire.

Results

Habitat availability was not limiting the distribution of eastern white cedar at its northern range boundary. White cedar did not occupy most sites with suitable habitat in the north, suggesting that other factors prevent white cedar from establishing more stands northward. White?cedar stands were older than adjacent stands without white cedar all along the gradient, but the difference was more pronounced in the north. This suggests that white?cedar stands in the north are restricted to undisturbed areas.

Main conclusions

Fire disturbance, more than habitat availability, limits the distribution of white cedar at its northern range boundary. Projections of white cedar distribution under climate change that ignore fire could overestimate the ability of warming temperatures to extend its northern range limit.

Increase in frost damage to trees due to earlier spring dehardening could outweigh the expected increase in forest productivity caused by climate warming. We quantified the impact of growing?season frosts on the performance of three spruce species (white, black, and Norway spruce) and various seed sources with different frost tolerance in two plantations, established on both sides of the eastern Canadian boreal?temperate forest ecotone. The objectives of this study were to determine (a) if spruce species and seed sources planted in sites far from their natural provenance would be less adapted to local site conditions, leading to increased frost damage and reduced height growth; (b) at which height above the ground growing?season frosts ceased to damage apical meristems; and (c) if height growth was best predicted by extreme climatic events (growing?season frosts) or by mean annual or summer temperature. At each site and for all spruce species and seed sources, we cross?sectioned spruce trees at different heights above the ground. Tree rings were cross?dated and screened for frost rings, which were then given a severity score based on cellular damage. Frost severity reduced height growth of all spruce species and provenances at both sites. Height growth of the non?native Norway spruce was the most reduced by frost severity and was the smallest species at both sites. Frost caused the highest growth reduction in white spruce at the boreal mixedwood site and had the least effect on black spruce at both sites. For all spruce species, height growth was affected up to 2 m above the ground. Model selection based on corrected Akaike's information criteria (AICc) identified that minimum temperature in May was by far the best climate variable predicting tree growth (AICc weight = 1), highlighting the importance of considering extreme climatic events, which are likely to increase in the future.

Climate warming-driven early leaf-out is expected to increase forest productivity but concurrently increases leaf exposure to spring frosts, which could reduce forests' net productivity. We hypothesized that due to their damaging effect on buds, spring frosts exert a stronger control on bud phenology than do growing degree-days. We monitored bud flush phenology of three white spruce seed sources (one local seed source from the boreal mixedwood forest and two seed sources from the temperate forest), one black spruce seed source originating from the boreal mixedwood forest and four nonlocal Norway spruce seed sources in 2016 and 2017 in two plantations located on both sides of the temperate-boreal mixedwood forest ecotone in eastern Canada (Quebec). We aimed to determine inter- and intraspecies variations in bud break timing and sensitivity to air temperature and photoperiod. We expected that bud break timing for boreal species and seed sources would be better synchronized with the decrease in frost probability than for nonlocal species and seed sources. We used mixed binomial regressions and AICc model selection to determine the best environmental variables predicting each transition from one stage of bud phenology to the next. At both plantation sites, white spruce bud flush began and ended earlier compared to black and Norway spruce. Buds of all spruce species were sensitive to frost probability for early phenological stages, whereas growing degree-days controlled the remaining stages. Photoperiod sensitivity was higher for white spruce compared to black and Norway spruce and reached its maximum in the temperate forest. At intraspecies level, the two southern white spruce seed sources opened their buds earlier than the local source and were more sensitive to photoperiod, which increased their exposure to spring frosts. Onset of spruce bud flush is driven by spring frosts and photoperiod, but once started, bud phenology responds to temperature. The high photoperiod sensitivity in white spruces could counterbalance climate warming and limit future premature leaf-out, whereas the low photoperiod sensitivity in black spruce should not restrain leaf-out advancement with climate warming. Our results call for adapting the temperature-driven hypotheses of ecophysiological models predicting leaf-out to include spring frost probability.

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.

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.

Aim

Distribution?wide trends in climate variability significantly influence the genetic diversity, differentiation and population structure of tree species. This study investigates the effects of disturbances such as fire, fragmentation and climate on modern?day genetic patterns and clonal structures of trembling aspen at the transcontinental scale.

Location

North American boreal zone.

Taxon

Trembling aspen (Populus tremuloides Michx.).

Methods

One thousand two hundred individuals in a 30?site network (40 trees per location) were genotyped with neutral genetic markers and studied in relation to regional differences in climate and surrounding site conditions (aridity, fire cycle, fragmentation). Multiple linear regression models and variance analysis were used to test relationships between genetic indices, structural parameters and the surrounding site factors.

Results

Overall, a high percentage of single ramet clones (SRC) and clonal diversity was detected and assumed to be the consequence of multiple sexual reproduction events that took place at all sites, together with suckering, which shapes the clonal structure of populations. Neutral genetic diversity and clonal structure suggested no substantial differences among sites, which were categorized into climate moisture index (CMI) classes; aspen stands across Canada were highly similar from a genetic point of view. Allelic richness (AR) and the average number of alleles (Na) varied significantly among clonal organization groups, and landscape fragmentation and a higher frequency of fires showed a negative influence on the levels of genetic diversity.

Main conclusions

Our results are inconsistent with the idea that the genotypic diversity of trembling aspen is related to the intensity of disturbance within the boreal forest. It appears that species?specific disturbance responses and post?fire recruitment mechanisms are more important than dominant ecological factors, such as climate and fire regimes, in shaping distribution?wide patterns of neutral genetic variation and clonal structure.

Contrary to the generally advanced spring leaf unfolding under global warming, the effects of the climate warming on autumn leaf senescence are highly variable with advanced, delayed, and unchanged patterns being all reported. Using one million records of leaf phenology from four dominant temperate species in Europe, we investigated the temperature sensitivities of spring leaf unfolding and autumn leaf senescence (ST, advanced or delayed days per degree Celsius). The ST of spring phenology in all of the four examined species showed an increase and decrease during 1951–1980 and 1981–2013, respectively. The decrease in the ST during 1981–2013 appears to be caused by reduced accumulation of chilling units. As with spring phenology, the ST of leaf senescence of early successional and exotic species started to decline since 1980. In contrast, for late successional species, the ST of autumn senescence showed an increase for the entire study period from 1951 to 2013. Moreover, the impacts of rising temperature associated with global warming on spring leaf unfolding were stronger than those on autumn leaf senescence. The timing of leaf senescence was positively correlated with the timing of leaf unfolding during 1951–1980. However, as climate warming continued, the differences in the responses between spring and autumn phenology gradually increased, so that the correlation was no more significant during 1981–2013. Our results further suggest that since 2000, due to the decreased temperature sensitivity of leaf unfolding the length of the growing season has not increased any more. These finding needs to be addressed in vegetation models used for assessing the effects of climate change.

The unburned forest remnants of boreal mixed-woods, also known as fire residuals, are essential in providing habitats for disturbance-sensitive species and function as the main source of recolonization of disturbed sites. Forest remnants have been identified as historical or potential reservoirs of genetic diversity in several tree species in fire-prone landscapes. In this study, we investigate the genetic diversity of eastern white cedar (EWC, Thuya occidentalis L.), in forest sites that were affected by different fire regimes. The study area is located in northwestern Quebec, in the Lake Duparquet Research and Teaching Forest (79°10?W–48°30?N). We used 18 polymorphic microsatellite loci to investigate the genetic diversity of eastern white cedar, in forest sites where the fire regime history has been reconstructed to examine the importance of the temporal and spatial continuity of the forest on the genetic diversity of EWC. Three types of landscapes, including; islands within a large lake, fragmented fire refuges (forest patches) and non-fragmented mainland forests were studied. Our results revealed a source-sink dynamic associated with a high level of gene flow. Two of the mainland sites served as the main source of migrants. The level of gene flow in island sites was sufficiently high to counteract the effect of isolation. The fire refuges showed the lowest allelic richness, the highest population differentiation and the fewest number of private alleles. The mean fire-free intervals are much longer in fire-refuges causing environmental isolation through time and higher genetic differentiation. The conservation of large mainland sites should be given careful attention because they maintained high genetic diversity and function as the main source of gene flow. Fire refuges have an intrinsic conservation value in landscapes that are affected by spatially heterogeneous fires because they are important for population persistence through disturbances.

Due to climate change, the ranges of many North American tree species are expected to shift northward. Sugar maple (Acer saccharum Marshall) reaches its northern continuous distributional limit in northeastern North America at the transition between boreal mixed-wood and temperate deciduous forests. We hypothesized that marginal fragmented northern populations from the boreal mixed wood would have a distinct pattern of genetic structure and diversity. We analyzed variation at 18 microsatellite loci from 23 populations distributed along three latitudinal transects (west, central, and east) that encompass the continuous–discontinuous species range. Each transect was divided into two zones, continuous (temperate deciduous) and discontinuous (boreal mixed wood), based on sugar maple stand abundance. Respective positive and negative relationships were found between the distance of each population to the northern limit (D_north), and allelic richness (AR) and population differentiation (FST). These relations were tested for each transect separately; the pattern (discontinuous–continuous) remained significant only for the western transect. structure analysis revealed the presence of four clusters. The most northern populations of each transect were assigned to a distinct group. Asymmetrical gene flow occurred from the southern into the four northernmost populations. Southern populations in Québec may have originated from two different postglacial migration routes. No evidence was found to validate the hypothesis that northern populations were remnants of a larger population that had migrated further north of the species range after the retreat of the ice sheet. The northernmost sugar maple populations possibly originated from long-distance dispersal.

We developed allometric equations for small-diameter woody species growing on mixed forest marginal lands, which are potential sources of biomass for bioenergy. Eleven species of trees and shrubs were sampled from a site located in eastern Canada. Equations derived in this study generally performed better than equations from the literature. Also, fixed-area plots (FAP) and line-intersect sampling (LIS) methods using both random or systematic selection of sampling units were compared to determine which method required the lowest number of measurements to estimate stand biomass for the same precision.

The fixed-area plots method was successfully used to estimate relatively accurately oven-dry biomass per hectare. Results indicated that potentially harvestable woody biomass (oven dry basis) varied between 33-41 and 12–13 t ha?1 for the most and least productive marginal sites respectively. On the most productive site, LIS estimates (between 20 and 42 t ha?1) were usually lower than those obtained using different FAP sampling methods (i.e. systematic or random, small (50 m2) or large (100 m2) plots), but similar on the more open sites (between 10 and 14 t ha?1). Small FAP resulted in a plot without measurements in one case. Moreover, estimates based on small FAP were generally higher, even if not significantly different from larger plot estimates. We therefore suggest using FAP with 100 m2 plots to estimate small-diameter woody biomass on marginal lands with dense vegetation, while LIS, even if promising for open stands, needs further evaluation before recommendation.

Background

In North America, the last ice age is the most recent event with severe consequences on boreal species’ ranges. Phylogeographic patterns of range expansion in trembling aspen (Populus tremuloides) suggested that Beringia is likely to be a refugium and the “ice-free corridor” in Alberta may represent a region where small populations persisted during the last glacial maximum (LGM). The purpose of this study was to ascertain whether the origins of trembling aspen in western North America are reflected in the patterns of neutral genetic diversity and population structure. A total of 28 sites were sampled covering the northwestern part of aspen’s distribution, from Saskatchewan to Alaska. Twelve microsatellite markers were used to describe patterns of genetic diversity. The genetic structure of trembling aspen populations was assessed by using multivariate analyses, Mantel correlograms, neighbor-joining trees and Bayesian analysis.

Results

Microsatellite markers revealed little to no neutral genetic structure of P. tremuloides populations in northwestern North America. Low differentiation among populations and small isolation by distance (IBD) were observed. The most probable number of clusters detected by STRUCTURE was K = 3 (?K = 5.9). The individuals in the populations of the 3 clusters share a common gene pool and showed a high level of admixture. No evidence was found that either Beringia or the “ice-free corridor” were refugia. Highest allelic richness (AR) and lowest heterozygosity (Ho) were observed in Alberta foothills of the Rocky Mountains.

Conclusions

Contrary to our hypothesis, our results showed that microsatellite markers revealed little to no genetic structure in P. tremuloides populations. Consequently, no divergent populations were observed near supposed refugia. The lack of detectable refugia in Beringia and in the “ice-free corridor” was due to high levels of gene flow between trembling apsen populations. More favorable environmental conditions for sexual reproduction and successful trembling aspen seedling establishment may have contributed to increase allelic richness through recombination in populations from the Albertan foothills of the Rocky Mountains.

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.

Balsam poplar seeds are short-lived and require moist seedbeds soon after they are released to germinate. In addition to sexual reproduction, balsam poplar stands can regenerate clonally by root suckering. The origin of stands will in turn affect their genetic structure and root system architecture, which are poorly understood for upland forest stands. Three stands were hydraulically excavated in Quebec (moist) and Alberta (dry) to determine the origin of trees and to characterize root systems with respect to presence of parental roots and root grafts connections. Clones were identified using single-nucleotide polymorphism (SNPs), and all stems, roots and root grafts were aged using dendrochronology techniques. All 82 excavated trees were of sucker origin, and four of the six stands contained a single clone. Parental root connections were found between 22% and 25% of excavated trees, and 53% and 48% of trees were linked with a root graft between the same or different clones, in Alberta and Quebec, respectively. Mean distance between trees connected by parental root was significantly lower than the distance between unconnected trees (0.47 ± 0.25 m vs. 3.14 ± 0.15 m and 1.55 ± 0.27 m vs. 4.25 ± 0.13 m) in Alberta and in Quebec, respectively. The excavations also revealed many dead stumps with live roots, maintained through root connections with live trees. This research highlights that balsam poplar growing in upland stands is a clonal species that can maintain relatively high genotypic diversity, with frequent root connections between trees at maturity. Maintaining an extensive root system through root connections increases the chances of a clone surviving when the above ground tree is dead and may also enhance the resilience of balsam poplar stands after disturbance.

Selection cutting is a treatment that emulates tree-by-tree replacement for forests with uneven-age structures. It creates small openings in large areas and often generates a more homogenous forest structure (fewer large leaving trees and defective trees) that differs from old-growth forest. In this study, we evaluated whether this type of harvesting has an impact on genetic diversity of sugar maple (Acer saccharum Marshall). Genetic diversity among seedlings, saplings, and mature trees was compared between selection cut and old-growth forest stands in Québec, Canada. We found higher observed heterozygosity and a lower inbreeding coefficient in mature trees than in younger regeneration cohorts of both forest types. We detected a recent bottleneck in all stands undergoing selection cutting. Other genetic indices of diversity (allelic richness, observed and expected heterozygosity, and rare alleles) were similar between forest types. We concluded that the effect of selection cutting on the genetic diversity of sugar maple was recent and no evidence of genetic erosion was detectable in Québec stands after one harvest. However, the cumulative effect of recurring applications of selection cutting in bottlenecked stands could lead to fixation of deleterious alleles, and this highlights the need for adopting better forest management practices.

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.

Intensively managed plantations are being established in a wide range of environmental conditions to satisfy the high demand for wood products and reduce the exploitation pressure on natural forests. In this study, we investigated the plasticity of four hybrid poplar (Populus spp.) clones established in 2005 along a latitudinal gradient in northwestern Quebec, Canada. The effect of latitudinal gradient on maximum rates of electron transfer (Jmax) and carboxylation (Vcmax), dark respiration (Rd), spring and fall bud phenology, net photosynthesis (Pn), specific leaf area (SLA), per mass nitrogen leaf concentration (Nm) were assessed in order to evaluate if clonal plasticity would result in increased overall productivity.

Growth season duration between the southernmost to the northernmost sites ranged 21–32 days, and was positively correlated to stem volume and negatively correlated to bud burst and bud set duration. Growth increment (stem volume) along the latitudinal gradient ranged 100–184% between the least and most productive clone. Clone 747215 had the most stable but the slowest growth. Leaf net photosynthesis decreased or did not change northwards except for the most productive clone for which it increased slightly likely due to a significant decrease in SLA. Maximum rates of carboxylation and photosynthesis electron transfer (Vcmax and Jmax) decreased northwards for three of the four clones, suggesting that photosynthesis of trees did not acclimate to lower temperatures from south to north. Plasticity of photosynthetic variables, measured with trait plasticity index was usually greater than that of SLA and Nm.

Clonal integration between ramets can be an ecological advantage of clonal plant species in environments where resources are patchily distributed. We investigated physiological integration among Populus balsamifera L. ramets under drought stress in order to demonstrate water sharing between connected ramets. Pairs of connected ramets were grown in separate pots in the greenhouse where half of ramets had the parental root connection severed and half were left intact. Drought stress was applied to one ramet, and growth, specific leaf area (SLA), net photosynthesis, stomatal conductance, leaf water potential and carbon isotopic composition (?13C) were measured after an 8-week growing period. Droughted ramets connected to watered ramets were able to maintain high gas exchange activity and water potential, similar to watered ramets. Leaf water potential and SLA results showed that the root connection was more beneficial for proximal compared with distal ramets. The parental root connection also allowed droughted ramets to discriminate more against 13C compared with severed ramets. In conclusion, this study shows compelling evidence of physiological integration of connected P. balsamifera ramets through water sharing.

Mixing tree cultivars or species in forest plantations can be efficient to reduce the risk of pest damages and could have a positive effect on yields if complementarity or facilitation between trees occurs. Four hybrid poplar clones (747215, Populus trichocarpa Torrey & A.Gray × P. balsamifera L.; 915004 and 915005, P. balsamifera × P. maximowiczii Henry; and 915319 P. maximowiczii × P. balsamifera) were planted in monoclonal and polyclonal plantations in three sites located in Quebec, Canada, to assess effects of clonal diversity on (i) aboveground biomass productivity, (ii) net photosynthesis and nutrient status of trees, and (iii) root spatial distribution. Stem growth was measured over five growing seasons, while root development, foliar nutrient concentrations and photosynthesis were measured during the fifth growing season. Results showed frequent but not general overyielding of trees in the polyclonal plots compared to monoclonal plots, five years after plantation establishment. Overall, stem volumes were 21% higher in the polyclonal (7.4 m3 ha?1) vs. monoclonal (6.1 m3 ha?1) plots. Effects of clone mixing on growth were greater in sites where soil nutrients were more limiting. However, foliar macronutrient concentrations (N, P, K, Ca and Mg) in trees growing in polyclonal plots were similar to those in monoclonal plots. Root development differed between the two plot layouts, with mean root:shoot ratio being greater in monoclonal (0.41:1) vs. the polyclonal (0.35:1) plots. Mixing clones increased biomass allocation aboveground, which we attributed to reduced competition between individuals of different clones and could explain overyielding in the polyclonal plots. The root fraction most distant from the stem (?60 cm) was greater in monoclonal (67% of total root biomass) compared to polyclonal (47% of total root biomass) plots, suggesting greater belowground competition in the former, which forced roots to extend further from the stems. Effects of plot layout on net assimilation rate (Pn) depended on site, with trees in polyclonal plots having greater Pn in two of the three sites. Root total non-structural carbohydrates were greater in the polyclonal (216 mg g?1) compared to the monoclonal (159 mg g?1) plots. Mixing hybrid poplar clones often resulted in greater aboveground growth, lower root:shoot ratios, and different spatial root distributions, when compared to clones planted in monocultures.

Natural root grafting has been observed in more than 150 tree species where up to 90 % of trees could be interconnected within a stand. Intraspecific root grafting was previously found in Pinus banksiana stands, ranging from 21 to 71 % of trees grafted with one another. It is not known why root grafting is frequent in some species and not in others, or why not all roots that cross form root grafts. We investigated genetic diversity of grafted and non-grafted trees to determine if there was a relationship between genetic distance and the probability of forming natural root grafts. Seven plots were hydraulically excavated in four natural forest stands and three plantations of P. banksiana in the western boreal forest of Quebec, Canada. At pairs scale, we studied the effect of geographic and genetic distances on root grafting occurrence. At stand level, we analysed the effect of tree density, soil type, stand type and mean pairwise relatedness on the mean number of grafts per tree and on the percentage of grafted trees per plot. At pairs scale, our analysis revealed that root grafting presence was influenced by spatial distance between trees and less importantly, by genetic distance between individuals. At stand level, root grafting frequency was correlated with stand type (greater in naturally regenerated stands), but not with genetic diversity between individuals. In conclusion, root grafting appears to be principally linked to tree proximity and slightly to genetic proximity between individuals.

Aspen clones were traditionally identified based on similarities in several phenotypic traits including leaf shape. This required several visits of the stands, laborious measurements and subjective visual assessments. In this study, we investigated a novel approach to clone identification using digital morphometrics of leaf shape complemented with bark characteristics and spring phenology. Aspen clones were delineated based on molecular (microsatellite loci), morphological (leaf shape, bark colour and type) and phenological (when first fully expanded leaves appeared) characteristics. Leaves were scanned and images were analyzed using normalized elliptic Fourier descriptors and principal component analysis. Using microsatellite loci, 18 clones were identified among 60 aspen trees in three sites investigated in this study. When employing digital morphometrics, foliar types in two out of the three sites matched the clones defined by microsatellite markers. Many ramets from the third site were clustered erroneously into incorrect clones. The reclassification test indicated that leaf shape contains features according to which very similar clones can be differentiated with low error rates. However, because it was not possible to set a threshold for maximum distances within clones, application of digital morphometrics of complex leaf shape for clone identification in natural aspen stands with a high number of multi-ramet clones and many singletons is unfeasible. We judged spring phenology as the least reliable trait for clone recognition and suggested possible causes of heterogeneous leaf flushing among ramets of the same genotype.

Aim

Several models have predicted that, with climate change, Northern Hemisphere species will migrate northwards from their present distribution ranges. Sugar maple (Acer saccharum Marshall) reaches its northern continuous distributional limit in north-eastern North America at the transition between boreal mixed-wood and temperate deciduous forest. Our objective was to determine whether lower sugar maple recruitment potential accounts for this gradual transition between the continuous and discontinuous zones of the distribution.

Location

The northern limit of sugar maple in eastern Canada.

Methods

We analysed the reproductive capacity of sugar maple populations along three latitudinal transects (located in the west, centre and east of Québec) in 24 sites located between 45°51′–48°59′ N and 70°21′–79°27′ W. The study area was divided into two zones, continuous and discontinuous, based on sugar maple stand abundance. We examined stand structure, sugar maple seed abundance and germination, and sugar maple seedling density and age structure.

Results

Sugar maple regeneration was uneven-aged and similar between continuous (south) and discontinuous (north) zones. For the western transect, more filled seeds and more seedlings were recorded in the continuous zone than in the discontinuous zone. Sugar maple seedling density was positively influenced by (1) basal area of mature sugar maple and saplings of all species, and (2) July mean temperature and precipitation. Four mast seed years were identified that were well synchronized across all sites; mast seeding covaried significantly with July mean temperature and July mean precipitation of the previous year.

Main conclusions

Our study clearly demonstrated an effect of climatic variables and stand characteristics on sugar maple regeneration. However, these factors did not explain the transition from a continuous to a discontinuous distribution for this species. Most of our northern sites exhibited constant sugar maple recruitment over time. These results highlight the importance of including non-climatic factors in models predicting species change in abundance.

Our aim was to test whether or not climate influence the northern distributional limit of eastern white cedar (Thuja occidentalis L.) by affecting its radial growth and regeneration. Twenty-two sites were selected along the bioclimatic gradient in Northwestern Québec, Canada. The gradient was divided into three zones based on the abundance of white cedar stands: continuous, discontinuous, and marginal zones. Radial growth and regeneration (numbers of sedds, saplings) were determined for each zone. Results showed that basal area increment (b.a.i.) was the same along the gradient. Seed production and germination did not differ between zones. One-year-old seedlings and small individuals (<30 cm) were less abundant in the northern sites than in stands from the continuous and discontinuous zones. More saplings were found in the northernmost sites (389/400 m2 in the marginal and 354.7/400 m2 in the discontinuous zones) than in the south (130.6/400 m2). Layering seemed to compensate for the low recruitment observed in the marginal sites. Recruitment of seedlings originating from sexual reproduction in the discontinuous zone was not different from the stands in the southern areas. Thus, a climatically driven decrease in recruitment cannot explain the observed decrease in white cedar abundance, which occurred at this latitude. Although seedlings were less abundant in isolated northern marginal stands, sexually based regeneration was still possible. Therefore, the direct effect of climate seems to only have a minor influence on white cedar northern distributional limit and other factors, such as natural disturbances, might better explain its actual boundary.

Aim

Several models have predicted that, with climate change, Northern Hemisphere species will migrate northwards from their present distribution ranges. Sugar maple (Acer saccharum Marshall) reaches its northern continuous distributional limit in north-eastern North America at the transition between boreal mixed-wood and temperate deciduous forest. Our objective was to determine whether lower sugar maple recruitment potential accounts for this gradual transition between the continuous and discontinuous zones of the distribution.

Location

The northern limit of sugar maple in eastern Canada.

Methods

We analysed the reproductive capacity of sugar maple populations along three latitudinal transects (located in the west, centre and east of Québec) in 24 sites located between 45°51–48°59 N and 70°21–79°27 W. The study area was divided into two zones, continuous and discontinuous, based on sugar maple stand abundance. We examined stand structure, sugar maple seed abundance and germination, and sugar maple seedling density and age structure.

Results

Sugar maple regeneration was uneven-aged and similar between continuous (south) and discontinuous (north) zones. For the western transect, more filled seeds and more seedlings were recorded in the continuous zone than in the discontinuous zone. Sugar maple seedling density was positively influenced by (1) basal area of mature sugar maple and saplings of all species, and (2) July mean temperature and precipitation. Four mast seed years were identified that were well synchronized across all sites; mast seeding covaried significantly with July mean temperature and July mean precipitation of the previous year.

Main conclusions

Our study clearly demonstrated an effect of climatic variables and stand characteristics on sugar maple regeneration. However, these factors did not explain the transition from a continuous to a discontinuous distribution for this species. Most of our northern sites exhibited constant sugar maple recruitment over time. These results highlight the importance of including non-climatic factors in models predicting species change in abundance.

In order to maximize early growth and establishment of planted hybrid poplars in the boreal region of Eastern Canada, growth response of four clones to fertilization was tested in two plantations. The first two fertilization treatments were based on Diagnosis and Recommendation Integrated System (DRIS), a method based on nutrient ratios: DRIS I was based on previously established norms from a study that had been conducted in the same area, and DRIS II was based on DRIS norms developed from hybrid poplars in northern Ontario, Canada. Nutrient status and growth of trees under these 2 treatments were compared to unfertilized trees and to trees under standard (STD) fertilization treatment (40 N-20P-20 K). Leaf nutrient concentrations and DRIS indices showed that fertilization treatments, and especially DRIS I corrected N deficiencies but failed to correct P deficiencies. Fertilization increased volume relative growth rate by 7.51, 4.76 and 13.25 % on average at the agricultural site for DRIS I, DRIS II and STD treatments respectively, compared to no fertilizer application. At the forest site, fertilization treatments based on DRIS indices (DRIS I and DRIS II) increased growth rates (6.67 %) slightly more than the standard treatment (5.80 %). Overall, although DRIS-based fertilization treatments generally increased growth rates, they were often equal to or less efficient than the STD treatment, and may not be as practical as using a standard fertilization recipe. © 2012 Springer Science+Business Media Dordrecht.

Trembling aspen (Populus tremuloides Michx.) as a clonal tree species possesses a complex root system through which trees of the same or different clones are connected. Root connections have been studied with respect to resource sharing, but the nature, quantities or extent of what is shared between trees is relatively unknown. In this study, we posed the hypothesis that systemic defense induction signals could also spread through these root networks and trigger defenses in neighboring ramets before arrival of pests. Temporal expression pattern of Kunitz trypsin inhibitor (KTI) and dihydroflavonol reductase (DFR) genes, two markers of poplar defense, was followed by quantitative real-time polymerase chain reaction. The expression was quantified in systemic leaves of wounded and healthy plants that shared the same parental root and in untreated controls grown in separate pots. Untreated interconnected plants did not show induced resistance upon herbivore-simulated attack. Although wound-treated ramets induced defense genes, untreated interconnected plants produced an expression pattern similar to non-connected controls. Root connections do not automatically lead to induction of defensive traits that are expressed in plants directly under damage thought to simulate herbivory. Rather, it seems that other communication means such as airborne volatiles can serve as signal transmission pathways among neighboring plants.

Il y a un intérêt pour les plantations d'espèces à croissance rapide de peupliers hybrides et de saules au Canada. Cet intérêt s'explique à la fois par le taux de croissance rapide et le potentiel élevé de rendement de ce type de système de production. Cependant, leur établissement et leur entretien requièrent des investissements considérables. Les efforts de recherche sur les comparaisons de productivité de différents clones ou espèces, les croisements, la physiologie, la nutrition, la sylviculture, la gestion des ravageurs, les méthodes de récolte, les aspects socio-économiques et la transformation industrielle contribuent à maximiser la production de fibre et à minimiser les coûts de production. L'objectif de cet article est de décrire brièvement des travaux de recherche en cours ou récemment complétés sur les plantations d'espèces à croissance rapide de peupliers hybrides et de saules et de discuter de leur utilité et de leurs impacts à des fins d'applications pratiques.

Dans un contexte où on souhaite réaliser un aménagement forestier plus intensif, le reboisement des terres en friche suscite beaucoup d'intérêt pour les producteurs forestiers. Ces plantations en milieu agricole se compa-rent-elles à celles établies en milieu forestier? Dans cette étude, nous avons comparé les caractéristiques de sols argileux de plantations établies sur des terres en friche à celles des sols de plantations en milieu forestier et nous avons vérifié dans quelle mesure les différences observées affectent la croissance des arbres. Vingt plan-tations de pin gris et vingt plantations d'épinette blanche, âgées de 9 à 27 ans, ont été échantillonnées dans chaque milieu. Nos analyses indiquent que les anciens sols agricoles sont plus compacts que les sols forestiers, mais qu'ils contiennent davantage de matière organique. Malgré des différences importantes au niveau des sols, les taux de croissance de l'épinette blanche et du pin gris demeurent comparables dans les deux milieux pour la période couverte par l'étude.

Soil organic matter loss and increased soil compaction have been identified as the factors most likely to directly impact tree growth in managed forests. We compared the soil quality of plantations established on former agricultural lands (n = 20) with plantations established following clear cutting of native forests (n = 20). Half of the plantations had been planted with jack pine (Pinus banksiana Lamb.) and half with white spruce [Picea glauca (Moench) Voss], 9 to 27 yr before the study. Theold field plantations had lower (at 0–10 and 10–20 cm) mineral soil macroporosity and higher field capacity than forest plantations, indicating more severe soil compaction. The old field plantations, however, also had higher soil C content, raising the permanent wilting point and canceling compaction effects on the available water holding capacity. An indicator of organic matter quality, namely the potential net mineralization per unit of soil Kjeldahl N, was lower in the old fields. Species also affected soil quality indicators—with lower values of macroporosity and higher values of field capacity observed under white spruce. Despite significant differences in soil conditions, no significant effect (P < 0.05) of plantation origin on tree growth could be found. Old fields can support productive plantations of both species.

Several models of the effects of silviculture, radial growth, and tree age on wood density have been developed, but they have rarely considered the roles of diverse seed origins and climate. We developed a model to test the effects of radial growth, tree age, climate, and seed-source origins on wood density in 21 diverse populations of jack pine in a common garden in Petawawa, Ontario, Canada over the last 24 years using a linear mixed-effects model. Although we found significant differences in wood density among diverse seed origins, there were no differences between seed origins having the same ring age and ring width, indicating an indirect effect on wood density of seed-source origin via radial growth. High variation in wood density among trees within the same population and between populations indicated high genetic control of wood density. The climate effect was significant on wood density in all populations, but smaller when radial growth was controlled. Climate effect did not differ significantly among populations. Precipitation in July negatively affected latewood density, whereas precipitation in May in the current year and September of the previous year negatively affected earlywood density. We concluded that a single model of jack pine wood density and radial growth could be used, either controlling for climate effects or not, as the relationship between wood density and radial growth is preserved among the diverse populations, and the climate effect controlling for radial growth in the model was only slight.

Planting stock type and quality can have an important impact on early growth rates of plantations. The goal of this study was to evaluate early growth and root/shoot development of different planting materials in typical heavy clay soils of northwestern Quebec. Using one-year-old bareroot hybrid poplar dormant stock, four planting materials were compared: (1) regular bareroot stock, (2) rootstock (stem pruned before planting), (3) whips (roots pruned before planting), and (4) cuttings (30 cm stem sections taken from the basal portion of bareroot trees, i.e. roots and shoot pruned). Rooted stock types (bareroot and rootstock) produced, on average, 1.2 times larger trees than unrooted stock types (cuttings and whips). However, shoot-pruned stock types (rootstocks and cuttings) reached similar heights and basal diameters as unpruned stock types (bareroots and whips), during the first growing season. Shoot pruning reduced leaf carbon isotopic ratios, suggesting that unpruned stock types were water-stressed during the first growing season. The stress was most likely caused by early leaf development while root growth occurred later in the summer. We conclude that shoot pruning bareroot stock is a useful management option to reduce planting stress without compromising early growth rates of hybrid poplars.

Trembling aspen ( Populus tremuloides ) is a clonal tree species, which regenerates mostly through root suckering. In spite of vegetative propagation, aspen maintains high levels of clonal diversity. We hypothesized that the maintenance of clonal diversity in this species can be facilitated by integrating different clones through natural root grafts into aspen ’ s communal root system. To verify this hypothesis, we analyzed root systems of three pure aspen stands where clones had been delineated with the help of molecular markers. Grafting between roots was frequent regardless of their genotypes. Root system excavations revealed that many roots were still living below trees that had been dead for several years. Some of these roots had no root connections other than grafts to living ramets of different clones. The uncovered root systems did not include any unique genotypes that would not occur among stems. Nevertheless, acquiring roots of dead trees helps to maintain extensive root systems, which increases the chances of clone survival. Substantial interconnectivity within clones as well as between clones via interclonal grafts results in formation of large genetically diverse physiological units. Such a clonal structure can signifi cantly affect interpretations of diverse ecophysiological processes in forests of trembling aspen.

Abstract:

Effect of interannual climate variations on radial growth was compared among jack pine (Pinus banksiana Lamb.) of diverse geographical origins in a 41-year-old common-garden experiment in Petawawa, Ontario. Provenance experiments established from seeds transferred from different parts of a species range (from the northern United States to northern Canada) to the same environment might be considered as a simulation model of climate change and a shift of climate zones. The following questions are addressed: Did the response of growth to interannual climate variations differ among the provenances transferred within the experimental site? What climatic factors affect interannual growth variations of jack pine provenances? Tree-ring chronologies for 16 populations were developed for the period 1970–2004. The best climate predictors of radial growth were precipitation of June and March of the current year and precipitation of December of the previous year. Although, climatic factors affecting growth were similar between the provenances, absolute radial growth was proportional to the growth potential of the provenances. We conclude that variability due to seeds origins is not a significant source of variation for dendroclimatic studies of jack pine. Increased frequency of summer droughts might result in a growth decrease of jack pine © 2008 NRC Canada.

Résumé

Nous avons étudié les relations entre le climat et la croissance radiale annuelle de provenance de pin gris (Pinus banksiana Lamb.) établis depuis 41 ans dans un jardin commun à Petawawa en Ontario. Des tests de descendance établis dans des conditions similaires à partir de graines provenant de différentes parties de l’aire de distribution de l’espèce (depuis le Nord des Etats-unis jusqu’au nord du Canada) simulent des changements climatiques et des transitions entre différentes zones bioclimatiques. Nous avons tentés de répondre aux questions suivantes : Est-ce que la relation entre la croissance radiale interannuelle et le climat varie en fonction des provenances? Quelles sont les facteurs climatiques qui contrôlent la croissance radiale des différentes provenances. Des chronologies de la croissance radiale couvrant la période 1970–2004 ont été développées pour 16 provenances. Les facteurs climatiques les plus corrélés avec la croissance radiale sont les précipitations pour juin et mars de l’année en cours et les précipitations de décembre pour l’année précédente. Bien que les facteurs climatiques contrôlant la croissance sont les mêmes pour toutes les provenances, la croissance absolue variait en fonction du potentiel de croissance de chaque provenance. Nous concluons que la variabilité due à la provenance des graines ne constitue pas une source importante de variation dans la réponse dendroclimatique chez le pin gris. Une augmentation de la fréquence des sécheresses durant la période estivale pourrait entraîner une diminution de la croissance du pin gris © 2008 NRC Canada.

The effects of seed transfer and climate change on the width and basal area of tree rings were studied in 21 provenances of jack pine (Pinus banksiana Lamb.) grown in a common-garden plantation in Petawawa, Ontario, Canada. Seed-source origin significantly influenced both mean tree-ring width and mean annual basal area increment over a 25-year growth period (1975–1999). Temperature and precipitation transfer functions were developed to predict width and basal area of tree rings of the jack pine populations. The best predictors of growth were the transfer distances mean annual maximum daily temperature and annual precipitation between the plantation site and the seed origins. Radial growth of the jack pine populations was mainly related to temperature at seed origin and, to a lesser degree, to precipitation at seed origin. Extension of the transfer functions to three sets of independent data revealed significant correlations between estimated and predicted mean radial growth characteristics. Seed sources of jack pine originating from warmer and drier climates than that of the plantation site in Petawawa had slightly higher mean ring widths and basal areas than the local populations. The application of different climate change scenarios derived from general circulation models the developed transfer functions indicated that radial growth of jack pine may decline only if significant climate changes occur, which might happen before the mid 21st century. Both a higher radial growth of southern seed sources and a potential negative effect of a significant temperature increase and precipitation decrease in future suggest restricting the northward transfer of southern seed sources to less than 18 latitude. However, provenance specific differences in survivorship, frost- and disease-resistance, and cone serotiny should also be taken into consideration.© 2007 Elsevier B.V. All rights reserved.

Abstract

The aim of this study was to evaluate if and how the previous land use (here: agriculture versus forest) has influenced the floristic composition now observed in the understory of jack pine plantations of Abitibi-Temiscamingue (North-Eastern Canada). Floristic inventories were conducted in jack pine plantations which had been established on forest sites after logging and on sites previously utilized for agriculture. These were compared to one another and to inventories conducted in natural forests and on agricultural sites which had not (not yet) been planted with trees (old fields). The vegetation community composition of the sites is analysed using several multivariate techniques. Results show that the flora of plantations on old agricultural fields is markedly different than that of plantations established on sites which were previously forested and that the two types of plantation are unlikely to become floristically the same before the anticipated harvest time. Comparing old and young plantations and old fields in light of natural forests sites, we show that the previous land-use is a key determinant of future compositional differences in plantations. This must be considered when we evaluate the impact of establishing plantations on sites with different land-use histories.

Résumé

Le but de cette étude est d’évaluer dans quelle mesure l’utilisation du sol a pu influencer la composition floristique observée aujourd’hui dans le sous-bois de plantations de pin gris en Abitibi-Témiscamingue (nord-ouest du Québec). Des inventaires floristiques ont été effectués dans des plantations établies sur d’anciennes friches agricoles et sur d’anciennes friches forestières. Une comparaison (à l’aide d’analyses multivariées) est faite entre les sous-bois de chaque type de plantation, ainsi qu’avec ceux de sites présentement en friche et de forêts naturelles. Nos résultats montrent que la flore du sous-bois de plantations réalisées sur d’anciennes terres agricoles est très différente du sous-bois de plantations réalisées en sites forestiers. Les deux types de sites ont ainsi peu de chances de se ressembler dans le futur, surtout compte tenu des délais d’exploitation. L’historique d’utilisation du sol est donc un facteur déterminant qui explique les différences de composition floristique dans le sous-bois des plantations, ce qui doit être pris en compte dans l’évaluation des impacts de plantations réalisées dans ces environnements fort différents.© 2007 Elsevier B.V. All rights reserved.

En forêt boréale, les effets bénéfiques du passage du feu sur le substrat de germination ont souvent été liés à la présence de charbon et à son effet adsorbant sur les composés phénoliques qui inhibent la germination et la croissance des semis. L'objectif de la présente étude visait à mettre en évidence l'influence directe du charbon actif sur la germination et la croissance primaire des plantules de pin gris (Pinus banksiana Lamb.), une espèce très bien adaptée au feu. Quatre concentrations de charbon actif (0, 50, 100 et 150 % par rapport à la masse des semences) combinées à deux températures, respectivement 26 à 27 °C et 18 à 20 °C, ont été testées in vitro à une humidité constante dans des boîtes de Pétri. Les résultats montrent que l'ajout de charbon actif dans le milieu n'a pas eu d'effet significatif sur la germination des semences de pin gris. Pour ce qui est de la croissance des plantules cependant, le charbon actif exerce un effet inhibiteur, lorsque sa concentration dans le milieu est supérieure à 100 %. Une augmentation significative de la croissance a été uniquement observée seulement avec 50 % de charbon combiné à la température plus élevée (26–27 °C). Nous avons noté l'apparition de malformations à l'apex de la radicule particulièrement aux températures basses associées à des concentrations élevées de charbon actif dans le milieu.

In the boreal forest, the beneficial effect of wildfire on germination substrates has often been linked to the adsorption by charcoal of phenolic compounds detrimental for seedling germination and growth. Our goal was to show that active charcoal has direct positive effects on germination and early growth of jack pine (Pinus banksiana Lamb.) plantlets, a species well adapted to fire. Four concentrations of active charcoal (0%, 50%, 100%, and 150% seed mass) along with two temperatures (26–27 °C and 18–20 °C) were tested in vitro, in Petri dishes, under constant moisture conditions. Results showed that adding active charcoal to the substrate had no significant effect on jack pine seed germination but inhibited plantlet growth at concentrations over 100%. Significant growth increase was observed only with 50% charcoal at the highest temperature (26–27 °C). Deformities were noted at the apex of radicles, particularly with low temperatures and high levels of active charcoal in the substrate. ©2006 NRC Canada

We built two models to follow clonal species genotypic diversity (G/N) over long periods of time at the stand and landscape levels. The models were then validated with empirical data from trembling aspen (Populus tremuloides) populations in Quebec’s boreal forest. Data was collected using a chronosequence approach in seven sites that burned in 1717, 1760, 1797, 1823, 1847, 1944, and 1916. Genetic identifi- cation was done by using four microsatellite loci. At the stand scale, simulations were repeated for a genet size of 5, 25, 50 and 100 ramets each. At the landscape level, we simulated the cumulative genet survival rate under different fire cycles (5–500 years) for 500 years after fire. Stand simulations indicated that ramet mortality within genets rather than genet mortality accounts for the increase in G/N with time since fire. Both the initial genet size and the recurrent suckering of some genets (or ramet recruitment) play an important role in maintaining high G/N levels for long periods of time. In general, the larger the number of ramets per genet, the longer the genet survives under a gap disturbance regime and a minimum of 100 ramets per genet is required to maintain aspen genet survival for 500 years. At the landscape level, genet loss increases as the fire cycle gets longer. In Quebec’s boreal forest, short rotation even-aged management practices seem to maintain a genet survival rate similar to that produced by the natural succession regime.

In the present study we investigated the genetic structure and genetic diversity of Pinus heldreichii populations in Bulgaria using chloroplast microsatellite markers and terpene analysis. We were interested in addressing the following questions: (1) can population structuring in Bosnian pine be detected via chloroplast microsatellite markers; (2) are there differences in population differentiation as determined by terpenes and microsatellites; and (3) how are the patterns of size variant frequencies and geographical distances related. Four provenances were chosen throughout the species' range in Bulgaria. Following DNA extraction, chloroplast microsatellite (cpSSR) loci were surveyed using 6 primer pairs. Between 2 and 5 size variants were identified at each locus. A total of 16 size variants at the 6 loci were identified, 4 occurring at low frequencies. They were combined in 21 different haplotypes including 11 that were unique. AMOVA analysis revealed that 18.25% of the variation was found among populations, while 81.75% was expressed within populations. The cpSSR analysis divided Bosnian pine populations into two groups, the first represented by populations B, C and D located in the south and north-western part of the Pirin and Slavianka mountains, while the second group, represented by population A, is located in the north-eastern Pirin mountain. Terpene analysis revealed that on average, 59% of the monoterpene pool in P. heldreichii is accounted for by limonene (range 36-48%) followed by a-pinene (range 16-17%). The presence of two distinct groups (Pop-A, Pop-D and Pop-B, Pop-C) is more consistent with physical distances between populations. No significant correlation between genetic distance determined by chloroplast microsatellites analysis and chemotype distance determined by terpenes was observed. Our results suggest that the structural pattern of genetic diversity of cpDNA in Bosnian pine populations is the consequence of historical and biogeographical processes. © 2004 Elsevier Ltd. All rights reserved.

In this study we investigated the genetic structure and diversity of jack pine populations in Quebec using chloroplast microsatellite markers. Six cpSSR loci were used to screen 15 Pinus banksiana populations from an economically important region of the boreal zone. The main objective was to quantify the variation within and among populations and to relate this to their geographic pattern. From 3 to 6 size variants were identified at each locus. A total of 29 size variants at the six loci were identified and 5 of the size variants occurred infrequently. The size variants were combined into 87 different haplotypes, of which 49 were unique. The AMOVA analysis revealed that 10.96% of the variation was distributed between populations and 89.04% within populations. Cluster analysis divided the studied populations into two large groups, one from north-western Quebec and the second from eastern Quebec. These results suggest that the genetic structure of the studied jack pine populations is the result of interactions between historical post-glacial migrations and climate-disturbance structuring factors.

To portray aspen clonal and spatial genetic structures, we mapped and genotyped trees in two 1-ha plots, each containing three aspen cohorts originating from fire or subsequent secondary disturbances. We used four microsatellite loci to identify aspen clones and increment core analysis to determine tree age. Clonal dimensions were measured by the maximum distance between two ramets and the number of ramets per genet. Standard normal deviate (SND) was used to assess the spatial distribution of aspen genets and cohorts, and multivariate spatial genetic autocorrelations to assess the spatial distribution of aspen genetic variation. Most aspen genets consisted of only one ramet (> 75%). Median clonal dimensions were 19 and 29 m (maxima: 104 and 72 m in the two plots). No segregation was observed between clones. Aspen cohorts were spatially segregated but trees were spatially aggregated within old and medium-aged cohorts. In contrast, trees were more randomly distributed within the youngest cohorts. This coincided with a spatial genetic autocorrelation at small scales (up to 30 m) in the older cohorts and a more random genetic distribution in the youngest ones. Our results suggest that aspen spatial genetic structuring reflects the spatial patterns produced by the regeneration of discrete cohorts at different stages of succession. Vegetative reproduction leads to aspen genetic spatial structuring at small scales (few metres) until midsuccession. However, as the stand gets older, the spatial distribution of aspen trees and genetic structure evolve from a structured pattern to a more random one under a gap disturbances regime.

Aim

For this study, we wanted to evaluate the reproductive potential of northern red maple (Acer rubrum L.) populations to identify the possible factors responsible for the scattered distribution pattern of these northern populations.

Location

Samara production and long-term establishment of seedlings were observed along a north-south transect crossing the transition zone between continuous and discontinuous stands of red maple (47°80'-49°27' N) in western Quebec.

Methods

Eleven populations of red maple were selected along a latitudinal gradient extending to the northern limit of the species. Seed traps were placed in each stand and distributed under the canopy of mature red maple trees. Seed abundance was tracked for 6 years from 1988 to 1993. Phenological observations were made in 1992 and 1993 at Roquemaure (Roq), a site located at the centre of the latitudinal gradient. Red maple trees were randomly selected within the population; counts of flower buds, pollinated buds and samaras produced were made in 1992-93. Samaras were collected from each branch immediately before dispersal and counted. During the summer of 1987, seedlings (< 1 cm d.b.h.) were collected and aged at each site in twenty 1 m² quadrants and age of the seedlings (< 1 cm d.b.h.) was determined by counting the annual scars left by terminal buds.

Results

Samaras were produced even at the northern limit but large yearly variations were observed. Over the 6-year period we counted 3 years (1989, 1990, 1993) when samara production was high, and 3 years (1988, 1991, 1992) when production was low. Phenological observations indicate that the occurrence of spring frosts at the time of flower bud flushing could contribute to decreasing the abundance of seeds. The age structure of southern localities had a relatively constant production of seedlings, as indicated by an inverse J-shaped distribution. However, the five northernmost localities show sporadic recruitment.

Main conclusions

Populations at the northern limit are maintained essentially through vegetative reproduction and infrequent sexual recruitment. Our results indicate that regeneration within established stands through sexual recruitment is possible in all of the populations we studied. This potential becomes very low at more northerly sites and sexual reproduction alone would be unlikely to ensure successful stand regeneration. Without major disturbances in those stands, shade tolerant conifer species such as balsam fir (Abies balsamea) or black spruce (Picea mariana) would readily dominate the canopy. The discontinuous distribution of red maple stands at the northern limit is the consequence of either a random colonization of few sites during a better climatic period or remnants of a much larger distribution that has been constrained because of climatic deterioration.

Samaras of red maple (Acer rubrum L.) were collected at six locations at the northern edge of its distribution. Two populations from southern Quebec, in the deciduous forest, and four populations at its northern limit in the boreal forest were selected. Germination responses of these populations were evaluated in growth chambers and in the field. The effects of cold-moist stratification, temperature (20/10 degree C, 10/1 degree C) and light intensity (750 mu-mol cntdot m-2 cntdot s-1, 160 mu-mol cntdot m-2 cntdot s-1) were evaluated in growth chambers. Seeds collected at the two most southern sites had 46% to 90% germination without stratification, while germination without stratification varied between 0% to 41% for the four most northern populations. In the six populations tested, the highest germination percentages were obtained at 10/1 degree C under low light conditions. Cold moist stratification enhanced germination of the four most northern populations. We observed in the field experiment that germination occurred during the summer months for sites located in the deciduous forest, while germination was delayed until the next spring for the sites located in the boreal forest. The percentages of seedling emergence in the field were similar under shade and nonshade conditions. Overall, our results indicate a marked degree of dormancy in the most northern populations. The heterogeneous germination response of northern red maple populations could be associated with the temperature regime at the collection site.