Integrating demographic niches and black spruce range expansion at subarctic treelines.

When investigating relationships between species' niches and distributions, niches can be divided demographically, resulting in unique niches for different life stages. This approach can identify changing substrate requirements throughout a species' life cycle. Using non-metric multidimensional scaling, we quantified microsite conditions associated with successful recruitment in the tundra landscape and successful seed production amongst adult trees of black spruce (Picea mariana) at subarctic treeline in Yukon, Canada to assess how life stage-specific requirements may impact the distribution of this widespread boreal tree species. Treeline ecotones in this region showed high heterogeneity in tundra microsites available for establishment. Black spruce exhibited changing microsite associations from germination to reproductive maturity, which were mainly driven by changes in plant community and soil moisture. These associations limit the microsites where individuals can establish and reproduce to a subset available within the heterogeneous landscape. Overall, we suggest that (1) substrates suitable for early recruitment are limited at the range edge; and (2) reproductive adults have a narrow niche, limiting successful seed production in adults and forming sink populations where suitable conditions are limited. Our multivariate assessment of microsite suitability can provide valuable insights into the spatial distribution of a species throughout its life cycle and identify life stage-specific constraints to range expansion.

Predictors of taxonomic and functional composition of black spruce seedling ectomycorrhizal fungal communities along peatland drainage gradients.

Many trees depend on symbiotic ectomycorrhizal fungi for nutrients in exchange for photosynthetically derived carbohydrates. Trees growing in peatlands, which cover 3% of the earth's terrestrial surface area yet hold approximately one-third of organic soil carbon stocks, may benefit from ectomycorrhizal fungi that can efficiently forage for nutrients and degrade organic matter using oxidative enzymes such as class II peroxidases. However, such traits may place a higher carbon cost on both the fungi and host tree. To investigate these trade-offs that might structure peatland ectomycorrhizal fungal communities, we sampled black spruce (Picea mariana (Mill.)) seedlings along 100-year-old peatland drainage gradients in Minnesota, USA, that had resulted in higher soil nitrogen and canopy density. Structural equation models revealed that the relative abundance of the dominant ectomycorrhizal fungal genus, Cortinarius, which is known for relatively high fungal biomass coupled with elevated class II peroxidase potential, was negatively linked to site fertility but more positively affected by recent host stem radial growth, suggesting carbon limitation. In contrast, Cenococcum, known for comparatively lower fungal biomass and less class II peroxidase potential, was negatively linked to host stem radial growth and unrelated to site fertility. Like Cortinarius, the estimated relative abundance of class II peroxidase genes in the ectomycorrhizal community was more related to host stem radial growth than site fertility. Our findings indicate a trade-off between symbiont foraging traits and associated carbon costs that consequently structure seedling ectomycorrhizal fungal communities in peatlands.

Solid mechanics of the torus-margo in conifer intertracheid bordered pits.

Bordered pits of many conifers include a torus-margo structure acting as a valve that prevents air from spreading between tracheids, although the extent of torus deflection as a function of applied pressure is not well known. Models were developed from images of pits in roots and stems of Picea mariana (Mill.) BSP. A computational solid mechanics approach was utilised to determine the extent of torus deflection from pressure applied to the torus and margo. Torus deflection increased in nonlinear fashion with applied pressure. The average pressure required for sealing the pit was 0.894 MPa for stems and 0.644 MPa for roots, although considerable variation was apparent between individual pits. The pits of roots were wider and deeper than those of stems. For stems, the pit depth did not increase with pit width; thus the torus displacement needed to seal the pit was less than for pits from roots. The pressure required to seal the pit depends upon anatomical characteristics such as pit width and pit depth. Although the torus displacement for sealing was greater for roots because of their greater depth, the pressures leading to sealing were not significantly different between roots and stems.

Improved performance of the eastern spruce budworm on black spruce as warming temperatures disrupt phenological defences.

Phenological shifts, induced by global warming, can lead to mismatch between closely interacting species. The eastern spruce budworm, Choristoneura fumiferana, an important outbreaking insect defoliator in North America, mainly feeds on balsam fir, Abies balsamea, which has historically been well synchronized with the insect. But as climate change pushes the northern range limit of the budworm further north into the boreal forest, the highly valuable black spruce, Picea mariana, historically protected against the budworm by its late budburst phenology, is suffering increased defoliation during the current outbreak. We tested the hypothesis that rising temperatures can lead, not to a mismatch, but to an improved match between the budworm and black spruce through differential phenological advancement. For 3 years, eastern spruce budworm larvae were reared from instar 2 to pupae, on both black spruce and balsam fir, in a temperature free-air controlled enhancement experiment (T-FACE) consisting in 24 field plots, half of which were heated at +2°C from March to October. Our results show that every year, larval development was faster on heated trees and pupation was earlier than on unheated trees. Bud development was also accelerated in heated trees of both species. However, there was no difference in mass between pupae that developed at +2°C and controls at the end of the season. Finally, we found no difference either in development rate or pupal mass between larvae reared on black spruce and those reared on balsam fir. This suggests that under higher temperature regimes, eastern spruce budworm will be as successful on black spruce as on balsam fir, as black spruce budburst becomes better synchronized with the insect's emergence from diapause. This could lead to critical changes in outbreak dynamics and severity with important ecological state shifts at the landscape level.

Development and characterization of Novel EST-based single-copy genic microsatellite DNA markers in white spruce and black spruce.

Due mainly to large genome size and prevalence of repetitive sequences in the nuclear genome of spruce (Picea Mill.), it is very difficult to develop single-copy genomic microsatellite markers. We have developed and characterized 25 polymorphic, single-copy genic microsatellites from white spruce (Picea glauca (Moench) Voss) EST sequences and determined their informativeness in white spruce and black spruce (Picea mariana (Mill.) B.S.P.) and inheritance in black spruce. White spruce EST sequences from NCBI dbEST were searched for the presence of microsatellite repeats. Forty-seven sequences containing dinucleotide, trinucleotide, tetranucleotide and compound repeats were selected to develop primers. Twenty-five of the designed primer pairs yielded scorable amplicons, with single-locus patterns, and were characterized in 20 individuals each of white spruce and black spruce. All 25 microsatellites were polymorphic in white spruce and 24 in black spruce. The number of alleles at a locus ranged from two to 18, with a mean of 8.8 in white spruce, and from one to 17, with a mean of 7.6 in black spruce. The expected heterozygosity/polymorphic information content ranged from 0.10 to 0.92, with a mean of 0.67 in white spruce, and from 0 to 0.93, with a mean of 0.59 in black spruce. Microsatellites with dinucleotide and compound repeats were more informative than those with trinucleotide and tetranucleotide repeats. Eighteen microsatellite markers polymorphic between the parents of a black spruce controlled cross inherited in a single-locus Mendelian fashion. The microsatellite markers developed can be applied for various genetics, genomics, breeding, and conservation studies and applications.

Contrasting acclimation responses to elevated CO2 and warming between an evergreen and a deciduous boreal conifer.

Rising atmospheric carbon dioxide (CO2 ) concentrations may warm northern latitudes up to 8°C by the end of the century. Boreal forests play a large role in the global carbon cycle, and the responses of northern trees to climate change will thus impact the trajectory of future CO2 increases. We grew two North American boreal tree species at a range of future climate conditions to assess how growth and carbon fluxes were altered by high CO2 and warming. Black spruce (Picea mariana, an evergreen conifer) and tamarack (Larix laricina, a deciduous conifer) were grown under ambient (407 ppm) or elevated CO2 (750 ppm) and either ambient temperatures, a 4°C warming, or an 8°C warming. In both species, the thermal optimum of net photosynthesis (ToptA ) increased and maximum photosynthetic rates declined in warm-grown seedlings, but the strength of these changes varied between species. Photosynthetic capacity (maximum rates of Rubisco carboxylation, Vcmax , and of electron transport, Jmax ) was reduced in warm-grown seedlings, correlating with reductions in leaf N and chlorophyll concentrations. Warming increased the activation energy for Vcmax and Jmax (EaV and EaJ , respectively) and the thermal optimum for Jmax . In both species, the ToptA was positively correlated with both EaV and EaJ , but negatively correlated with the ratio of Jmax /Vcmax . Respiration acclimated to elevated temperatures, but there were no treatment effects on the Q10 of respiration (the increase in respiration for a 10°C increase in leaf temperature). A warming of 4°C increased biomass in tamarack, while warming reduced biomass in spruce. We show that climate change is likely to negatively affect photosynthesis and growth in black spruce more than in tamarack, and that parameters used to model photosynthesis in dynamic global vegetation models (EaV and EaJ ) show no response to elevated CO2 .

Wood anatomical traits in black spruce reveal latent water constraints on the boreal forest.

The effects of climate change on high-latitude forest ecosystems are complex, making forecasts of future scenarios uncertain. The predicted lengthening of the growing season under warming conditions is expected to increase tree growth rates. However, there is evidence of an increasing sensitivity of the boreal forest to drought stress. To assess the influence of temperature and precipitation on the growth of black spruce (Picea mariana), we investigated long-term series of wood anatomical traits on 20 trees from four sites along 600 km, the latitudinal range of the closed boreal forest in Quebec, Canada. We correlated the anatomical traits resolved at intraring level with daily temperature, vapor pressure deficit (VPD), and precipitation during the 1943-2010 period. Tree-ring width, number of cells per ring and cell wall thickness were positively affected by spring and summer daily mean and maximum temperature at the northern sites. These results agree with the well-known positive effect of high temperatures on tree ring formation at high latitudes. However, we captured, for the first time in this region, the latent impact of water availability on xylem traits. Indeed, in all the four sites, cell lumen area showed positive correlations with daily precipitation (mostly at low latitude), and/or negative correlations with daily mean and maximum temperature and VPD (mostly at high latitude). We inferred that drought, due to high temperatures, low precipitations, or both, negatively affects cell enlargement across the closed boreal forest, including the northernmost sites. The production of tracheids with narrower lumen, potentially more resistant to cavitation, could increase xylem hydraulic safety under a warmer and drier climate. However, this would result in lower xylem conductivity, with consequent long-term hydraulic deterioration, growth decline, and possibly lead to tree dieback, as observed in other forest ecosystems at lower latitudes.

Is size an issue of time? Relationship between the duration of xylem development and cell traits.

BACKGROUND AND AIMS: Secondary growth is a process related to the formation of new cells that increase in size and wall thickness during xylogenesis. Temporal dynamics of wood formation influence cell traits, in turn affecting cell patterns across the tree ring. We verified the hypothesis that cell diameter and cell wall thickness are positively correlated with the duration of their differentiation phases. METHODS: Histological sections were produced by microcores to assess the periods of cell differentiation in black spruce [Picea mariana (Mill.) B.S.P.]. Samples were collected weekly between 2002 and 2016 from a total of 50 trees in five sites along a latitudinal gradient in Quebec (Canada). The intra-annual temporal dynamics of cell differentiation were estimated at a daily scale, and the relationships between cell traits and duration of differentiation were fitted using a modified von Bertalanffy growth equation. KEY RESULTS: At all sites, larger cell diameters and cell wall thicknesses were observed in cells that experienced a longer period of differentiation. The relationship was a non-linear, decreasing trend that occasionally resulted in a clear asymptote. Overall, secondary wall deposition lasted longer than cell enlargement. Earlywood cells underwent an enlargement phase that lasted for 12 d on average, while secondary wall thickness lasted 15 d. Enlargement in latewood cells averaged 7 d and secondary wall deposition occurred over an average of 27 d. CONCLUSIONS: Cell size across the tree ring is closely connected to the temporal dynamics of cell formation. Similar relationships were observed among the five study sites, indicating shared xylem formation dynamics across the entire latitudinal distribution of the species.The duration of cell differentiation is a key factor involved in cell growth and wall thickening of xylem, thereby determining the spatial variation of cell traits across the tree ring.

Contrasting drivers and trends of coniferous and deciduous tree growth in interior Alaska.

The boreal biome represents approximately one third of the world's forested area and plays an important role in global biogeochemical and energy cycles. Numerous studies in boreal Alaska have concluded that growth of black and white spruce is declining as a result of temperature-induced drought stress. The combined evidence of declining spruce growth and changes in the fire regime that favor establishment of deciduous tree species has led some investigators to suggest the region may be transitioning from dominance by spruce to dominance by deciduous forests and/or grasslands. Although spruce growth trends have been extensively investigated, few studies have evaluated long-term radial growth trends of the dominant deciduous species (Alaska paper birch and trembling aspen) and their sensitivity to moisture availability. We used a large and spatially extensive sample of tree cores from interior Alaska to compare long-term growth trends among contrasting tree species (white and black spruce vs. birch and aspen). All species showed a growth peak in the mid-1940s, although growth following the peak varied strongly across species. Following an initial decline from the peak, growth of white spruce showed little evidence of a trend, while black spruce and birch growth showed slight growth declines from ~1970 to present. Aspen growth was much more variable than the other species and showed a steep decline from ~1970 to present. Growth of birch, black and white spruce was sensitive to moisture availability throughout most of the tree-ring chronologies, as evidenced by negative correlations with air temperature and positive correlations with precipitation. However, a positive correlation between previous July precipitation and aspen growth disappeared in recent decades, corresponding with a rise in the population of the aspen leaf miner (Phyllocnistis populiella), an herbivorous moth, which may have driven growth to a level not seen since the early 20th century. Our results provide important historical context for recent growth and raise questions regarding competitive interactions among the dominant tree species and exchanges of carbon and energy in the warming climate of interior Alaska.

Cross-scale controls on carbon emissions from boreal forest megafires.

Climate warming and drying is associated with increased wildfire disturbance and the emergence of megafires in North American boreal forests. Changes to the fire regime are expected to strongly increase combustion emissions of carbon (C) which could alter regional C balance and positively feedback to climate warming. In order to accurately estimate C emissions and thereby better predict future climate feedbacks, there is a need to understand the major sources of heterogeneity that impact C emissions at different scales. Here, we examined 211 field plots in boreal forests dominated by black spruce (Picea mariana) or jack pine (Pinus banksiana) of the Northwest Territories (NWT), Canada after an unprecedentedly large area burned in 2014. We assessed both aboveground and soil organic layer (SOL) combustion, with the goal of determining the major drivers in total C emissions, as well as to develop a high spatial resolution model to scale emissions in a relatively understudied region of the boreal forest. On average, 3.35 kg C m-2 was combusted and almost 90% of this was from SOL combustion. Our results indicate that black spruce stands located at landscape positions with intermediate drainage contribute the most to C emissions. Indices associated with fire weather and date of burn did not impact emissions, which we attribute to the extreme fire weather over a short period of time. Using these results, we estimated a total of 94.3 Tg C emitted from 2.85 Mha of burned area across the entire 2014 NWT fire complex, which offsets almost 50% of mean annual net ecosystem production in terrestrial ecosystems of Canada. Our study also highlights the need for fine-scale estimates of burned area that represent small water bodies and regionally specific calibrations of combustion that account for spatial heterogeneity in order to accurately model emissions at the continental scale.

Fuel-reduction management alters plant composition, carbon and nitrogen pools, and soil thaw in Alaskan boreal forest.

Increasing wildfire activity in Alaska's boreal forests has led to greater fuel-reduction management. Management has been implemented to reduce wildfire spread, but the ecological impacts of these practices are poorly known. We quantified the effects of hand-thinning and shearblading on above- and belowground stand characteristics, plant species composition, carbon (C) and nitrogen (N) pools, and soil thaw across 19 sites dominated by black spruce (Picea mariana) in interior Alaska treated 2-12 years prior to sampling. The density of deciduous tree seedlings was significantly higher in shearbladed areas compared to unmanaged forest (6.4 vs. 0.1 stems/m2 ), and unmanaged stands exhibited the highest mean density of conifer seedlings and layers (1.4 stems/m2 ). Understory plant community composition was most similar between unmanaged and thinned stands. Shearblading resulted in a near complete loss of aboveground tree biomass C pools while thinning approximately halved the C pool size (1.2 kg C/m2 compared to 3.1 kg C/m2 in unmanaged forest). Significantly smaller soil organic layer (SOL) C and N pools were observed in shearbladed stands (3.2 kg C/m2 and 116.8 g N/m2 ) relative to thinned (6.0 kg C/m2 and 192.2 g N/m2 ) and unmanaged (5.9 kg C/m2 and 178.7 g N/m2 ) stands. No difference in C and N pool sizes in the uppermost 10 cm of mineral soil was observed among stand types. Total C stocks for measured pools was 2.6 kg C/m2 smaller in thinned stands and 5.8 kg C/m2 smaller in shearbladed stands when compared to unmanaged forest. Soil thaw depth averaged 13 cm deeper in thinned areas and 46 cm deeper in shearbladed areas relative to adjacent unmanaged stands, although variability was high across sites. Deeper soil thaw was linked to shallower SOL depth for unmanaged stands and both management types, however for any given SOL depth, thaw tended to be deeper in shearbladed areas compared to unmanaged forest. These findings indicate that fuel-reduction management alters plant community composition, C and N pools, and soil thaw depth, with consequences for ecosystem structure and function beyond those intended for fire management.

Ecological responses to forest age, habitat, and host vary by mycorrhizal type in boreal peatlands.

Despite covering vast areas of boreal North America, the ecological factors structuring mycorrhizal fungal communities in peatland forests are relatively poorly understood. To assess how these communities vary by age (younger vs. mature), habitat (fen vs. bog), and host (conifer trees vs. ericaceous shrub), we sampled the roots of two canopy trees (Larix laricina and Picea mariana) and an ericaceous shrub (Ledum groenlandicum) at four sites in northern Minnesota, USA. To characterize the specific influence of host co-occurrence on mycorrhizal fungal community structure, we also conducted a greenhouse bioassay using the same three hosts. Root samples were assessed using Illumina-based high-throughput sequencing (HTS) of the ITS1 rRNA gene region. As expected, we found that the relative abundance of ectomycorrhizal fungi was high on both Larix and Picea, whereas ericoid mycorrhizal fungi had high relative abundance only on Ledum. Ericoid mycorrhizal fungal richness was significantly higher in mature forests, in bogs, and on Ledum hosts, while ectomycorrhizal fungal richness did not differ significantly across any of these three variables. In terms of community composition, ericoid mycorrhizal fungi were more strongly influenced by host while ectomycorrhizal fungi were more influenced by habitat. In the greenhouse bioassay, the presence of Ledum had consistently stronger effects on the composition of ectomycorrhizal, ericoid, and ericoid-ectomycorrhizal fungal communities than either Larix or Picea. Collectively, these results suggest that partitioning HTS-based datasets by mycorrhizal type in boreal peatland forests is important, as their responses to rapidly changing environmental conditions are not likely to be uniform.

CNVs into the wild: screening the genomes of conifer trees (Picea spp.) reveals fewer gene copy number variations in hybrids and links to adaptation.

BACKGROUND: Copy number variations (CNVs) have been linked to different phenotypes in human, including many diseases. A genome-scale understanding of CNVs is available in a few plants but none are wild species, leaving a knowledge gap regarding their genome biology and evolutionary role. We developed a reliable CNV detection method for species lacking contiguous reference genome. We selected multiple probes within 14,078 gene sequences and developed comparative genome hybridization on arrays. Gene CNVs were assessed in three full-sib families from species with 20 Gb genomes, i.e., white and black spruce, and interior spruce - a natural hybrid. RESULTS: We discovered hundreds of gene CNVs in each species, 3612 in total, which were enriched in functions related to stress and defense responses and narrow expression profiles, indicating a potential role in adaptation. The number of shared CNVs was in accordance with the degree of relatedness between individuals and species. The genetically mapped subset of these genes showed a wide distribution across the genome, implying numerous structural variations. The hybrid family presented significantly fewer CNVs, suggesting that the admixture of two species within one genome reduces the occurrence of CNVs. CONCLUSIONS: The approach we developed is of particular interest in non-model species lacking a reference genome. Our findings point to a role for CNVs in adaptation. Their reduced abundance in the hybrid may limit genetic variability and evolvability of hybrids. We propose that CNVs make a qualitatively distinct contribution to adaptation which could be important for short term change.

Bud break responds more strongly to daytime than night-time temperature under asymmetric experimental warming.

Global warming is diurnally asymmetric, leading to a less cold, rather than warmer, climate. We investigated the effects of asymmetric experimental warming on plant phenology by testing the hypothesis that daytime warming is more effective in advancing bud break than night-time warming. Bud break was monitored daily in Picea mariana seedlings belonging to 20 provenances from Eastern Canada and subjected to daytime and night-time warming in growth chambers at temperatures varying between 8 and 16 °C. The higher advancements of bud break and shorter times required to complete the phenological phases occurred with daytime warming. Seedlings responded to night-time warming, but still with less advancement of bud break than under daytime warming. No advancement was observed when night-time warming was associated with a daytime cooling. The effect of the treatments was uniform across provenances. Our observations realized under controlled conditions allowed to experimentally demonstrate that bud break can advance under night-time warming, but to a lesser extent than under daytime warming. Prediction models using daily timescales could neglect the diverging influence of asymmetric warming and should be recalibrated for higher temporal resolutions.

Volcano-induced regime shifts in millennial tree-ring chronologies from northeastern North America.

Dated records of ice-cap growth from Arctic Canada recently suggested that a succession of strong volcanic eruptions forced an abrupt onset of the Little Ice Age between A.D. 1275 and 1300 [Miller GH, et al. (2012) Geophys Res Lett 39(2):L02708, 10.1029/2011GL050168]. Although this idea is supported by simulation experiments with general circulation models, additional support from field data are limited. In particular, the Northern Hemisphere network of temperature-sensitive millennial tree-ring chronologies, which principally comprises Eurasian sites, suggests that the strongest eruptions only caused cooling episodes lasting less than about 10 y. Here we present a new network of millennial tree-ring chronologies from the taiga of northeastern North America, which fills a wide gap in the network of the Northern Hemisphere's chronologies suitable for temperature reconstructions and supports the hypothesis that volcanoes triggered both the onset and the coldest episode of the Little Ice Age. Following the well-expressed Medieval Climate Anomaly (approximately A.D. 910-1257), which comprised the warmest decades of the last millennium, our tree-ring-based temperature reconstruction displays an abrupt regime shift toward lower average summer temperatures precisely coinciding with a series of 13th century eruptions centered around the 1257 Samalas event and closely preceding ice-cap expansion in Arctic Canada. Furthermore, the successive 1809 (unknown volcano) and 1815 (Tambora) eruptions triggered a subsequent shift to the coldest 40-y period of the last 1100 y. These results confirm that series of large eruptions may cause region-specific regime shifts in the climate system and that the climate of northeastern North America is especially sensitive to volcanic forcing.

Optimization of Bioactive Polyphenols Extraction from Picea Mariana Bark.

Reported for its antioxidant, anti-inflammatory and non-toxicity properties, the hot water extract of Picea mariana bark was demonstrated to contain highly valuable bioactive polyphenols. In order to improve the recovery of these molecules, an optimization of the extraction was performed using water. Several extraction parameters were tested and extracts obtained analyzed both in terms of relative amounts of different phytochemical families and of individual molecules concentrations. As a result, low temperature (80 °C) and low ratio of bark/water (50 mg/mL) were determined to be the best parameters for an efficient polyphenol extraction and that especially for low molecular mass polyphenols. These were identified as stilbene monomers and derivatives, mainly stilbene glucoside isorhapontin (up to 12.0% of the dry extract), astringin (up to 4.6%), resveratrol (up to 0.3%), isorhapontigenin (up to 3.7%) and resveratrol glucoside piceid (up to 3.1%) which is here reported for the first time for Picea mariana. New stilbene derivatives, piceasides O and P were also characterized herein as new isorhapontin dimers. This study provides novel information about the optimal extraction of polyphenols from black spruce bark, especially for highly bioactive stilbenes including the trans-resveratrol.

Allometric exponents as a tool to study the influence of climate on the trade-off between primary and secondary growth in major north-eastern American tree species.

BACKGROUND AND AIMS: Trees invest in both primary (e.g. height) and secondary (e.g. diameter) growth. The trade-off between these investments varies between species and changes with the tree growing environment. To better establish this trade-off, readily available allometric exponents relating height to diameter at breast height (γ(h,dbh)) and stem volume to diameter at breast height (α(v,dbh)) were simultaneously studied. METHODS: Allometric exponents α(v,dbh) and γ(h,dbh) were obtained from 8893 individual tree stem analyses from two broadleaved species (Betula papyrifera, Populus tremuloides) and four conifers (Picea glauca, Picea mariana, Pinus banksiana, Abies balsamea) in the temperate and boreal forests of the province of Quebec, Canada. α(v,dbh) and γ(h,dbh) were related to tree age, stand density index (SDI), and mean temperature (TGS) and total precipitation (PGS) of the growing season. KEY RESULTS: α(v,dbh) and γ(h,dbh) were found to be invariant with PGS and positively related to SDI and TGS for all species except Pinus banksiana. The parameter values associated with SDI and TGS were of higher value for conifers than for broadleaved species. CONCLUSIONS: This suggests that conifers and broadleaved species have different growth patterns. This could be explained by their different mode of development, the conifer species having a stronger apical dominance than broadleaved species. Such results could be further considered in allocation studies to quantify future carbon stocks in managed forests.

Fire-severity effects on plant-fungal interactions after a novel tundra wildfire disturbance: implications for arctic shrub and tree migration.

BACKGROUND: Vegetation change in high latitude tundra ecosystems is expected to accelerate due to increased wildfire activity. High-severity fires increase the availability of mineral soil seedbeds, which facilitates recruitment, yet fire also alters soil microbial composition, which could significantly impact seedling establishment. RESULTS: We investigated the effects of fire severity on soil biota and associated effects on plant performance for two plant species predicted to expand into Arctic tundra. We inoculated seedlings in a growth chamber experiment with soils collected from the largest tundra fire recorded in the Arctic and used molecular tools to characterize root-associated fungal communities. Seedling biomass was significantly related to the composition of fungal inoculum. Biomass decreased as fire severity increased and the proportion of pathogenic fungi increased. CONCLUSIONS: Our results suggest that effects of fire severity on soil biota reduces seedling performance and thus we hypothesize that in certain ecological contexts fire-severity effects on plant-fungal interactions may dampen the expected increases in tree and shrub establishment after tundra fire.

Tracking the progression of speciation: variable patterns of introgression across the genome provide insights on the species delimitation between progenitor-derivative spruces (Picea mariana × P. rubens).

The genic species concept implies that while most of the genome can be exchanged somewhat freely between species through introgression, some genomic regions remain impermeable to interspecific gene flow. Hence, interspecific differences can be maintained despite ongoing gene exchange within contact zones. This study assessed the heterogeneous patterns of introgression at gene loci across the hybrid zone of an incipient progenitor-derivative species pair, Picea mariana (black spruce) and Picea rubens (red spruce). The spruce taxa likely diverged in geographic isolation during the Pleistocene and came into secondary contact during late Holocene. A total of 300 SNPs distributed across the 12 linkage groups (LG) of black spruce were genotyped for 385 individual trees from 33 populations distributed across the allopatric zone of each species and within the zone of sympatry. An integrative framework combining three population genomic approaches was used to scan the genomes, revealing heterogeneous patterns of introgression. A total of 23 SNPs scattered over 10 LG were considered impermeable to introgression and putatively under diverging selection. These loci revealed the existence of impermeable genomic regions forming the species boundary and are thus indicative of ongoing speciation between these two genetic lineages. Another 238 SNPs reflected selectively neutral diffusion across the porous species barrier. Finally, 39 highly permeable SNPs suggested ancestral polymorphism along with balancing selection. The heterogeneous patterns of introgression across the genome indicated that the speciation process between black spruce and red spruce is young and incomplete, albeit some interspecific differences are maintained, allowing ongoing species divergence even in sympatry. The approach developed in this study can be used to track the progression of ongoing speciation processes.

Stable carbon isotope analysis reveals widespread drought stress in boreal black spruce forests.

Unprecedented rates of climate warming over the past century have resulted in increased forest stress and mortality worldwide. Decreased tree growth in association with increasing temperatures is generally accepted as a signal of temperature-induced drought stress. However, variations in tree growth alone do not reveal the physiological mechanisms behind recent changes in tree growth. Examining stable carbon isotope composition of tree rings in addition to tree growth can provide a secondary line of evidence for physiological drought stress. In this study, we examined patterns of black spruce growth and carbon isotopic composition in tree rings in response to climate warming and drying in the boreal forest of interior Alaska. We examined trees at three nested scales: landscape, toposequence, and a subsample of trees within the toposequence. At each scale, we studied the potential effects of differences in microclimate and moisture availability by sampling on northern and southern aspects. We found that black spruce radial growth responded negatively to monthly metrics of temperature at all examined scales, and we examined ∆(13)C responses on a subsample of trees as representative of the wider region. The negative ∆(13)C responses to temperature reveal that black spruce trees are experiencing moisture stress on both northern and southern aspects. Contrary to our expectations, ∆(13)C from trees on the northern aspect exhibited the strongest drought signal. Our results highlight the prominence of drought stress in the boreal forest of interior Alaska. We conclude that if temperatures continue to warm, we can expect drought-induced productivity declines across large regions of the boreal forest, even for trees located in cool and moist landscape positions.