Functional diversity: quantifying patterns across the tundra biome

Abstract

Widespread vegetation change is underway throughout the northern high latitudes in response to pervasive and accelerating Arctic warming. Such changes are becoming increasingly well documented with key aspects such as phenology, species composition and trait make-up known to be shifting across the tundra biome. However, one area that remains underrepresented in tundra vegetation studies is functional diversity, known to be a principal determinant of ecosystem function and consequently, services. Communities comprising higher functional diversity are considered more stable and resistant to global change impacts and as such, any loss of functional diversity under the influence of warming could have cascading impacts on ecosystem services and resultant feedbacks. Tundra functional diversity is hence an overlooked subject area with the potential to strongly influence ecosystems and communities throughout the far north in the coming decades. This thesis tackles this knowledge gap by: characterising its biome-scale biogeographic patterns and potential drivers (Chapter 3); identifying limitations in currently accepted gap-filling methodologies (Chapter 2); and developing new remotely-sensed approaches to better understand patterns in tundra functional diversity (Chapter 4).

In Chapter 2, I used in situ trait data collected on individuals of multiple species under nearidentical environmental and temporal conditions to determine the influence of explicitly incorporating spatial hierarchies on gap-filling performance in tundra trait matrices. I found that gap-filling across progressively higher spatial and taxonomic hierarchies reduced the accuracy of trait estimates, although such patterns were seen to be both scale and trait-specific. In Chapter 3, I undertook a biome-wide, in situ, cross-site synthesis of over 2,000 plots spanning ~40 years to determine, for what I believe is the first time, biome-scale biogeographic patterns in tundra vascular plant functional diversity across space and time and identify potential drivers of such patterns. I found that spatial patterns in functional diversity conform to those seen in species and functional diversity across latitudes globally and that whilst functional diversity exhibited no net directional change over time, plot-scale changes were strongly related to changes in functional group cover. Finally, in Chapter 4, I used airborne hyperspectral imagery from the Front Range, Colorado, USA to determine whether optical remote sensing can accurately track fine-scale differences in functional diversity throughout alpine tundra ecosystems across both space and time. I found that the method showed promise across space, tracking patterns in functional betadiversity within years across our sample region, but exhibited limited potential over time, highlighting continued issues with remotely sensed time series in assessments of biodiversity. Overall, I believe this thesis has helped tackle large unknowns surrounding tundra functional diversity and has highlighted key research areas to target in the near future as rapid Arctic warming continues.