Widespread mortality of trembling aspen (Populus tremuloides) throughout interior Alaskan boreal forests resulting from a novel canker disease

Over the past several decades, growth declines and mortality of trembling aspen throughout western Canada and the United States have been linked to drought, often interacting with outbreaks of insects and fungal pathogens, resulting in a “sudden aspen decline” throughout much of aspen’s range. In 2015, we noticed an aggressive fungal canker causing widespread mortality of aspen throughout interior Alaska and initiated a study to quantify potential drivers for the incidence, virulence, and distribution of the disease. Stand-level infection rates among 88 study sites distributed across 6 Alaska ecoregions ranged from <1 to 69%, with the proportion of trees with canker that were dead averaging 70% across all sites. The disease is most prevalent north of the Alaska Range within the Tanana Kuskokwim ecoregion. Modeling canker probability as a function of ecoregion, stand structure, landscape position, and climate revealed that smaller-diameter trees in older stands with greater aspen basal area have the highest canker incidence and mortality, while younger trees in younger stands appear virtually immune to the disease. Sites with higher summer vapor pressure deficits had significantly higher levels of canker infection and mortality. We believe the combined effects of this novel fungal canker pathogen, drought, and the persistent aspen leaf miner outbreak are triggering feedbacks between carbon starvation and hydraulic failure that are ultimately driving widespread mortality. Warmer early-season temperatures and prolonged late summer drought are leading to larger and more severe wildfires throughout interior Alaska that are favoring a shift from black spruce to forests dominated by Alaska paper birch and aspen. Widespread aspen mortality fostered by this rapidly spreading pathogen has significant implications for successional dynamics, ecosystem function, and feedbacks to disturbance regimes, particularly on sites too dry for Alaska paper birch.

Regional variation in wood discoloration in paper birch trees

Wood discoloration was investigated in 721 paper birch trees from 146 sites across the managed forest of Quebec, Canada. Discoloration was present at breast height in 85% of trees, but its impact was limited in terms of tree basal area and volume, with proportions of 6.4% and 3.6%, respectively. These two measures were strongly correlated. Discoloration changed wood appearance but had no effect on microfibril angle and modulus of elasticity. Discolored wood was nevertheless denser, probably due to the accumulation of colored extractives. Predictive models were also developed and applied to 415,711 paper birch trees from 51,689 inventory plots across the province to study regional variation. Higher proportions of discolored basal area were obtained in the southwestern areas of the province where the climate is warmer and drier. In these areas, paper birch trees are larger and, likely, support bigger branches which can cause larger columns of discoloration when broken. Lower proportions were found in eastern regions where snowfall is more abundant. Smaller, more flexible branches in trees growing in these areas could explain this result. This study confirms the feasibility of large-scale mapping of wood discoloration in standing trees based on forest inventory and climate data.

Limited physiological acclimation to recurrent heatwaves in two boreal tree species

The intensity of extreme heat and drought events has drastically risen in recent decades and will likely continue throughout the century. Northern forests have already seen increases in tree mortality and a lack of new recruitment, which is partially attributed to these extreme events. Boreal species, such as paper birch (Betula papyrifera) and white spruce (Picea glauca), appear to be more sensitive to these changes than lower-latitude species. Our objectives were to investigate the effects of repeated heatwaves and drought on young paper birch and white spruce trees by examining (i) responses in leaf gas exchange and plant growth and (ii) thermal acclimation of photosynthetic and respiratory traits to compare ecophysiological responses of two co-occurring, yet functionally dissimilar species. To address these objectives, we subjected greenhouse-grown seedlings to two consecutive summers of three 8-day long, +10 °C heatwaves in elevated atmospheric CO2 conditions with and without water restriction. The data show that heatwave stress reduced net photosynthesis, stomatal conductance and growth—more severely so when combined with drought. Acclimation of both photosynthesis and respiration did not occur in either species. The combination of heat and drought stress had a similar total effect on both species, but each species adjusted traits differently to the combined stress. Birch experienced greater declines in gas exchange across both years and showed moderate respiratory but not photosynthetic acclimation to heatwaves. In spruce, heatwave stress reduced the increase in basal area in both experimental years and had a minor effect on photosynthetic acclimation. The data suggest these species lack the ability to physiologically adjust to extreme heat events, which may limit their future distributions, thereby altering the composition of boreal forests.

Effects of manual brushing on 10-year survival and growth of Douglas-fir in the mixed broadleaf – shrub complex of southern interior British Columbia

Manual brushing is used to minimize the competitive effects of paper birch (Betula papyrifera Marsh) and associated broadleaved trees on young Douglas-fir (Pseudotsuga menziesii var. glauca) in southern interior British Columbia. Effects of brushing broadleaved trees, predominantly birch, on interior Douglas-fir survival and growth were studied on four sites. Treatments were applied when plantations were five to nine years old. Through 10 years post treatment, brushing did not affect Douglas-fir survival, but increase height by 22 % and stem diameter by 31 % and the differences were greater than seen at five years. After 10 years, linear models described a declining Douglas-fir height or diameter with increasing broadleaved tree density. Boundary line analysis was used to describe maximum treatment response to broadleaved density and two distance independent competition indices for birch and broadleaves, combining either cover or density with relative heights (CRH, DRH, respectively). A negative exponential relationship was fit to 10-year Douglas-fir heights and diameters with increasing values CRH or DRH. Competition thresholds for density, CRH and DRH were not apparent. The quantile regression results indicated the 10-year response of young Douglas-fir diameter to brushing occurred primarily with the largest 55 % to 85 % of the population, CRH and DRH respectively.

Spatial Distribution of a Tree Trunk Specialist Spider: Relative Role of Landscape Versus Microhabitat Drivers

By completely censusing a 1 ha forest dynamics plot it was possible to identify the variables (spider mass, size, sex and tree species, size, and bark roughness) that influenced the spatial distribution of adult Drapetisca alteranda Chamberlin 1909 (Araneae: Linyphiidae), a sheet web spider that specializes in lower tree trunks in North American forests. To account for spatial autocorrelation, a conditional autoregressive random effect was included in the zero-inflated Poisson generalized linear mixed model. Parameters estimated were produced by Bayesian inference with vague prior probability distributions and the best of 16 models were selected using Watanabe-Akaike Information Criterion. The best model showed that larger diameter trees located at higher plot elevations were more likely to have D. alteranda present. Smooth bark tree species such as paper birch and American basswood tended to have the most spiders while rough bark species had the least. The relationship between tree diameter and D. alteranda abundance also varied by tree species. Paper birch and quaking aspen tend to produce a greater slope compared to the other species, indicating that as these trees get larger, the abundance of D. alteranda increases at a higher rate than on other tree species. Spider sex and size were not associated with height on the trunk or tree species selection, nor were they associated with microhabitats such as bark furrow depth. Landscape-level factors largely predict D. alteranda abundance and distribution, suggesting that spatial autocorrelation should be considered when modeling the abundance of even small organisms, such as spiders.

Metabolite Composition of Paper Birch Buds after Eleven Growing Seasons of Exposure to Elevated CO2 and O3

Research Highlights: Long-term exposure of paper birch to elevated carbon dioxide (CO2) and ozone (O3) modified metabolite content of over-wintering buds, but no evidence of reduced freezing tolerance was found. Background and Objectives: Atmospheric change may affect the metabolite composition of over-wintering buds and, in turn, impact growth onset and stress tolerance of perennial plant species in spring. Materials and Methods: Low molecular weight compounds of paper birch (Betula papyrifera) buds, including lipophilic, polar and phenolic compounds were analyzed, and freezing tolerance (FT) of the buds was determined prior to bud break after 11 growing seasons exposure of saplings to elevated concentrations of CO2 (target concentration 560 µL L−1) and O3 (target concentration 1.5 × ambient) at the Aspen FACE (Free-Air CO2 and O3 Enrichment) facility. Results: The contents of lipophilic and phenolic compounds (but not polar compounds) were affected by elevated CO2 and elevated O3 in an interactive manner. Elevated O3 reduced the content of lipids and increased that of phenolic compounds under ambient CO2 by reallocating carbon from biosynthesis of terpenoids to that of phenolic acids. In comparison, elevated CO2 had only a minor effect on lipophilic and polar compounds, but it increased the content of phenolic compounds under ambient O3 by increasing the content of phenolic acids, while the content of flavonols was reduced. Conclusions: Based on the freezing test and metabolite data, there was no evidence of altered FT in the over-wintering buds. The impacts of the alterations of bud metabolite contents on the growth and defense responses of birches during early growth in spring need to be uncovered in future experiments.