2007 Annual Report
Implications of extreme precipitation events for grassland carbon balance: Climate change driven by increasing atmospheric CO2 concentrations is causing measurable changes in precipitation patterns. Most climate change scenarios forecast continuing increases in extreme precipitation patterns for North American terrestrial ecosystems, manifest as larger precipitation events separated by longer dry periods. Changes in the size of precipitation events may differentially affect the processes controlling uptake and release of carbon (C) from terrestrial ecosystems, and therefore could alter carbon sequestration on grasslands and other ecosystems. Scientists at the Grassland, Soil & Water Research Laboratory in Temple, Texas, together with university collaborators found that more extreme precipitation patterns (longer intervals between events combined with larger events) shifted experimental grasslands toward greater net uptake of C and made C fluxes less responsive to variation in event size. More extreme precipitation regimes thus may reinforce increases in grassland C-sequestration expected to result from increasing atmospheric CO2, but may also lower plant water status and productivity. Benefits of greater carbon storage on grasslands likely will be offset by reductions in forage quantity and quality. (NP 204; Component IV, Changes in Weather and the Water Cycle at Farm, Ranch and Regional Scales; Objective 5, Climate and Weather Variability and Extremes)
Photosynthetic traits of C3 and C4 grassland species: Scientists at the Grassland, Soil & Water Research Laboratory in Temple, Texas, together with university collaborators collected data to test a basic hypothesis in grassland ecology, that grasses with the C4 photosynthetic metabolism dominate in the tallgrass prairie because physiological resource use efficiency is greater than in co-occurring plant species with C3 photosynthesis. Leaf level physiological processes related to carbon gain and water loss were measured for seven C3 and C4 species, both in quasi-natural microcosms with high resource availability and in intact grasslands where plants have been shown to experience multiple resource limitations. C3 and C4 species failed to show expected differences in resource use efficiency in field compared to microcosm conditions. Our findings suggest instead that C4 species shift their resource use strategies as conditions change through the growing season. Results provide a partial mechanistic explanation for the seasonal variation in forage quality observed on tallgrass prairie. (NP 204; Component III, Agricultural Ecosystem Impacts; Objective 3, Grazinglands (Range and Pastures))
Species abundances affect grassland productivity: Humans are changing the relative abundances of plant species on much of the landscape so that some species are becoming very abundant and others are becoming rare. Whether the increasing disparity in species abundances is affecting the capacity of grasslands and other ecosystems to meet human needs is not known. To determine effects of changing species abundances on grasslands, scientists at the Grassland, Soil & Water Research Laboratory in Temple, Texas, experimentally varied the proportional contributions of species to the total number of plants in small plots. Aboveground biomass produced by species mixtures varied with changes in species ratios in only one of the six species combinations studied. For three of six species mixtures, however, species abundances determined whether the biomass of mixtures exceeded the biomass expected based on the yield of each species when grown alone. Our results indicate that the productivity of grasslands may be sensitive in the short-term to changes in species ratios caused by grazing, fire, or herbicide application. (NP 204; Component III, Agricultural Ecosystem Impacts; Objective 3, Grazinglands (Range and Pastures))
Nippert, J.B., Fay, P.A., Knapp, A.K. 2007. Photosynthetic traits in C3 and C4 grassland species in mesocosm and field environments. Environmental and Experimental Botany. 60:412-420.
Polley, H.W., Dugas, W.A., Mielnick, P.C., Johnson, H.B. 2007. C3-C4 composition and prior carbon dioxide treatment regulate the response of grassland carbon and water fluxes to carbon dioxide. Functional Ecology. 21:11-18.
Polley, H.W., Wilsey, B.J., Tischler, C.R. 2007. Species abundances influence the net biodiversity effect in mixtures of two plant species. Basic and Applied Ecology 8:209-218.