Sevilleta Rainfall Manipulation Study

The overall goal of the rainfall manipulation project is to understand the coupled ecological and hydrological responses of grassland, shrubland and the ecotone between them to extended periods of increased or decreased rainfall. Rainfall manipulation plots have been established in each of these three communities at the Five Points area of Sevilleta NWR. In each vegetation type three control plots, three drought treatment plots, and three water addition plots have been installed, each approximately 10 x 15 m in size. Control plots have been trenched on three sides to approximately 1 m depth and lined with a double layer of plastic before being refilled. Galvanized steel edging has been installed to prevent water flow on and off of the plot. A gutter leading to a collection tank has been installed across the fourth edge, always on the downhill side of the plot, to permit automated measurements of plot-scale runoff. In each plot, vertical profiles of soil moisture probes have been installed under each cover type (canopy and interspace in grassland and shrubland; grass canopy, shrub canopy and interspace at the ecotone site). The probes measure differences in infiltration and soil water content and potential associations with these different cover types. In addition, TDR probes have been installed diagonally in each cover type to integrate the water content of the top 15 cm of soil. Each plot contains a variable number of quads (see below). Each spring and fall, the following parameters are measured in every quad: live plant cover, height, and abundance by species; dead plant cover; soil cover; litter cover; and rock cover. Data collection began in the drought and control plots in the spring of 2002. Data collection began in the water addition plots in the spring of 2004. In the grassland and shrubland communities, all nine currently established plots are located together. The three drought plots are located under a single large roof with a 0.5 m path separating each plot. The control plots and water addition plots are similarly grouped without the shelter structure. In the ecotone community, the plots are in three groups; each group is comprised of one drought plot covered by a roof, one water addition plot, and one control plot. Control plots receive no experimental treatment, while the sliding roofs over the drought plots are used to divert precipitation, producing a long-term drought. The roofs covering the drought plots are lowered when there is no precipitation so that the amount of sunlight received by the drought plots is minimally affected. Water addition is intended to impose a complementary increase in water supply on the water addition plots. At all plots in all vegetation communities, permanent pieces of rebar have been installed to mark reference corners outside of the plots. The distance from the downhill corner (at the gutter end of each plot) to the uphill corner is used as a reference distance to locate and relocate vegetation transects. Three additional pairs of rebar, traversing the lengths of each plot, mark the endpoints of the transects to be measured