Tree assisted migration in a browsed landscape: can we predict susceptibility to herbivores?
To promote the sustainability of forest ecosystems and maintain ecosystem services to human populations in the context of climate change, forest managers are considering several adaptation tools. One of those, assisted migration, consists of displacing tree species and/or populations to locations with suitable future climate conditions. Assisted migration plantations, however, could fail to produce viable forests under high herbivory pressure from mammalian herbivores. Thus, selecting species and genotypes with low susceptibility to herbivores could be a key condition for the recruitment of translocated seedlings. We developed an approach to predict susceptibility to mammalian herbivores, based on the seedlings' chemical content (a proxy of phytochemical defence and susceptibility to herbivores). We used the approach on eight North American tree species of three climate analogues regions each (i.e. locations where the current climate is similar to the future climate at plantation site). We built chemical profiles and ranked species and climate analogue based on their potential susceptibility. To assess the reliability of our approach, we compared the chemical profiles to a systematic review of these species' chemistry and of mammalian browsing throughout their native geographic range. For most species, our chemical profiles and browse susceptibility rankings were congruent with information available in the literature, both for phytochemical defence and for browsing. Two of the eight species (Pinus strobus and Thuja occidentalis) were more susceptible than predicted based on their chemical profile. These discrepancies could be linked to specific mammalian herbivores, that were unaffected by the phytochemical defence of these species. We observed a generally higher susceptibility of broadleaf species, which could be taken into account when devising adaptive silvicultural strategies. Furthermore, we propose the chemical profiling approach as a preliminary screening tool to identify species more resistant to mammalian herbivores, but also potentially to other herbivores and pathogens. Our chemical profile approach, based on the objective assessment of multivariate analyses results, could be replicated to compare the potential susceptibility of other species. This approach could be especially useful when contending with novel plant-herbivore relationships, such as forestry and conservation assisted migration or species invasion.