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Wildfires influence many temperate terrestrial ecosystems worldwide. Historical environmental heterogeneity created by wildfires has been altered by human activities and will be impacted by future climate change. Our ability to predict the impact of wildfire-created heterogeneity on biodiversity is limited because few studies have investigated variation in community composition (beta-diversity) in response to fire. Wildfires may influence beta-diversity through several ecological mechanisms. First, high-severity fires may decrease beta-diversity by homogenizing species composition when they create landscapes dominated by disturbance-tolerant or rapidly colonizing species. In contrast, mixed-severity fires may increase beta-diversity by creating mosaic landscapes containing habitats that support species with differing environmental tolerances and dispersal traits. Moreover, the effects of fire severity on beta-diversity may change depending on site conditions. Disturbance is hypothesized to increase local species richness at higher productivity and decrease local species richness at lower productivity, a process that can have important, but largely unexamined, consequences on beta-diversity in fire-prone ecosystems. We tested these hypotheses by comparing patterns of beta-diversity and species richness across 162 plant communities in three sites that span a large-scale gradient in climate and productivity in the Northern Rockies of Montana. Within each site, we used spatially explicit fire-severity data to stratify sampling across unburned forests and forests burned with mixed- and high-severity wildfires. We found that beta-diversity (community dispersion) of forbs was higher in mixed-severity compared to high-severity fire, regardless of productivity. Counter to our predictions, local species richness of forbs was higher in burned landscapes compared to unburned landscapes at the low-productivity site, but lower in burned landscapes at the high-productivity site. This pattern may be explained by rapid regeneration of woody plants after fire in high-productivity forests. Moreover, forbs and woody plants had disproportionately higher overall species richness in mixed-severity fire compared to high-severity fire, but only at the low-productivity site. These patterns suggest that mixed-severity fires promote higher landscape-level biodiversity in low-productivity sites by increasing species turnover across landscapes with a diverse mosaic of habitats. Our study illustrates the importance of understanding the mechanisms by which patterns of wildfire severity interact with environmental gradients to influence patterns of biodiversity across spatial scales.

Understanding the complex and reciprocal ways in which ecosystem functions (such as productivity) and biodiversity interact may provide important insights into sustaining ecosystem services and protecting species diversity. Despite years of research, observed relationships between productivity and biodiversity have been varied and hypotheses explaining patterns are diverse and equivocal. The ways in which patterns of productivity-biodiversity relationships can inform conservation may depend on understanding how gradients in human land use map onto gradients of productivity and biodiversity. Here, we used diverse publicly available spatial datasets on ecosystem productivity, tree diversity, climate, and land cover to explore two broad questions in ecoregions of Montana: (1) what is the relationship between productivity and tree diversity and (2) are land use gradients (from timber harvesting to wilderness protection) related to gradients of productivity and diversity? We also investigated the possible climatic and topographic drivers of these ecological and anthropogenic patterns. We found the most productive ecoregions support greater numbers of tree species, which appears to be influenced by precipitation but not temperature gradients. The most productive and diverse ecoregions included greater amounts of timber harvesting disturbance and associated road densities. The amount of protected area within ecoregions was highest at intermediate levels of productivity and diversity and was strongly related to mean elevation of ecoregions. Our results suggest that climate-influenced productivity gradients may influence patterns of biodiversity and are related to patterns in human land use. Further, given the existing conditions and current distribution of protected areas, we recommend that the most altered productive and diverse ecoregions may offer opportunities both for additional protection and for holistic restoration programs.

Ecosystems can shift between alternative states characterized by persistent differences in structure, function, and capacity to provide ecosystem services valued by society. We examined empirical evidence for alternative states in a semiarid grassland ecosystem where topographic complexity and contrasting management regimes have led to spatial variations in levels of livestock grazing. Using an inventory data set, we found that plots (n = 72) cluster into three groups corresponding to generalized alternative states identified in an a priori conceptual model. One cluster (biocrust) is notable for high coverage of a biological soil crust functional group in addition to vascular plants. Another (grass-bare) lacks biological crust but retains perennial grasses at levels similar to the biocrust cluster. A third (annualized-bare) is dominated by invasive annual plants. Occurrence of grass-bare and annualized-bare conditions in areas where livestock have been excluded for over 30 years demonstrates the persistence of these states. Significant differences among all three clusters were found for percent bare ground, percent total live cover, and functional group richness. Using data for vegetation structure and soil erodibility, we also found large among-cluster differences in average levels of dust emissions predicted by a wind-erosion model. Predicted emissions were highest for the annualized-bare cluster and lowest for the biocrust cluster, which was characterized by zero or minimal emissions even under conditions of extreme wind. Results illustrate potential trade-offs among ecosystem services including livestock production, soil retention, carbon storage, and biodiversity conservation. Improved understanding of these trade-offs may assist ecosystem managers when evaluating alternative management strategies.