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Press-pulse interactions: Effects of warming, N-deposition, altered winter precipitation and fire on desert grassland community structure and dynamics

Collins, S. L., L. M. Ladwig, M. D. Petrie, S. K. Jones, J. M. Mulhouse, J. R. Thibault, ,W. T. Pockman. 2016.

Abstract

Global environmental change is altering temperature, precipitation patterns, resource availability, and disturbance regimes. Theory predicts that ecological presses will interact with pulse events to alter ecosystem structure and function. In 2006, we established a long-term, multifactor global change experiment to determine the interactive effects of nighttime warming, increased atmospheric nitrogen (N) deposition, and increased winter precipitation on plant community structure and aboveground net primary production (ANPP) in a northern Chihuahuan Desert grassland. In 2009 a lightning-caused wildfire burned through the experiment. Here we report on the interactive effects of these global change drivers on pre- and post-fire grassland community structure and ANPP. Our nighttime warming treatment increased winter nighttime air temperatures by an average of 1.1 °C and summer nighttime air temperature by 1.5 °C. Soil N availability was 2.5 times higher in fertilized compared to control plots. Average soil volumetric water content (VWC) in winter was slightly but significantly higher (13.0% vs. 11.0%) in plots receiving added winter rain relative to controls, and VWC was slightly higher in warmed (14.5%) compared to control (13.5%) plots during the growing season even though surface soil temperatures were significantly higher in warmed plots. Despite these significant treatment effects, ANPP and plant community structure were highly resistant to these global change drivers prior to the fire. Burning reduced cover of the dominant grasses by more than 75%. Following the fire forb species richness and biomass increased significantly, particularly in warmed, fertilized plots that received additional winter precipitation. Thus, although unburned grassland showed little initial response to multiple ecological presses, our results demonstrate how a single pulse disturbance can interact with chronic alterations in resource availability to increase ecosystem sensitivity to multiple drivers of global environmental change.

Key Words

Aboveground net primary production Bouteloua eriopoda Bouteloua gracilis Desert grassland Fire Nighttime warming Nitrogen addition Soil moisture Species richness