Ecological Significance of Within- Species Leaf Trait Variability: A Test Using an Island Area Gradient

There is growing recognition of the need to incorporate within-species trait variability into trait-based studies to improve understanding of community assembly and how plant communities drive ecosystem processes. Given that many plant species can occupy a wide range of environmental conditions, studies that have traditionally focused solely on between-species trait variability and neglected within-species trait variability could lead to an incomplete picture of how plant traits influence communityand ecosystemlevel properties. In this thesis, within-species trait variability of all component species across a well-studied system of 30 forested lake islands in the boreal zone of northern Sweden were characterized to understand how differences among individual species traits contribute to community level properties and community assembly. Collectively, the islands represent a long-term chronosequence across which there are large changes in plant community composition, diversity and aboveand belowground resource availability and heterogeneity. Significant within-species trait variability was found among all dominant species that were widespread across the chronosequence. In addition, within-species trait variability was highly responsive to differences in environmental conditions among ecosystems, in a manner mostly consistent with patterns observed at the acrossspecies level. Across contrasting environments, trait variability within species sometimes explained a greater amount of variation in overall community-level responses than did amongspecies variation. There was also significant within-species variation in biomass allocation patterns of co-occurring dominant dwarf shrub species across the chronosequence. This, together with directional shifts in withinand between-species functional trait diversity of both dominant and subordinate species across the gradient, provides insights on how changes in resource availability drive community trait composition, species coexistence and consequently community responses. These findings overall highlight the importance of within-species variability for understanding the responses of whole plant communities to environmental changes, and potentially to ongoing global changes. Further, given the importance of plant traits in governing ecosystem processes such as net primary productivity, carbon sequestration, biogeochemical cycling and decomposition, knowledge of the extent and magnitude of within-species trait variability is imperative for better understanding these processes and their drivers, especially in ecosystems with low species diversity and turnover such as boreal forests.