Ecosystem functioning determined by plant functional group composition

Species extinctions and the resulting loss of biodiversity are one of the largest impacts humans are making on ecosystems. Biodiversity-ecosystem function research has focused on potential changes in ecosystem functioning that this loss of species may impose. Species extinctions do not happen at random, however, and the identity of the lost species may determine changes that the ecosystem undergoes. I investigated the role that plant functional group composition plays in determining ecosystem function and whether this role is influenced by changes in environmental conditions. To investigate this question, I established a removal experiment in a natural grassland in northern Canada in 2003 with four continuous removal treatments: a control (no-removal), and removal of legumes, non-leguminous forbs, and grasses. Removals were crossed with an environmental change (fertilization). I examined a variety of ecosystem properties, including soil moisture, light interception and nutrient availability between 2003 and 2006. Removal treatments had a significant effect on all variables examined, but effects depended highly on year. Removal plots had higher soil moisture, with removal of grasses creating the largest soil moisture effect. Significant effects of removal were found for soil nutrients including total N, NO3-, P, and S. Effects of fertilization only became evident in the 3^rd season of treatment application. In addition, for some variables there was a significant interaction between removal treatments and fertilization indicating the role of a functional group is partially determined by the environment in which it is found. This study highlights the importance of species composition in biodiversity-ecosystem functioning research, but also the importance of multi-year experiments, especially in northern ecosystems.