Tuesday, August 4, 2009: 8:20 AM
Cinnarron, Albuquerque Convention Center
Background/Question/Methods: Global environmental change is increasing nighttime low temperatures and precipitation variability, and altering resource availability in aridland ecosystems. The hierarchical response framework proposes that ecosystems will respond to chronic environmental presses (N deposition, increased temperature) via physiological acclimation, species reordering, and eventually community change. To test aspects of this model, we established a multifactor global change experiment to determine the interactive effects of nighttime warming, increased atmospheric N deposition, and increased winter rainfall on plant community dynamics in a northern Chihuahuan Desert grassland. To do so, we used thermal blankets to increase nighttime temperature, added 2 g N m-2 yr-1 in the form of NH4NO3, and increased winter precipitation by 50% in a replicated (N=5), fully crossed experiment in Chihuahuan Desert grassland in central New Mexico, USA. Treatment plots were instrumented with temperature probes at 20 cm aboveground, and 2 and 8 cm belowground, and moisture probes at 5 and 15cm belowground. We tested the following hypotheses: (1) increased nighttime temperature will increase aboveground biomass of Bouteloua eriopoda, a dominant grass in the northern Chihuahuan Desert, (2) increased N availability will increase aboveground biomass of B. gracilis, the dominant grass in shortgrass steppe, and (3) warming plus increased winter precipitation will increase the biomass of C3 shrubs, but not C4 grasses.
Results/Conclusions: Our passive nighttime warming treatment increased monthly average nighttime temperatures by 1.4 to 3.0 oC. Our warming treatment increased nighttime soil temperature at 2 cm depth and increased soil temperatures during the morning, as well. Both N addition and warming increased soil N availability. Cover of B. eriopoda increased significantly in response to nighttime warming. Cover of B. gracilis increased with N fertilization alone, but this response to increased N availability was supressed by increased nighttime temperatures. Cover of Gutierrezia sarothrae, a C3 shrub, increased in response to warming and winter rains. Our results demonstrate that this semiarid desert grassland will respond rapidly to nighttime warming, N fertilization and altered precipitation regimes, and that global environmental drivers will interact to generate complex species and community responses in aridland ecosystems. Overall, our results support the hierarchical response model in that a reordering of plant community structure occurs relatively rapidly in response to chronic but subtle global change drivers, such as increase N deposition, altered precipitation regimes, and elevated nighttime temperatures.
Results/Conclusions: Our passive nighttime warming treatment increased monthly average nighttime temperatures by 1.4 to 3.0 oC. Our warming treatment increased nighttime soil temperature at 2 cm depth and increased soil temperatures during the morning, as well. Both N addition and warming increased soil N availability. Cover of B. eriopoda increased significantly in response to nighttime warming. Cover of B. gracilis increased with N fertilization alone, but this response to increased N availability was supressed by increased nighttime temperatures. Cover of Gutierrezia sarothrae, a C3 shrub, increased in response to warming and winter rains. Our results demonstrate that this semiarid desert grassland will respond rapidly to nighttime warming, N fertilization and altered precipitation regimes, and that global environmental drivers will interact to generate complex species and community responses in aridland ecosystems. Overall, our results support the hierarchical response model in that a reordering of plant community structure occurs relatively rapidly in response to chronic but subtle global change drivers, such as increase N deposition, altered precipitation regimes, and elevated nighttime temperatures.
