Leaf and root functional traits, phylogeny, and the structure of plant populations and communities

I present a study of relationships among functional traits of plant leaves and roots, their evolutionary histories, and their influence on the ecology of populations and communities. My first objective was to describe how leaf and root functional traits vary and covary among and within plant species and communities. I measured the leaf and root traits, phylogenetic relationships and community structure of species at sites in mixedgrass and fescue grasslands in Alberta. Plants growing in mixedgrass communities, and at drier locations within each site, tended to have a suite of stress-tolerant traits, including smaller, thicker leaves with lower specific leaf area (area per unit mass) and dense roots with lower specific root length (length per unit mass). Phylogenetic comparative methods indicated that suites of leaf and root traits evolved independently in these grasslands. The traits of individual species varied more along environmental gradients than mean trait values did across entire communities, and trait variation among communities was much smaller than the magnitude of trait variation within communities. In a growth chamber experiment conducted with a single plant species (Arabidopsis thaliana), root traits such as root diameter and specific root length were more important for maintaining plant fitness when soil nutrients were limiting, but otherwise above- and belowground vegetative traits were highly integrated regardless of nutrient availability.; My second objective was to determine how patterns of trait evolution interact with ecological processes to determine the trait similarity and phylogenetic structure of ecological communities. In Alberta grasslands, I found consistent leaf and root trait clustering within communities, but variation in the pattern of trait evolution in different lineages and communities led to changes in patterns of phylogenetic relatedness at different scales. In a study of the relatedness of co-occurring trees in a neotropical forest, phylogenetic relatedness within communities differed among habitats, and measures of phylogenetic similarity were affected by null model choice. In general, communities in more stressful environments contained species that were more similar in terms of their traits and phylogenetic relatedness, indicating the potential importance of environmental filtering of traits during community assembly.