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Changing patterns of warming and drying would decrease or reverse Evernia growth at its southern range margins, with potential consequences for the maintenance of local and regional populations. Negative carbon balances among persisting individuals could physiologically commit these epiphytes to local extinction. Our findings illuminate the processes underlying local extinctions of epiphytes and suggest broader consequences for range shrinkage if dispersal and recruitment rates cannot keep pace.

Aim Communities with many species at their climatic niche limits may be vulnerable to species losses, suggesting that niche‐based measures can inform vulnerability assessments. We aimed to quantify spatial variation in community‐level vulnerability indices based on niche limits, not just central tendencies, to predict where climate changes will produce rapid, pronounced shifts in ecological communities. Location North America. Taxon Epiphytic lichenized fungi. Methods We integrated broad‐extent herbarium data (46,343 North American sites, for estimation of realized niches) with systematic, whole‐community macrolichen surveys from the U.S. Forest Inventory and Analysis inventory (6,474 U.S. sites, for standardized vulnerability estimation). We introduce three niche‐based vulnerability indices describing (a) percentage of vulnerable species occurring at their upper climatic limits, (b) community‐mean percentile, and (c) deviation of local conditions from community‐mean upper limits. We also performed sensitivity analyses and evaluated vulnerability in future climate change scenarios. Results Sensitivity of indices to uncertainty in climate variables was minor. Present‐day vulnerability was greatest in north‐central California, in the Southwest along the western Colorado Plateau, and on the Southeastern coastal plain—locations where climatically‐induced compositional changes will become most evident. Under future warming scenarios (+0.5 to + 3.6°C increases), the percentage of U.S. epiphytic macrolichen communities exceeding critical values grew from about 2%–20%. Main conclusions In all scenarios the most vulnerable communities were concentrated in low‐elevation and southerly locations, suggesting that many ostensibly “warm‐adapted” communities may be close to exceeding their climatic limits. Findings indicate that warming and moisture changes will modify community compositions through the local loss of 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 diversity will occur.

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Summary Dissimilarity measures, which gauge compositional resemblance between sample units, tend to lose information with increasing distance along ecological gradients. This undesirable property is especially common in high‐beta‐diversity community data, yet analysts seldom acknowledge it when relating multivariate attributes (e.g. species composition) to underlying gradients. With 1000 simulated and 14 real community data sets, I systematically varied beta‐diversity to evaluate the effects of seven dissimilarity adjustment methods (plus one baseline method) on outcomes from multivariate gradient analyses. Performance was determined by plotting dissimilarities vs. environmental distances, by ordination vs. environmental distance correlations and by Procrustean concordance of ordinations with known environmental grid structure. Performance of all methods declined as narrower niches and greater competition asymmetry led to greater beta‐diversity. With simulated data, methods based on probabilities of joint occurrences (Beals smoothing, Swan's method) most effectively resolved the loss‐of‐information problem, followed closely by three step‐across methods (Shortest‐path, Extended, Geodesic); the CY and Diffusion methods did not outperform unmodified Bray–Curtis dissimilarities. With real community data sets, the Beals, Shortest‐path and Extended methods excelled. Electing to adjust dissimilarities is recommended when many pairs of sample units share few species in common, although unmodified dissimilarities may still be appropriate when beta‐diversity is low. Dissimilarity adjustments are appropriate not only for species diversity and ecological purposes, but also for other applications where nonlinear relationships among attributes and gradients are expected in zero‐rich and highly variable multivariate data.

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Questions As changing wildfire regimes modify North American deserts, can fires of greater severity and frequency negatively impact the recovery of native bryophyte communities, which are not adapted to such disturbances? Does post‐fire recovery result from the survival of existing surface plants, dormant propagules in sub‐surface soil banks (dispersal in time) or aerial immigration into burned sites (dispersal in space)? Do wildfires negatively affect the survival of propagules in bryophyte soil banks? Location Continental arid shrublands, Mojave Desert, southwestern US. Methods We characterized bryophyte communities along a post‐fire chronosequence spanning three decades across sites where fires had different severities, ages and potential fuels. Three community profiles (surface, soil and aerial) were surveyed with a combination of on‐site surveys and emergence germination techniques. We tested for differences in beta diversity, species composition and richness with PERMDISP, PERMANOVA and linear mixed models, respectively. Results Burn severity was associated with differences in beta diversity, species composition and richness, while burn age was associated only with different composition and richness. No effect of potential fuel availability was found. More variation in composition was explained by significant differences among profiles than by other fire attributes. Species richness (but not beta diversity) was higher in soil profiles than in aerial spore rain or existing surface communities. Soils from the oldest and least severe burns had a greater number of species than soils from recent and more severe burns. Conclusions Bryophyte soil banks are common elements of desert soils that facilitate post‐disturbance recovery of communities, but soil banks are themselves threatened by the intensifying frequency and severity of wildfires in North American deserts. Recovery of desert bryophyte communities seems to begin (but not necessarily conclude) within 30 yr after wildfires. In the near future, communities may become perturbed from historical patterns as contemporary fire regimes undergo extensive changes.