Background & Objectives

Rainfall Mesocosm Experiment

Together with large ungulate grazers and periodic fires, climate is a key driver of ecosystem structure and function in grasslands worldwide (Knapp et al. 1998). Climatic variability is a hallmark of the North American Central Plains grasslands. Paleoclimatic records and historical accounts from early settlers document a long history of climatic extremes in the Central Plains (Miner 1986, Gates 1993, Graham 1999). These extremes continue into modern times, with severe droughts in the 1930s and 1950s, and excessive rainfall in the early 1950s and 1990s (Easterling et al. 2000).

Current climate models suggest that climatic extremes in North America may intensify as a consequence of global climate change, driven by the accumulation of CO₂ and other ‘greenhouse’ gases in the atmosphere (Easterling et al. 2000, Alley et al. 2001). An increasingly extreme climate implies changes in the means and increases in the temporal variability in rainfall, temperature, and other climate parameters regulating ecosystem structure and function. Changes in rainfall quantity on an annual or growing season basis are often accompanied by characteristic changes in within-season rainfall patterns (here defined as per rain event quantity and rain event frequency, or its inverse, dry interval length), with drier locations and drier years exhibiting generally smaller and less frequent rainfall events. Field data indicates that a similar change in productivity can result from different combinations of rain event quantity and event frequency (Fay et al. 2000). Thus, understanding the impacts of rainfall quantity and frequency variation on productivity is extremely important for developing a correct understanding of grassland rainfall/productivity relationships.

The objective of the research described here is to experimentally determine the impact of rainfall patterns on above- and belowground grassland net primary productivity (ANPP, BNPP), using tallgrass prairie as a model system. Altered rainfall patterns will accompany warming and elevated CO₂, and together they will impact the future productivity and biodiversity of grassland ecosystems. Rainfall is one of the most strongly limiting resources in grasslands, governing both the productivity and geographic extent of this biome. Climate model predictions regarding future rainfall regimes in the Great Plains are considerably uncertain, with some models predicting wetter (larger, more frequent rainfall events), and others predicting dryer (smaller, less frequent rainfall) weather patterns during the 21^st century (Giorgi et al. 1994, National Assessment Synthesis Team 2000).

The research plan is to examine the above- and belowground responses of synthetic tallgrass prairie plant assemblages grown in microcosms subjected to sixteen combinations of growing season rainfall quantity and rainfall frequency. The treatments will include both decreases and increases in variability from present rainfall patterns in eastern Kansas tallgrass prairie. The experiment will examine several specific hypotheses about the relationships between rainfall quantity and frequency, ANPP, BNPP, and the two dominant plant functional groups, warm season grasses and herbaceous dicots. The two plant groups have functional and morphological differences that should lead to distinct responses to altered rainfall patterns. This research has implications for grassland biodiversity, carbon sequestration, atmosphere/vegetation feedbacks, and interactions among climatic elements in global change scenarios (rainfall, temperature, and elevated CO₂). These issues are crucial to developing proactive conservation and management plans for these endangered systems.

References

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Easterling DR, Meehl GA, Parmesan C, Changnon SA, Karl TR, Mearns LO (2000) Climate extremes: observations, modeling, and impacts. Science 289:2068-2074.

Fay, PA, Carlisle JD, Knapp AK, Blair JM, Collins SL (2000) Altering rainfall timing and quantity in a mesic grassland ecosystem: Designing and performance of rainfall manipulation shelters. Ecosystems 3:308-319.

Gates, DM (1993) Climate change and its biological consequences. Sinauer, Sunderland, MA

Giorgi F, Brodeur CS, Bates GT (1994) Regional climate change scenarios over the United States produced with a nested vegetation climate model. J. Clim. 7:375-399.

Graham A (1999) Late Cretaceous and Cenozoic history of North American vegetation, north of Mexico. Oxford University Press, New York

Knapp AK, Briggs JM, Hartnett DC, Collins SL (1998) Grassland dynamics: long-term ecological research in tallgrass prairie. Oxford University Press, New York

Miner C (1986) West of Wichita: settling the high plains of Kansas, 1865-1890. University Press of Kansas, Lawrence, KS

National Assessment Synthesis Team. Climate change impacts on the United States: the potential consequences of climate variability and change. 2000. US Global Change Research Program, 400 Virginia Avenue, SW, Suite 750, Washington DC, 20024.