Global change effects on grass-shrub interaction in an arid ecosystem

Supported by NSF DEB 0516458 (J. Fargione, W. Pockman and S. Collins).

Humans are causing significant global environmental changes, including climate change, nitrogen (N) deposition, and invasion of woody species into grasslands. Although each of these factors has major impacts on plant community composition, it is not known how these factors will interact to affect ecosystem dynamics in the future. Furthermore, although arid and semi-arid systems cover approximately a third of the earth's surface, few experimental warming studies have been conducted in these systems. This study is a multi-factorial global change field experiment in an arid grassland community at the ecotone between shortgrass prairie and desert grassland, which is undergoing shrub encroachment. Specifically, our experiment simulates predicted future environmental conditions with increased nighttime temperatures, N deposition, and El NiƱo frequency (which increases winter precipitation by 50% at our field site). These are all ongoing changes both globally and locally that may influence community composition. We hypothesize that warmer summer temperatures and increased evaporation will favor growth of the desert grass black grama ( Bouteloua eriopoda ), but that increased winter precipitation and/or available nitrogen fertilization will favor the growth of shortgrass prairie grass blue grama ( Bouteloua gracilis ). Furthermore, we hypothesize that growth and survival of introduced seeds and seedlings of the shrub creosote ( Larrea tridentata ) will be favored by increased winter precipitation, N addition, and warmer temperatures. Thus, the direct effects of these factors favor different species. Because it is unknown how these factors will interact, the net outcome of these perturbations is not easily predicted. We are measuring treatment effects on limiting resources (soil moisture, nitrogen mineralization), species growth (photosynthetic rates, creosote shoot elongation), and species abundance (aboveground biomass harvests of grasses and allometric biomass estimates of shrubs) to determine the interactive effects of three key global change drivers on plant community dynamics in an aridland ecosystem.