Top-down effects, like predation, are drivers of insect outbreaks, but bottom-up effects, like host nutritional quality, also influence outbreaks and could in turn be altered by insect-caused defoliation. We evaluated the prediction that herbivory leads to a positive feedback on outbreak severity as nutrient concentration in plant tissues increases through improved soil nutrient availability from frass and litter deposition. Over seven years of a spruce budworm outbreak, we quantified litter nutrient fluxes, soil nitrogen availability, and host tree foliar nutrient status along a forest susceptibility gradient. As the outbreak progressed, both soil nutrient fluxes and availability increased which, in turn, improved foliage quality in surviving host trees. This is consistent with boosted insect fitness and increased population density and defoliation as outbreaks grow. Our results suggest that a positive bottom-up feedback to forest ecosystems from defoliation may result in conditions favorable to self-amplifying population dynamics in insect herbivores that can contribute to driving broad-scale outbreaks.

 

Aim

The main objective of this study was to evaluate the influence of leachates from three typical boreal forest ground layers on young tree growth and to explore the linkages between the chemical composition of the leachates, tree growth, the allocation between belowground and aboveground parts, and ectomycorrhizal colonization.

Methods

An original 6-month greenhouse experiment was set up to investigate (i) the effects of lichen (Cladonia spp.) and feather moss (Pleurozium schreberii [Brid.] Mitt.) leachates on jack pine (Pinus banksiana Lamb.) growth and (ii) the effects of feather moss and Sphagnum spp. leachates on black spruce (Picea mariana [Mill.] B.S.P.) growth.

Results

Belowground growth and root allocation was reduced by lichen leachates in 2-year-old pine seedlings, while the impact was significant on both below- and aboveground growth in 6-month-old pine seedlings. A substance having the same migration time as usnic acid was detected in the lichen leachates by high-performance liquid chromatography. Sphagnum spp. and feather moss leachates were not found to have any effect on black spruce seedling growth, despite a higher supply of dissolved inorganic N in the feather moss leachates compared to the leachates of Sphagnum spp. and the control.

Conclusions

These results demonstrate that ground layer composition plays a crucial role in shaping the plant community in boreal ecosystems by influencing the chemical composition of the soil solution. They suggest that chemical interference may be another mechanism by which lichens promote the self-perpetuation of open woodlands in the closed-crown boreal forest.

Following a wildfire, organic carbon (C) accumulates in boreal-forest soils. The long-term patterns of accumulation as well as the mechanisms responsible for continuous soil C stabilization or sequestration are poorly known. We evaluated post-fire C stock changes in functional reservoirs (bioreactive and recalcitrant) using the proportion of C mineralized in CO2 by microbes in a long-term lab incubation, as well as the proportion of C resistant to acid hydrolysis. We found that all soil C pools increased linearly with the time since fire. The bioreactive and acid-insoluble soil C pools increased at a rate of 0.02 and 0.12?MgC?ha?1?yr?1, respectively, and their proportions relative to total soil C stock remained constant with the time since fire (8?% and 46?%, respectively). We quantified direct and indirect causal relationships among variables and C bioreactivity to disentangle the relative contribution of climate, moss dominance, soil particle size distribution and soil chemical properties (pH, exchangeable manganese and aluminum, and metal oxides) to the variation structure of in vitro soil C bioreactivity. Our analyses showed that the chemical properties of podzolic soils that characterize the study area were the best predictors of soil C bioreactivity. For the O layer, pH and exchangeable manganese were the most important (model-averaged estimator for both of 0.34) factors directly related to soil organic C bioreactivity, followed by the time since fire (0.24), moss dominance (0.08), and climate and texture (0 for both). For the mineral soil, exchangeable aluminum was the most important factor (model-averaged estimator of ?0.32), followed by metal oxide (?0.27), pH (?0.25), the time since fire (0.05), climate and texture (?0 for both). Of the four climate factors examined in this study (i.e., mean annual temperature, growing degree-days above 5??C, mean annual precipitation and water balance) only those related to water availability – and not to temperature – had an indirect effect (O layer) or a marginal indirect effect (mineral soil) on soil C bioreactivity. Given that predictions of the impact of climate change on soil C balance are strongly linked to the size and the bioreactivity of soil C pools, our study stresses the need to include the direct effects of soil chemistry and the indirect effects of climate and soil texture on soil organic matter decomposition in Earth system models to forecast the response of boreal soils to global warming.

The increase in open-crown forest stands in the closed-crown boreal forest of Quebec over the last 50 years prompts us to identify and understand the drivers responsible for stand opening. To do so, we studied 37 jack pine plots with varying degrees of canopy opening in the Eastern Canadian boreal forest to answer four questions: (1) Does stand opening result from a deficit in pine regeneration, from poor tree growth, or from both processes simultaneously? (2) In the event that pine stand opening results at least in part from poor tree growth, how early following stand initiation does the tree growth divergence occur between unproductive and productive plots? (3) Is poor tree growth in the unproductive plots related to water stress? Finally, (4) are there predisposing site factors and, if so, what are their contributions versus non-permanent factors such as disturbance history, vegetation, and soil dynamics? In the study area, jack pine stand openings resulted from both a poor regeneration density and weak tree growth. Tree growth divergence between productive and unproductive plots occurred very early during the post-disturbance forest succession and is not likely to result from water limitation during the early development of the trees as revealed by ?13C analysis of tree rings. Low-productivity plots were exclusively found on substrates with low base cation reserves. However, because plots of higher productivity were also found on these substrates, we conclude that stand susceptibility to regeneration failures may be greater on sites with such conditions. Variations in tree cover were mainly related to non-permanent environmental variables, suggesting that restoration of forest productivity is theoretically possible in the low-productivity sites investigated.

Identifying geochemical paleo?proxies of vegetation type in watersheds could become a powerful tool for paleoecological studies of ecosystem dynamics, particularly when commonly used proxies, such as pollen grains, are not suitable. In order to identify new paleological proxies to distinguish ecosystem types in lake records, we investigated the differences in the sediment geochemistry of lakes surrounded by two boreal forest ecosystems dominated by the same tree species: closed?canopy black spruce?moss forests (MF) and open?canopy black spruce?lichen woodlands (LW). This study was designed as a first calibration step between terrestrial modern soils and lacustrine sediments (0–1000 cal yr BP) on six lake watersheds. In a previous study, differences in the physical and geochemical properties of forest soils had been observed between these two modern ecosystems. Here we show that the geochemical properties of the sediments varied between the six lakes studied. While we did not identify geochemical indicators that could solely distinguish both ecosystem types in modern sediments, we observed intriguing differences in concentrations of C:N ratio, carbon isotopic ratio, and aluminum oxide species, and in the stabilization of their geochemical properties with depth. The C accumulation rates at millennial scale were significantly higher in MF watersheds than in LW watersheds. We suggest that these variations could result from organic matter inflows that fluctuate depending on forest density and ground vegetation cover. Further investigations on these highlighted geochemistry markers need to be performed to confirm whether they can be used to detect shifts in vegetation conditions that have occurred in the past.

In the managed portion of the boreal forest of eastern North America, logging has replaced fire as the most important disturbance agent. There, a large proportion of timber is harvested in forests susceptible of accumulating a thick Sphagnum layer that decreases forest productivity, a process called paludification. In such a context, understanding how disturbance type and severity of soil disturbances may affect post-disturbance microhabitat characteristics and understory community composition is critical for forest management. Different management techniques have been used such as careful logging and clearcutting, as well as winter and summer harvests, with various impacts on soils and forest regeneration. In the current study, we used 55 study sites representing a gradient of soil disturbance severity by harvesting (winter and summer) and fire (low and high severity) to compare their impacts on understory plant communities in the Clay Belt area of eastern Canada. At each site, understory community composition (vascular plants, bryophytes, and lichens) was assessed. We found a strong response of communities to overall severity as represented by disturbance type (careful logging, clearcutting, and fire), but little impact of fine scale disturbance severity (winter vs. summer, low vs. high severity disturbance) within each type of disturbance. Differences in community composition were reflected in the abundance of the various plant functional types, with invaders being more common in harvested sites, endurers being common in all disturbances except high severity fires, and avoiders being more common in older sites. Understory communities in harvested sites (<40?years old) were similar to communities typical of old sites originating from natural wildfire disturbances (75–100?years old low severity fires or 200?years old high severity fires) in terms of composition, but also Sphagnum spp. abundance. In order to maintain long-term forest productivity and manage forests in ways that more closely reproduce post-fire conditions, logging operations should aim at increasing soil disturbances, for example by using prescribed burns, in the Clay Belt area of eastern North America.

In the boreal forest, open lichen woodlands have been described as an alternative stable state to closed-crown feather moss forest. In this study, we addressed the role of terricolous lichens in stabilizing open woodlands by hindering tree regeneration and/or growth. Based on field and greenhouse experiments, we compared germination and growth of jack pine (Pinus banksiana) on feather mosses (primarily Pleurozium schreberi) and lichens (primarily Cladonia stellaris), using bare mineral soil as a control. Drivers were investigated by (1) manipulating nutrient supply, (2) simulating shade of a closed canopy on the ground layer with the assumption this would mitigate lichen influence on pine growth, and (3) examining pine root ectomycorrhizal colonization and diversity as indicators of pine ability to take up nutrients. Total growth of 6-month-old greenhouse and 2–3-year-old field seedlings, as well as belowground growth of 2-year-old greenhouse seedlings, was significantly greater in moss than in lichen. Seed germination was not affected by ground cover type. Although field phosphorus and base cation availability was greater in mosses than in lichens, fertilization did not entirely compensate for the negative effects of lichens on pine growth in the greenhouse. Ground layer shading had no impact on pine growth. Lichens were associated with reduced abundance and modified composition of the root ectomycorrhizal community. By suggesting that terricolous lichens constitute a less favorable growth substrate than mosses for pine, our results support the hypothesis that lichens contribute to open woodland stability in the potentially closed-crown feather moss forest.

Wildfire is the primary abiotic disturbance in the boreal forest, and its long-term absence can lead to large changes in ecosystem properties, including the availability and cycling of nutrients. These effects are, however, often confounded with the effects of successional changes in vegetation toward nutrient-conservative species. We studied a system of boreal forested lake islands in eastern Canada, where time since last fire ranged from 50 to 1500 years, and where the relative abundance of the most nutrient-conservative tree species, black spruce, was largely independent of time since last fire. This allowed us to disentangle the effects of time since fire and the dominant vegetation on ecosystem properties, including nutrient stocks and concentrations. Effects of time since fire independent of vegetation composition mostly involved an increase in the thickness of the organic layer and in nitrogen concentration in both soil and leaves. Domination by black spruce had strong negative effects on nutrient concentrations and was associated with a shift toward more fungi and Gram-positive bacteria in the soil microbial community. Path modeling showed that phosphorus concentration was inversely related to organic layer thickness, which was in turn related to both time since fire and black spruce abundance, while nitrogen was more directly related to time since fire and the composition of the overstory. We conclude that discriminating between the effects of vegetation and time since fire is necessary for better understanding and predicting the long-term changes that occur in forest nutrient availability and ecosystem properties.

Forest management strongly influences the carbon (C) budget of boreal forests and their potential to mitigating greenhouse gas emissions. A better quantification of the net changes of carbon pools with time since harvesting is necessary to guide the development of climate-friendly forest management practices. The objective of this study was to assess the evolution of forest C pools, with a special focus on detrital biomass, in an 80-year post-harvesting chronosequence consisting of 36 very homogenous stem-only harvested plots from a humid boreal balsam fir forest of eastern Canada. Dead wood C stocks comprised of snags, stumps, downed woody debris and buried wood averaged 37?Mg?C?ha?1 and evolved according to an upward-facing «boomerang» shape pattern throughout the chronosequence (rapid decrease in the first years followed by a constant increase until the end of the time horizon). In contrast, soil C stocks (FH and mineral) averaged 156?Mg?C?ha?1 and remain constant through time. Stand C sequestration increased rapidly in the early stages up to age 50 when it reached about 250?Mg?C?ha?1, and then continued to accumulate at a slower rate. The temporal trends observed in C pools suggest that C originating from aboveground dead wood (snags, stumps, downed woody debris) is either leaving the system (respired or leached) or transferred into buried wood, and does not appear to influence the C stocks of the fine fraction of the organic and mineral soil horizons. However, the ultimate fate of dead wood C is still poorly understood and further research is needed in this field. We recommend fixing the length of harvest rotation at a minimum of 50?years for this ecosystem to allow the build-up of its dead wood capital, and to promote dead wood retention on site. We also recommend including buried wood in carbon inventories as this pool represents an important share of the detrital C stock in these humid boreal forests.

The accumulation of soil carbon (C) is regulated by a complex interplay between abiotic and biotic factors. Our study aimed to identify the main drivers of soil C accumulation in the boreal forest of eastern North America. Ecosystem C pools were measured in 72 sites of fire origin that burned 2–314 years ago over a vast region with a range of ? mean annual temperature of 3°C and one of ? 500 mm total precipitation. We used a set of multivariate a priori causal hypotheses to test the influence of time since fire (TSF), climate, soil physico?chemistry and bryophyte dominance on forest soil organic C accumulation. Integrating the direct and indirect effects among abiotic and biotic variables explained as much as 50% of the full model variability. The main direct drivers of soil C stocks were: TSF >bryophyte dominance of the FH layer and metal oxide content >pH of the mineral soil. Only climate parameters related to water availability contributed significantly to explaining soil C stock variation. Importantly, climate was found to affect FH layer and mineral soil C stocks indirectly through its effects on bryophyte dominance and organo?metal complexation, respectively. Soil texture had no influence on soil C stocks. Soil C stocks increased both in the FH layer and mineral soil with TSF and this effect was linked to a decrease in pH with TSF in mineral soil. TSF thus appears to be an important factor of soil development and of C sequestration in mineral soil through its influence on soil chemistry. Overall, this work highlights that integrating the complex interplay between the main drivers of soil C stocks into mechanistic models of C dynamics could improve our ability to assess C stocks and better anticipate the response of the boreal forest to global change.