Rainfall Manipulation Plot Study

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     Climatic variability is a major factor controlling the structure and function of grassland ecosystems worldwide. The grasslands of North America were formed by climate changes originating during the Miocene-Pliocene transition (Axelrod 1985), and their present day distribution and attributes are determined primarily by regional temperature and precipitation gradients (Lauenroth 1979, Sala et al. 1988), and interactions with fire and grazing (Risser et al. 1981, Knapp et al. 1998a).  For mesic tallgrass prairies in the eastern Central Plains (tallgrass prairie), the mean and variability of precipitation strongly affects most ecosystem processes (Axelrod 1985, Anderson 1990, Hayden 1998). On a continental scale, variability in precipitation affects productivity more in these grasslands than in any other biome in North America (Knapp and Smith 2001).  Forecast changes in rainfall and temperature are expected to rapidly affect individual organisms, community composition (Alward et al. 1999), ecosystem processes [net primary productivity (NPP; Sala et al. 1988), decomposition and mineralization rates (Jenny 1930, Parton et al. 1993)], and the susceptibility of grasslands to invasion by exotic species (Dukes and Mooney 1999, Buckland et al. 2001).  Because grasslands and savannas comprise up to 40% terrestrial land cover, altered temperature and precipitation regimes are important drivers of terrestrial responses to climate change regionally and globally.

      Altered rainfall patterns and warming are integral components of most global climate change scenarios. The Inter-Governmental Panel on Climate Change (IPCC) has suggested that potential increases in temporal variability in rainfall will impact biotic responses and processes in North American grasslands more than changes in annual precipitation quantity alone (Houghton et al. 1996; IPCC 2001). Further, the combined effects of increased temperatures and temporal variability in rainfall patterns on soil moisture and plant growth may be much larger, and in opposition to, the well-documented effects of elevated CO₂ (Newton et al. 1996, Clark et al 1999, Huang et al. 2001). The sensitivity of grassland ecosystems to climatic variability, and the expected changes in temperature and rainfall variability predicted by general circulation models for North American grasslands renders uncertain future productivity and sustainability of these agriculturally and biologically important ecosystems.  Hence, ecosystem responses to changes in climate means (temperature), and variability (rainfall), and interactions between these need to be assessed to develop a more complete and predictive understanding of climate/ecosystem dynamics.

       Rainfall Manipulation Plots facility (RaMPs) is a unique experimental infrastructure that allows us to manipulate precipitation events and temperature, and assess population community, and ecosystem responses in native grassland. This facility allows us to manipulate the amount and timing of individual precipitation events in replicated field plots at the Konza Prairie Long-Term Ecological Research (LTER) site. Questions we are addressing include:

1. What is the relative importance of more extreme precipitation patterns (increased climatic variability) vs. increased temperatures (increased climatic mean) with regard to their impact on grassland ecosystem structure and function? Both projected climate change factors are predicted to decrease soil water availability (see below) but the mechanisms by which this resource depletion occurs differ. 

2. Will altered precipitation patterns, increased temperatures and their interaction increase opportunities for invasion by exotic species?

3. Will long-term (6-10 yr) trajectories of community and ecosystem change in response to more extreme precipitation patterns continue at the same rate as initial responses from years 1-6? Or will non-linear change occur as potential ecological thresholds are crossed? And will increased temperatures accelerate these responses?