Tundra species diversity and plant traits in a changing Arctic

Abstract

Global air temperature is unequivocally increasing and will keep rising, more rapidly in the Arctic than in other regions. Climate warming may affect not only soil factors, e.g. temperature, moisture and nutrient availability for plants, but also vegetation. Changes in species diversity, distribution, and plant traits are expected as a consequence of direct and indirect effects of climate warming, especially in high-latitude ecosystems. Two of the main changes expected in arctic tundra are shrub expansion and loss of cryptogam diversity. Tundra vegetation shifts will result in altered feedbacks with atmosphere and permafrost through the surface energy budget and the water and carbon cycle, which might affect regional climate. Despite the high vulnerability of arctic species to climate change and the importance of tundra vegetation‒climate feedbacks, uncertainties remain in relation to species diversity and plant trait response to climate. In order to identify changes in species diversity, community composition, and plant traits that might take place under climate change in arctic tundra ecosystems, I combined observational, experimental, and dendroecological approaches. I established a set of observational plots in two contrasting habitats in northeastern Siberia, finding that species diversity and community composition were closely related to edaphic factors. These relationships were different among plant functional types, suggesting a higher vulnerability of cryptogam diversity to changes in edaphic factors and, therefore, to climate warming. Moreover, by sampling shrub individuals from experimental soil warming and fertilization plots, I found faster shrub growth with enhanced nutrient availability, a decrease in bark investment with faster growth rates, and a coordinated response of shrub traits to nutrient addition. These findings suggest a shift in growth strategy and resource acquisition towards more rapid ones with climate warming. Although shrub expansion is expected in the short term due to faster growth and denser covers, in the long term, shrubs might become more vulnerable to herbivory, pathogens, and climate extremes because of shifts in shrub resource allocation towards growth (growth‒defence trade-off). This thesis contributes, therefore, to increase our knowledge of species diversity vulnerability and plant trait shifts in a changing Arctic, which is a first step to better understand vegetation effects on the surface radiation budget in tundra ecosystems. Such an understanding is essential for reducing the uncertainties in direction and magnitude of future vegetation‒climate feedbacks.