No abstract

A replicated factorial experiment was designed to test the hypothesis that manipulating inputs of water and mineral nitrogen to a semiarid grassland would disrupt existing interactions resulting in alteration of the structure of the primary producer community. Alteration of community structure was measured as either changes in growing season average biomass of 6 functional groups of plants or their relative contribution to total biomass.Additions of water greatly increased total biomass and resulted in the replacement of one of the dominant functional groups by a subordinate group. The water plus nitrogen treatment resulted in large biomass increases in two of the dominant functional groups, elimination of succulents as an important component of community structure, and establishment of several introduced weedy species. Continuation of the experiment will likely result in complete dominance of the water plus nitrogen treatment by these introduced species.Despite the large changes in community structure observed as a result of water- and nitrogen-induced stresses we conclude that the shortgrass prairie in northcentral Colorado is asymptotically stable with respect to these influences.

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No abstract

[1] Gross primary production (GPP) is one of the most important characteristics of an ecosystem. At present, no empirically based method to estimate GPP is available, other than measurements of net CO 2 exchange and calculations of respiration. Data sets from continuous CO 2 flux measurements in a number of ecosystems (Ameriflux, AgriFlux, etc.) for the first time provide an opportunity to obtain empirically based estimates of GPP. In this paper, using the results of CO 2 flux tower measurements during the 1997 season at four sites in Oklahoma (tallgrass prairie, mixed prairie, pasture, and winter wheat crop), we describe a method to evaluate the average daytime rate of ecosystem respiration, R d , by estimation of the respiration term of the nonrectangular hyperbolic model of the ecosystem-scale light-response curve. Comparison of these predicted daytime respiration rates with directly measured corresponding nighttime values, R n , after appropriate length of the night and temperature correction, demonstrated close linear relationship, with 0.82 R 2 0.98 for weekly averaged fluxes. Daily gross primary productivity, P g , can be calculated as P g = P d + R d , where P d is the daytime integral of the net ecosystem CO 2 exchange, obtained directly from measurements. Annual GPP for the sites, obtained as the sum of P g over the whole period with P g > 0 were: tallgrass prairie, 5223 g CO 2 m À2 ; winter wheat, 2853 g CO 2 m À2 ; mixed prairie, 3037 g CO 2 m À2 ; and pasture, 2333 g CO 2 m À2. These values are in agreement with published GPP estimates for nonforest terrestrial ecosystems.