Role of Mixed-Species Stands in Attenuating the Vulnerability of Boreal Forests to Climate Change and Insect Epidemics

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.

Site index as a predictor of the effect of climate warming on boreal tree growth

“…Species mixture can enhance litter decomposition and nutrient cycling (Cavard et al, 2010;Prescott et al, 2000), increase soil nutrient availability (Houle et al, 2014;Oboite & Comeau, 2019), but also increase competition if there is a strong niche overlap between species (Oboite & Comeau, 2019, 2021. Species mixture can also influence the impact of climate on growth (Chavardès et al, 2021;Oboite & Comeau, 2019). The D'Orangeville et al, 2018).…”

Insect defoliation modulates influence of climate on the growth of tree species in the boreal mixed forests of eastern Canada

“…Higher densities of host species and warmer climates can increase the intensity of tree defoliation, and promote growth reduction and mortality in the host trees. The resulting canopy openings will increase the availability of light and water to promote the growth of the hon‐hosts (Chavardès et al, 2021; Steckel et al, 2020).…”

Greater tree species diversity and lower intraspecific competition attenuate impacts from temperature increases and insect epidemics in boreal forests of western Quebec, Canada

“…Specifically, we focused temperaturegrowth relations on two surrogates of the growing season length and summer heat stress. These surrogates are two important temperature variables for tree growth in the study area (Chavardès et al 2021) and were obtained by calculating the mean of average temperatures for April and September and June-August, respectively, between 1955 We measured growth characteristics of focal trees (orange boxes) and environmental variables (blue boxes; only variables that were not excluded due to high multicollinearity according to Pearson's correlation coefficients are shown). Growth characteristics were represented by diameter at breast height (DBH), average growth rates over 1955-2018 (GR), tree response to growing season length (RespGSL) or summer heat stress (RespSHS), and growth response during spruce budworm epidemics (GSBW) or forest tent caterpillar epidemics (GFTC).…”

“…Testing relations between monthly to seasonal temperature and tree growth (i.e., temperature-growth relations) and estimating the impact of insect epidemics on tree growth are approaches that can be used independently or in combination (Drobyshev et al 2013;Chavardès et al 2021). Such approaches offer information on how temperature benefits or limits growth of different tree species, and whether these species have positive or negative growth responses associated with insect epidemics.…”

“…Diversity within stands can benefit growth of trees in several ways, for example through niche partitioning over space, including via rooting systems (Houle et al 2014;Mekontchou et al 2020) and canopy layers (Jucker et al 2020), or over time such as via distinct start and end dates for vegetative growth across tree species (Huang et al 2010). Diverse tree species in forest stands can also exhibit differing growth responses to disturbances such as insect epidemics (Chavardès et al 2021), thereby stabilizing tree biomass (Aussenac et al 2016). Conversely, dominance by tree species with given traits can affect ecosystem processes (i.e., selection effects), and functional identity of some species rather than species richness per se can be the most important promoting factor of ecosystem multifunctionality (i.e., species identify effect) (Mouillot et al 2011;Jochum et al 2020).…”