Changes in local-scale intraspecific trait variability of dominant species across contrasting island ecosystems

There is growing recognition of the need to incorporate intraspecific trait variability (ITV) into trait-based studies to improve understanding of community assembly and how plant communities drive ecosystem processes. However, most studies have focused on ITV across plant communities, with few quantifying it at local scales. Further, little is known about how ITV at local scales differs among communities or across environmental gradients. Here, we studied a well characterized 5000-year-old chronosequence involving 30 islands that differed greatly in size, soil fertility, spatial heterogeneity, and species diversity. On each island we measured foliar and litter (including nutrient resorption) traits for ten individuals for each of three species that occur on all islands, i.e., Betula pubescens, Vaccinum myrtillus and Vaccinium vitis-idaea. For each trait for each species we estimated its within- island mean, and coefficient of variation as a measure of local scale ITV. We predicted within- island ITV should be highest for larger islands that have the greatest spatial heterogeneity of soil resources but lowest species diversity. Further, we predicted that the species most dominant in the most resource heterogeneous environment (V. myrtillus) should have the greatest within- island ITV. First, we found that within- island ITV of foliar traits was generally unresponsive to island size, while for litter traits it was responsive to island size only for V. myrtillus in a direction consistent with our prediction. The within- island ITV of resorption efficiency of C,N and P for the three species was often responsive to island size, but not in a consistent direction. Second, against our prediction, V. myrtillus had the lowest within- island ITV for most traits, but its within- island ITV was generally more responsive than that of the other species to island size. While plant traits are well known to vary among and within species at broad spatial scales across environmental gradients, our results show that at local scales, ITV can also be highly responsive to fine scale environmental gradients, particularly for litter and nutrient resorptive traits. Understanding how local scale ITV varies across communities offers opportunities for improving our understanding of what drives community assembly, functional diversity and ecosystem processes.