Constraints on Production and Decomposition in Temperate Grassland
Bill Lauenroth, Dept of Forest, Range, and Watershed Stewardship, Colorado State University, Ft. Collins, CO 80523
970-491-7581 billl@cnr.colostate.edu
Indy Burke, Dept of Forest, Range, and Watershed Stewardship, Colorado State University, Ft. Collins, CO 80523
970-491-1620 indy@cnr.colostate.edu
Objective: The objective of this research is to utilize two long-term ecological research sites, one in South America and the other in North America (the CPER), to investigate the controls on ANPP and decomposition in grasslands. Hypothesis 1: Ecosystem structure as represented by species composition, plant density, and tiller density constrants the ANPP response of semiarid grasslands to fluctuations in water availability. Hypothesis 2: Temperature affects decomposition much more than precipitation or plant litter quality. Hypothesis 3a: Increases in temperature have a direct effect on N availability by enhancing decomposition and minerlization rates. These increases in N availability cause increased plant N uptake, decreased nitrogen-use-efficiency, and increased plant litter quality, and slight to moderate increases in plant production. The production responses will be small in low-precipitation treatments and years, and large in treatments or years when precipitation is high. Hypothesis 3b: Increases in precipitation augment plant production more than they increase decomposition and N availability over annual scales. Thus, in treatments and years when precipitation is high, plants become N-limited, and %N and litter quality decrease, which constrains the ability to respond to higher water availability (relative to the predictions of spatial model). Plants in systems with higher N availability respond much more to increases in precipitation.
Methods: We will be evaluating the effects of demographic manipulation, increased water availability and increased temperatures on plant demography, net aboveground primary production, decomposition, a suite of soil organic matter variables, and net nitrogen mineralization. We will conduct two separate experiments. The first will be a factorial with two levels of water addition (0 and +) and three levels of reduction of plant and tiller density (0, 25%, and 50%). The second experiment will be a factorial combination of temperature (ambient and elevated) and water addition (0 and +) treatments. The first experiment will use 180-1m2 plots. Sixty of these plots will have no demographic manipulation, 60 will have tiller or plant density reduced by 25%, and 60 will have tiller or plant density reduced by 50%. Ten plots from each treatment will be harvested (aboveground biomass) at the end of each of the three growing seasons over which this experiment will be conducted. The second experiment will use 120-1.2m2 plots (30 for each treatment). Forty of these plots will be harvested at the end of each growing season. In addition, we will remove 6.5cm diameter cores from each of 40 plots each year. These will be used to estimate soil organic matter and net nitrogen mineralization.
Study Area: We would like to use an area of approximately 2.25 ha (150 X 150 m; 5.5 acres) in section 21SE (see companion long-term irrigation and fertilization experimental summary for additional justification of this size). The exact location is directly east of the east fence of the LTER headquarters exclosure. An important reason for choosing this location is that we need to be close to the irrigation well at the LTER headquarters. We plan to use an electric fence to exclude cattle.
03/21/2002