Intraspecific trait variation and the intensity of species interactions

Background/Question/Methods

Recently community ecology has placed a strong emphasis on using functional traits to help understand the processes structuring communities.  These studies typically focus on observations of species co-occurrence patterns in relation to key traits and hypothesize that particular nonrandom trait distribution patterns within communities are the product of ecological processes such as habitat filtering, competition or facilitation.  However, these pattern-to-process links are rarely tested experimentally, and uncertainty about these linkages has restricted our ability to draw inferences from observational studies. Here we use an experimental system of fast-growing trees to test if functional trait differences between individuals help predict variation in the intensity of interactions.  Given the current dearth of knowledge about the impact of intraspecific trait variation in community assembly processes in general, we focus on intraspecific interactions among 15 accessions of Black cottonwood (Populus trichocarpa) grown in a common garden in Vancouver, British Columbia.  Prior work has characterized the functional traits and key physiological properties of a number of accessions, and those in our study were chosen to optimize the range of intraspecific trait differences.  Small clones of each accession were planted alone and in competition in sunk pots in an outdoor common garden, and growth was measured over one season.

Results/Conclusions

Preliminary results indicate that accessions varied in terms of their sensitivity to neighbors- some accession were relatively unaffected by neighbors, while others showed a strong reduction in end of season biomass typical of competitive interactions.  Among those accessions suffering competition, there was a weak positive correlation between the intensity of competition suffered by a plant and the degree of aggregate trait similarity (measured as axis scores along a principle components analysis of all traits) between the two accessions in the interaction.  Our results highlight that functional differences between individuals can help explain variation in the intensity of competitive interactions.