|dc.description.abstract||The climate is changing across the globe at unprecedented magnitudes, and temperatures in the Arctic are increasing at three times the rate of the global average. Climate change impacts are being felt across the tundra biome, both at northern latitudes and high elevations. Examples of these impacts include northward species
range shifts, changing community composition and increasing shrub growth, height and expansion, a phenomenon also known as shrubification. While multiple reports of plot- and landscape-level transformations have been described from locations across the tundra biome, uncertainty remains about 1) the extent of climate as a driver of biotic change, 2) the identity and traits of species that are currently shifting their ranges, and 3) how precisely is plant diversity changing over time. By combining decades-long large-scale tundra datasets of community composition, abundance, change over time, functional traits, species distributions and gridded climate data, I delve into these key questions about how climate change is reshaping the tundra biome.
My research has demonstrated that, while rates of woody encroachment do not generally correspond with rates of climatic change, climate is still a driver of shrub expansion in the tundra. I found that warmer and wetter sites have provided fertile ground for shrubs to expand across tundra landscapes. Increased precipitation at dry
sites was also associated with greater woody encroachment in a structurally similar open biome, the savanna. Contrary to initial expectations, trait values and intraspecific variation in three key functional traits (plant height, specific leaf area and seed mass) were not consistently related to current and projected tundra shrub species ranges. Likewise, the identity of climate change ‘winner’ and ‘loser’ shrub species differed depending on whether they were considered through past-observed change or projected range shifts, highlighting discrepancies arising from the use of different methods. Additionally, I found that there is a latitudinal biodiversity gradient in the tundra, with greater species richness at lower latitudes, and that tundra species richness has increased by a small amount over the last four decades. Climate influenced species trajectories, with warmer and drier areas containing more persisting and less extinct species. Species abundance has shifted, with consistent winner and loser functional groups, including an increasing dominance of shrubs influencing biodiversity change over time across plots. However, some sites appear resistant to change, with more resilient plant communities at plots that are more diverse and have a more even composition.
Overall, my findings indicate that the tundra biome is experiencing a transformation that is heavily underpinned by the expansion of shrubs, partly due to a warming climate, although microclimate, non-climate drivers and time lags can also influence woody encroachment rates. Species’ abundance changes and projected range shifts
will likely not lead to directional modifications in shrub functional trait composition or variation with future warming, since ‘winner’ and ‘loser’ species share relatively similar trait compositions. Importantly, I observed heterogeneous individual-level responses when examining traits, distributions and biodiversity patterns. This might be partly due to site-level conditions not being sufficiently captured by large-scale climate and species monitoring data, which are essential to adequately represent and predict biome-level shifts. Taken together, my results suggest biodiversity change driven by shifts in plant composition across the tundra biome, and indicate early signs of directional biotic changes that could ultimately result in tipping points in tundra biodiversity. These can alter the carbon cycle and ecosystem feedbacks, leading to further temperature increases, thawing of the permafrost, and its consequent release of carbon and methane in the Arctic. Ultimately, these could pose serious threats to the biotic communities, human populations and ecosystem services across the tundra biome and beyond.||en