Anthony R. Taylor

Climate‐induced changes in host tree–insect phenology may drive ecological state‐shift in boreal forests

Climate change is altering insect disturbance regimes via temperature-mediated phenological changes and trophic interactions among host trees, herbivorous insects, and their natural enemies in boreal forests. Range expansion and increase in outbreak severity of forest insects are occurring in Europe and North America. The degree to which northern forest ecosystems are resilient to novel disturbance regimes will have direct consequences for the provisioning of goods and services from these forests and for long-term forest management planning. Among major ecological disturbance agents in the boreal forests of North America is a tortricid moth, the eastern spruce budworm, which defoliates fir (Abies spp.) and spruce (Picea spp.). Northern expansion of this defoliator in eastern North America and climate-induced narrowing of the phenological mismatch between the insect and its secondary host, black spruce (Picea mariana), may permit greater defoliation and mortality in extensive northern black spruce forests. Although spruce budworm outbreak centers have appeared in the boreal black spruce zone historically, defoliation and mortality were minor. Potential increases in outbreak severity and tree mortality raise concerns about the future state of this northern ecosystem. Severe spruce budworm outbreaks could decrease stand productivity compared with their occurrence in more diverse, southern balsam fir forest landscapes that have coevolved with outbreaks. Furthermore, depending on the proportion of balsam fir and deciduous species present and fire recurrence, changes in regeneration patterns and in nutrient cycling could alter ecosystem dynamics and replace black spruce by more productive mixed-wood forest, or by less productive ericaceous shrublands. Long-term monitoring, manipulative experiments, and process modeling of climate-induced phenological changes on herbivorous insect pests, their host tree species, and natural enemies in northern forests are therefore crucial to predicting species range shifts and assessing ecological and economic impacts.

Climate change impacts on forest landscapes along the Canadian southern boreal forest transition zone

ContextForest landscapes at the southern boreal forest transition zone are likely to undergo great alterations due to projected changes in regional climate.ObjectivesWe projected changes in forest landscapes resulting from four climate scenarios (baseline, RCP 2.6, RCP 4.5 and RCP 8.5), by simulating changes in tree growth and disturbances at the southern edge of Canada’s boreal zone.MethodsProjections were performed for four regions located on an east–west gradient using a forest landscape model (LANDIS-II) parameterized using a forest patch model (PICUS).ResultsClimate-induced changes in the competitiveness of dominant tree species due to changes in potential growth, and substantial intensification of the fire regime, appear likely to combine in driving major changes in boreal forest landscapes. Resulting cumulative impacts on forest ecosystems would be manifold but key changes would include (i) a strong decrease in the biomass of the dominant boreal species, especially mid- to late-successional conifers; (ii) increases in abundance of some temperate species able to colonize disturbed areas in a warmer climate; (iii) increases in the proportions of pioneer and fire-adapted species in these landscapes and (iv) an overall decrease in productivity and total biomass. The greatest changes would occur under the RCP 8.5 radiative forcing scenario, but some impacts can be expected even with RCP 2.6.ConclusionsWestern boreal forests, i.e., those bordering the prairies, are the most vulnerable because of a lack of species adapted to warmer climates and major increases in areas burned. Conservation and forest management planning within the southern boreal transition zone should consider both disturbance- and climate-induced changes in forest communities.

The Contribution of Litterfall to Net Primary Production During Secondary Succession in the Boreal Forest

Litterfall is a fundamental process in the nutrient cycle of forest ecosystems and a major component of annual net primary production (NPP). Despite its importance for understanding ecosystem energetics and carbon accounting, the dynamics of litterfall production following disturbance and throughout succession remain poorly understood in boreal forest ecosystems. Using a replicated chronosequence spanning 209 years following fire and 33 years following logging in Ontario, Canada, we examined the dynamics of litterfall production associated with stand development, overstory composition type (broadleaf, mixedwood, and conifer), and disturbance origin. We found that total annual litterfall production increased with stand age following fire and logging, plateauing in post-fire stands approximately 98 years after fire. Neither total annual litterfall production nor any of its constituents differed between young fire- or logging-originated stands. Litterfall production was generally higher in broadleaf stands compared with mixedwood and conifer stands, but varied seasonally, with foliar litterfall highest in broadleaf stands in autumn, and epiphytic lichen litterfall highest in conifer stands in spring. Contrary to previous assumptions, we found that the contribution of litterfall production to net primary production increased with stand age, highlighting the need for modeling studies of net primary productivity to account for the effects of stand age on litterfall dynamics.

Intensive forest harvesting increases susceptibility of northern forest soils to carbon, nitrogen and phosphorus loss

Summary 1.Understanding the impact of forest harvesting is critical to sustainable forest management, yet there remains much uncertainty regarding how harvesting affects soil carbon (C), nitrogen (N) and phosphorus (P) dynamics. 2.Here we conducted a global meta-analysis of 808 observations from 49 studies to test the effects of harvesting on the stocks and concentrations of soil C, N, and P and C:N:P ratios relative to uncut control stands. 3.With all harvesting intensities combined, C stock was unaffected by harvesting in either the forest floor or mineral soil, while harvesting reduced forest floor [C], [N], and [P] and C:N ratio, increased the mineral soil [C] and C:N ratio, but reduced mineral soil N stock. The impacts of harvesting on forest floor C and N stocks, C:P and N:P and mineral soil [C] and [N] changed from no effects by partial, stem-only and whole-tree harvesting to significantly negative effects by the harvesting coupled with fire. Stem-only and whole-tree harvesting similarly reduced forest floor [P]. The negative effects of harvesting were most pronounced in conifer stands. Soil [C], [N] and C:N decreased with time since harvesting, but soil [P] did not, resulting in an increase in forest floor N:P. 4.Synthesis and applications. Our findings highlight the importance of harvest intensity and rotation length on long-term soil nutrient availability when managing forests. Furthermore, the lag in the recovery of phosphorus concentration following harvesting may indicate a decoupling of the phosphorus cycle from those of carbon and nitrogen and a potential concern in managed forests. This article is protected by copyright. All rights reserved.

Harvesting interacts with climate change to affect future habitat quality of a focal species in eastern Canada’s boreal forest.

Many studies project future bird ranges by relying on correlative species distribution models. Such models do not usually represent important processes explicitly related to climate change and harvesting, which limits their potential for predicting and understanding the future of boreal bird assemblages at the landscape scale. In this study, we attempted to assess the cumulative and specific impacts of both harvesting and climate-induced changes on wildfires and stand-level processes (e.g., reproduction, growth) in the boreal forest of eastern Canada. The projected changes in these landscape- and stand-scale processes (referred to as “drivers of change”) were then assessed for their impacts on future habitats and potential productivity of black-backed woodpecker (BBWO; Picoides arcticus), a focal species representative of deadwood and old-growth biodiversity in eastern Canada. Forest attributes were simulated using a forest landscape model, LANDIS-II, and were used to infer future landscape suitability to BBWO under three anthropogenic climate forcing scenarios (RCP 2.6, RCP 4.5 and RCP 8.5), compared to the historical baseline. We found climate change is likely to be detrimental for BBWO, with up to 92% decline in potential productivity under the worst-case climate forcing scenario (RCP 8.5). However, large declines were also projected under baseline climate, underlining the importance of harvest in determining future BBWO productivity. Present-day harvesting practices were the single most important cause of declining areas of old-growth coniferous forest, and hence appeared as the single most important driver of future BBWO productivity, regardless of the climate scenario. Climate-induced increases in fire activity would further promote young, deciduous stands at the expense of old-growth coniferous stands. This suggests that the biodiversity associated with deadwood and old-growth boreal forests may be greatly altered by the cumulative impacts of natural and anthropogenic disturbances under a changing climate. Management adaptations, including reduced harvesting levels and strategies to promote coniferous species content, may help mitigate these cumulative impacts.

Stand‐level drivers most important in determining boreal forest response to climate change

1.Forest ecosystems contain several climate-sensitive drivers that respond differentially to changes in climate and climate variability. For example, growth and regeneration processes are “stand-scale” drivers, while natural disturbances operate at “landscape-scale”. The relative contributions of these different scale drivers of change in ecosystems create great uncertainty when simulating potential responses of a forest to changes in climate. 2.Here we assess those contributions, along with harvesting effects, on biomass (both total and of individual species) in the southern boreal forest of Canada under three climate scenarios (RCP 2.6, RCP 4.5 and RCP 8.5). 3.Projections were performed for three future 30-year time periods, in four study regions located on an east-west transect, using a forest landscape model (LANDIS-II), parameterized using a forest patch model (PICUS). Projected future impacts were assessed for each driver of change, and found to vary greatly among regions, species, future period, and forcing scenarios. Fire, and stand-scale climate-induced impacts, had the strongest effects on forest vegetation, as well as on total and species’ biomass under most RCP scenarios, but the largest impacts occurred mostly after 2050, particularly with the RCP 8.5 scenario. 4.The relative importance and trends in species-specific impacts varied, both spatially and according to the different RCP scenarios. Western regions were generally more sensitive to stand-scale climate-induced changes whereas eastern regions were more sensitive to changes in fire regime. Our study also highlights the importance of considering the prevalence of species-level functional traits when assessing the sensitivity of forest landscapes to a given driver of change in the context of increasing anthropogenic climate forcing. 5.Synthesis. Increases in fire activity, and direct impacts of climate change on forest growth and regeneration, will be the most important drivers of future changes in southern boreal forest landscapes. This article is protected by copyright. All rights reserved.