Abstract
We investigated whether stand species mixture can attenuate the vulnerability of eastern Canada’s boreal forests to climate change and insect epidemics. For this, we focused on two dominant boreal species, black spruce [Picea mariana (Mill.) BSP] and trembling aspen (Populus tremuloides Michx.), in stands dominated by black spruce or trembling aspen (“pure stands”), and mixed stands (M) composed of both species within a 36 km2 study area in the Nord-du-Québec region. For each species in each stand composition type, we tested climate-growth relations and assessed the impacts on growth by recorded insect epidemics of a black spruce defoliator, the spruce budworm (SBW) [Choristoneura fumiferana (Clem.)], and a trembling aspen defoliator, the forest tent caterpillar (FTC; Malacosoma disstria Hübn.). We implemented linear models in a Bayesian framework to explain baseline and long-term trends in tree growth for each species according to stand composition type and to differentiate the influences of climate and insect epidemics on tree growth. Overall, we found climate vulnerability was lower for black spruce in mixed stands than in pure stands, while trembling aspen was less sensitive to climate than spruce, and aspen did not present differences in responses based on stand mixture. We did not find any reduction of vulnerability for mixed stands to insect epidemics in the host species, but the non-host species in mixed stands could respond positively to epidemics affecting the host species, thus contributing to stabilize ecosystem-scale growth over time. Our findings partially support boreal forest management strategies including stand species mixture to foster forests that are resilient to climate change and insect epidemics.
Highlights
The boreal forest is the second largest biome on Earth, providing humans with ecosystem services that include sustainably harvested wood, carbon storage, and freshwater resources (Gauthier et al, 2015)
Black spruce trees in mixed stands had high baseline mean basal area increment (BAI) relative to spruce in pure stands (Figure 5A), but their long-term growth trend was impacted by the spruce budworm (SBW) epidemic in the 1970s, which coincided with several years of above average summer drought within the region (Environment Canada, 2020b; Figure 5B)
Black spruce is a potential host for SBW (Blais, 1957), we found spruce in mixed stands and pure aspen stands showed a greater decrease in mean BAI and higher vulnerability to the 1970s SBW epidemic than in pure spruce stands (Figure 5E)
Summary
The boreal forest is the second largest biome on Earth, providing humans with ecosystem services that include sustainably harvested wood, carbon storage, and freshwater resources (Gauthier et al, 2015). The role of species mixture was investigated in reviews that compile a suite of findings identifying the beneficial effects of mixed-species stands over single-species stands including increased stand-level biodiversity and productivity (e.g., carbon sequestration, nutrient cycling, and water-use efficiency), decreased risk of damage caused by some disturbances (e.g., pathogens, pests, and windthrow), and diversified forestry production over time (Lilles and Coates, 2013; Felton et al, 2016; Liu et al, 2018). Studies that focus on understanding the dynamics in mixedspecies stands and how these compare with single-species stands remain few, and limit our assessments of the benefits and disadvantages of these stands with respect to climate change and its impacts (Liu et al, 2018)
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