A risk-based model of climate change threat: hazard, exposure, and vulnerability in the ecology of lichen epiphytes

This review positions the biodiversity response to climate change within a social-sciences risk-based framework, integrating the parameters of hazard, exposure, and vulnerability. It uses lichen epiphytes as a case study. In treating human-induced climate change as a hazard, the exposure of lichen epiphytes is considered as their sensitivity to spatial climatic variation, while also seeking congruence between bioclimatic models and observational data supporting distributional change. Improved understanding of exposure could be generated through functional response models, and climate sensitivity should be carefully interpreted against co-occurring hazards (pollution, habitat degradation). Where negative impacts result from exposure to climate change, species vulnerability may be reduced through adaptive forest management. This opportunity is based on a cross-scale interaction between microhabitat specificity and macroclimatic setting. Certain stand types (e.g., old-growth stands) offer greater opportunity for establishment and growth in suboptimal climates, because high microhabitat heterogeneity generates a broader spectrum of microclimatic niches, which buffer an unsuitable macroclimate. Lichen epiphyte vulnerability will nevertheless be dependent on an amalgam of ecological processes considered at the stand scale, including trophic interactions, acclimation, and evolutionary adaptation, and at the landscape scale, including gene flow and dispersal limitation. A trait-focused approach could provide an opportunity to generalize these processes.