Functional diversity of tundra vascular plants over space and time

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Aim: Rapid warming across the tundra biome is driving widespread changes in vascular plant community composition. While species turnover is well-documented, the ramifications for tundra functional diversity are unknown. Here, we quantify biome-scale spatial gradients and temporal trends in the functional diversity of tundra vegetation for the first time. Location: A biome-scale synthesis of in situ vegetation surveys and resurveys from 2087 plots across 45 sites throughout the high-latitude tundra. Time Period: 1984-2022 Major Taxa Studied: 352 vascular plant species encompassing shrub, graminoid and forb functional groups Methods: We used tundra species trait data alongside long-term, plot-based sampling of species composition to estimate three functional diversity metrics: functional richness, functional evenness and functional dispersion. We used Bayesian mixed-models to test for latitudinal gradients in functional diversity, temporal trends in functional diversity and major abiotic and biotic correlates of functional diversity over space and time. Results: Mirroring biogeographic gradients in species diversity, functional richness declined at high latitude and colder sites. However, functional richness exhibited no net directional change across the three-decade study period. Plots dominated by single growth forms had reduced functional diversity when compared with plots where individual growth forms had intermediate abundance. Changes in temperature and precipitation were not linked to temporal changes in functional diversity. Where shrubs were increasing in abundance, functional richness and dispersion declined, whereas increases in forbs were accompanied by increases in both aspects of functional diversity. Main Conclusions: The functional diversity of tundra plants is currently lowest in colder and high latitude sites. Despite rapid warming of the tundra biome, we have yet to see broad-scale changes in functional diversity over time. However, where shrubification occurs, we anticipate accompanying reductions in functional diversity. Our results highlight the potential consequences of changes in tundra species composition for ecosystem functioning over the coming decades.

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