Spatial and Temporal Variation in Phylogenetic Diversity in Rocky Mountain Plant Communities
Abstract
Phylogenetic diversity is an axis of biodiversity that captures the variation in evolutionary relationships present in a community, and is associated with important ecosystem functions. Interestingly, recent work has shown that phylogenetic diversity is often decoupled from other biodiversity metrics, such as richness and functional diversity. Phylogenetic diversity correlates more positively with biomass than does species richness or functional diversity, and promotes ecosystem stability, productivity and resistance to invasion. However, phylogenetic diversity is threatened by species loss and phylogenetically clustered extinction. Given the ecosystem benefits of phylogenetic diversity and its independence of other diversity metrics, it is critical to better understand its distribution and how it may respond to climate change. In this thesis, I investigate spatial and temporal variation in phylogenetic diversity in Rocky Mountain plant communities at the Rocky Mountain Biological Laboratory (RMBL; Gothic, CO, USA). Plant communities in this region are well-characterized, but phylogenetic patterns of species change are under-studied. Montane ecosystems are predicted to experience disproportionately intense impacts of climate change, therefore understanding the distribution of and variation in phylogenetic diversity in these communities is essential. First, I tested how evolutionary relationships between co-flowering species change over the growing season. I found that forb communities across an elevational gradient formed statistically significant groups of co-flowering species that flower in the early, middle and late growing season. Their phylogenetic diversity declined from early to late season at all elevations, indicating that late season conditions may constrain diversity. Next, I tested whether rare species, quantified by range size and local abundance, contribute disproportionately to phylogenetic diversity. I found that rare species by both metrics do not disproportionately contribute to phylogenetic diversity in this plant community. Instead, rare species might provide phylogenetic redundancy with common species, and a deeper understanding of functional differentiation is needed to understand contributions of rare species to this system. Third, I used experimentally warmed and cooled plant communities to ask how different aspects of community diversity change with the environment, and whether phylogenetic relationships predict individual species’ responses to change. I found that species’ responses were not phylogenetically conserved, and that changes in phylogenetic diversity were decoupled from changes in species richness and Shannon diversity. My results suggest that species losses in cooled plots were phylogenetically dispersed, increasing plots’ phylogenetic diversity, even as richness and Shannon diversity declined. In warmed plots, increasing richness and Shannon diversity suggested that new species from across the plant phylogeny can colonize after transplantation, stabilizing phylogenetic diversity under warming. Lastly, I investigated how phylogenetic diversity changed in four different elevational community types from 1950 to 2014 and whether individual species’ changes were phylogenetically conserved. I found that individual species’ responses to climate change were only phylogenetically conserved for two of three metrics in one community type each. Phylogenetic diversity changes varied by community type, with decreases in the sagebrush, increases in the spruce-fir and upland-herb, and no changes in the alpine, indicating unique evolutionary trajectories for each community type. Altogether, my dissertation studies highlight that aspects of the environment, whether seasonal variation, transplanting over space, or change over time, impact phylogenetic diversity, but sometimes in ways contrary to theoretical predictions. Understanding the distribution of phylogenetic diversity independently of other metrics is essential for tracking and forecasting changes and threats to diversity.
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