Press-pulse interactions and long-term community dynamics in a Chihuahuan Desert grassland

Questions Reordering of dominant species is an important mechanism of community response to global environmental change. We asked how wildfire (a pulse event) interacts with directional changes in climate (environmental presses) to affect plant community dynamics in a Chihuahuan Desert grassland. Location Sevilleta National Wildlife Refuge, Socorro County, New Mexico, USA. Methods Vegetation cover by species was measured twice each year from 1989 to 2019 along two permanently located 400-m long line intercept transects, one in Chihuahuan Desert grassland, and the second in the ecotone between Chihuahuan Desert and Great Plains grasslands. Trends in community structure were plotted over time, and climate sensitivity functions were used to predict how changes in the Pacific Decadal Oscillation (PDO) affected vegetation dynamics. Results Community composition was undergoing gradual change in the absence of disturbance in the ecotone and desert grassland. These changes were related to the reordering of abundances between two foundation grasses, Bouteloua eriopoda and B. gracilis, that together account for >80% of above-ground primary production. However, reordering varied over time in response to wildfire (a pulse) and changes in the PDO (a press). Community dynamics were initially related to the warm and cool phases of the PDO, but in the ecotone these relationships changed following wildfire, which reset the system. Conclusion Species reordering is an important component of community dynamics in response to ecological presses. However, reordering is a complex, non-linear process in response to ecological presses that may change over time and interact with pulse disturbances.