This project addresses the idea that 1) structural and functional differences between shrubs and grasses are responsible for different short-term and long-term physiological and growth responses and 2) that these differences govern vegetation and hydrologic changes when grassland, shrubland and the transition (ecotone) ecosystems are subjected to climate extremes such as extended drought or prolonged periods of above-average precipitation. Understanding these differences is key to explaining the controls on the grassland-shrubland biome transition, grassland responses to long term climatic fluctuations as well as the potential responses to earth’s changing climate. If extended drought differentially reduces grass productivity and increases grass mortality while periods of above-average precipitation increase grass productivity and cover, these differential responses may control the vegetation dynamics at the ecotone between desert shrubland and semiarid grassland. We are addressing two broad questions: 1) Do the structural and functional differences between grasses and shrubs lead immediately to differential physiological and growth responses to climate forcing in the form of extended periods of drought or above-average precipitation? 2) Are the immediate responses of grass and shrub to climate forcing maintained as treatment continues or do grass and shrub physiological and growth responses continue to diverge over time as the direct and indirect effects of climate forcing accumulate?
To impose periods of extended drought or above-average precipitation, rainout shelters and water addition plots are being used to impose the extremes of the historical climate record, drought with 50% of average precipitation like the 1950’s drought and wet periods with 150% of average precipitation as occurred during the 1990’s. These manipulations are repeated in grassland, shrubland, and the intervening ecotone sites within 2 km of each other near the 5 Points meteorological station. Drought plots utilize removable roof panels to deflect a portion of the precipitation from plots, ultimately under the control of a datalogger in response to individual storms. Water addition plots utilize a sprinkler system to add precipitation quality water with drop size and intensity as close to natural precipitation as possible. Adjacent control plots, which receive no treatment, are available for comparison with treated plots. All plots are roughly 10 m x 15 m to allow sufficiently large plots to observe the effects of changing vegetation structure on water redistribution, infiltration and runoff.