Causes and consequences of functional trait diversity: Plant community assembly and leaf decomposition
This thesis focuses on the connections between the environment, plant traits, and an ecosystem function---the decomposition of leaf litter. The connections between the environment and plant traits can be described using community assembly theory, with two collections of community assembly processes structuring the distribution of functional traits at a given site. First, habitat filtering restricts the range of viable trait values. Second, a limit to the similarity of coexisting species promotes even spacing of species in trait space. In a coastal California system I present empirical evidence---in part using a novel statistical technique---for both processes.
In the same system, I examined whether functional characteristics of plant species correlate with the species' abundance and rarity at two different scales. We measured the abundance of woody plant species at landscape and local scales. We then compared the species abundance to it leaf, wood, and seed traits. At the landscape scale we found no significant relationship between functional traits and abundance. In contrast, at the local scale we found significant relationships between abundance and two traits: specific leaf area and seed mass. At the local scale there are relationships between functional trait values and which species are abundant and which are rare.
I also tested whether the diversity of functional traits and the trends across a precipitation gradient affect the rate at which litter decomposes. In paired studies in Hawai'i and Australia I found that litter from dry sites was more decomposable and released N and P at a faster rate. The size of the plant-trait effect was of the same order of magnitude but smaller than to the site effect: in the litter transplant experiment species explained 34% of the variance in decomposition rates, while the site of decomposition explained 59%. This work suggests that precipitation and soil fertility have important indirect effects on C, N, and P cycling via plant traits.