Biomass dynamics and water use efficiencies of five plant communities in the shortgrass steppe

Summary Standing crop biomass and water-use efficiency were estimated for five plant communities of the Central Plains Experimental Range in north central Colorado. Aboveground biomass by functional groups, surface litter amounts, and standing dead biomass were compared, as were vertical and size-class distributions of belowground biomass. Greater production and water-use efficiency values were found: (1) at coarse-textured sites, indicating the importance of the inverse texture effect, and (2) wherever site characteristics favored the establishment of lifeforms other than grasses, e.g., succulents, and shrubs. Seasonal aboveground biomass and water-use efficiencies for the grass component were similar among sites, even though the mixes of C₃ and C₄ grass species were different. Similar grass biomass values in very different communities suggested that high biomass and high water-use efficiencies were related less to grass types than to the abundance of non-grass life-forms.     

Soil carbon flux following pulse precipitation events in the shortgrass steppe

Pulses of water availability characterize semiarid and arid ecosystems. Most precipitation events in these ecosystems are small (≤10 mm), but can stimulate carbon flux. The large proportion of carbon stored belowground and small carbon inputs create the potential for these small precipitation events to have large effects on carbon cycling. Land-use change can modify these effects through alteration of the biota and soil resources. The goal of our research was to determine how small precipitation events (2, 5, and 10 mm) affected the dynamics of soil carbon flux and water loss in previously cultivated Conservation Reserve Program (CRP) fields and undisturbed shortgrass steppe. Total carbon loss and duration of elevated carbon flux increased as event size increased in all field types. Time since cultivation increased in importance for carbon flux as event size increased. A comparison of water loss rates to carbon flux suggests that water is limiting to carbon flux for the smallest events, but is less limiting for events above 5 mm. We also describe how water availability interacts with temperature in controlling carbon flux rate. We conclude that small precipitation events have the potential for large short-term losses of carbon in the shortgrass steppe.     

Patterns of Production and Precipitation-Use Efficiency of Winter Wheat and Native Grasslands in the Central Great Plains of the United States

The Great Plains of the United States is characterized by a large west–east gradient in annual precipitation and a similar large north–south gradient in annual temperature. Native grasslands and winter wheat are found over a large portion of the precipitation and temperature gradients. In this article, we use long-term data to analyze the differences in the patterns in aboveground net primary production and precipitation-use efficiency between wheat and native grassland ecosystems in the central portion of Great Plains, and their relationships to potential water availability (precipitation). Aboveground net primary production of native grasslands shows a large response to precipitation. Aboveground net primary production of winter wheat has a smaller response to changing precipitation. Annual precipitation-use efficiency of native grasslands is unaffected by increases in average annual precipitation, but precipitation-use efficiency of summer-fallow wheat ecosystems decreases substantially with increased average precipitation. Our results suggest that in the wetter portion of the central Great Plains, summer-fallow wheat management is relatively inefficient, because increased water availability results in diminishing returns. Comparisons with data from continuously cropped wheat confirmed this result. Shifts across the region to continuous cropping of wheat potentially could have significant impacts on regional wheat yield, carbon balance, and economic status. Received 15 October 1999; accepted 10 March 2000.     

Zoospore interspecific signaling promotes plant infection by <Emphasis Type="Italic">Phytophthora</Emphasis>

Background  

Oomycetes attack a huge variety of economically and ecologically important plants. These pathogens release, detect and respond to signal molecules to coordinate their communal behaviors including the infection process. When signal molecules are present at or above threshold level, single zoospores can infect plants. However, at the beginning of a growing season population densities of individual species are likely below those required to reach a quorum and produce threshold levels of signal molecules to trigger infection. It is unclear whether these molecules are shared among related species and what their chemistries are.