PARENT SESSION
Oral Session #41: Agroecology and Urban Ecology. Presiding: M. Liebman.
Wednesday, August 8, 2001. 8:00 AM to 12:15 PM. Hall of Ideas L&M.
Productivity and stability relationships in clipped pasture communities of varying diversity.
TRACY, BENJAMIN1, SANDERSON, MATT2, 1 2
ABSTRACT- Recent studies have shown that plant diversity strongly affects ecosystem function in grasslands. Few studies, however, have evaluated these relationships under grazing or simulated grazing (i.e., clipping). In May 1998, we initiated a field experiment to address questions about the relationship between plant diversity and ecosystem function in pasture communities that we frequently clipped to simulate rotational grazing. The experiment consisted of 15 different cool season (C3), pasture forages planted into eight mixtures of 1, 2, 3, 4, 6, 8, 10 and 15 species, respectively. The eight mixtures were seeded into small plots (1.5 m x 1.5 m) arranged in a randomized complete block design and clipped at four week intervals. In 1999 and 2000 growing seasons, we measured changes in aboveground production, tissue C and N concentration and species composition. Root biomass, microbial respiration and nitrate concentrations were measured from soil cores collected at the end of the growing seasons. The simplest communities (1 and 2 species mixtures) were 50-80% less productive than more complex mixtures. The low productivity was a result of the simple mixtures containing only grasses and no legume functional group. Functional group diversity of the mixtures similarly explained ~ 45% of the variation in aboveground production during the growing season while species diversity explained less than 1%. Root biomass, rates of microbial respiration and soil nitrate concentrations were largely unrelated to plant diversity. Increased plant functional group diversity improved the stability of aboveground biomass production during drought conditions of 1999. Simple mixtures, however, were also comparatively stable mainly due to presence of stress tolerant species. We concluded that pasture communities should contain at least three different plant functional groups (perennial grasses, legumes and broadleaf forbs)to help maximize sustainability.
KEY WORDS: pastures, diversity, production, stability