Resolving how species differ in their tolerances to climatic constraints can help anticipate how global changes will cause range shifts. Communities with many species at or near their niche limits may be vulnerable to species losses, suggesting that niche-based measures can inform vulnerability assessments. In a two-stage process, we first estimated realized niches as empirical cumulative distribution functions for 443 epiphytic macrolichen species across fifteen climate gradients based on rasterized herbarium records (46,343 sites in Mexico, Canada and the U.S.). We then estimated thermal vulnerability based on systematic, whole-community lichen surveys from the U.S. Forest Inventory and Analysis (FIA) program (6,474 U.S. sites). Herbarium data addressed possible niche truncation, while systematic FIA data resolved sampling bias concerns. We developed three niche-based thermal vulnerability indices (TVIs) respectively showing i) percentage of vulnerable species at their upper thermal limits, ii) community-mean thermal percentile rank, and iii) thermal bias measuring deviation of local temperatures from thermal tolerances.
Sensitivity of indices to uncertainty in climate variables was minor (1.9–6.2% change in noise-added TVI values as a percentage of original TVI range). Present-day thermal vulnerability was greatest in north‑central California, in the Southwest along the western Colorado Plateau, and on the Southeastern coastal plain, suggesting that warming-induced species losses will become most evident in these areas. Under proposed warming scenarios (increases of +0.5 to +3.6 °C), the percentage of U.S. lichen communities exceeding thermal safety margins grew from about 2% to 20%. In all scenarios the most vulnerable communities were concentrated in low-elevation and southerly locations, which suggests that lichens otherwise assumed to be "warm-adapted" may nevertheless be commonly on the verge of exceeding their upper thermal limits. Our findings indicate that warming will modify community compositions through the loss of warming‑intolerant species pushed beyond their niche limits. Assessing vulnerability of bioindicators such as lichens will help prioritize locations where the greatest climate-induced changes in species distributions and forest diversity will occur.