Resilience and resistance of ecosystem functional response to a precipitation pulse in a semi-arid grassland

1 In water-limited ecosystems, discrete precipitation events trigger brief but important episodes of biological activity. Differential responses of above- and below-ground biota to precipitation may constrain biogeochemical transformations at the ecosystem scale. 2 We quantified short-term dynamics of whole ecosystem response to 39 mm irrigation events (precipitation pulses) during June 2002 and 2003 using plant physiological and ecosystem gas-exchange measurements as state variables in a principal components analysis (PCA). Experimental plots consisted of either native (Heteropogon contortus L.) or non-native (Eragrostis lehmanniana Nees) bunchgrasses planted in monoculture on two distinct geomorphic surfaces in a semi-arid grassland. 3 For 15 days, treatments followed similar, non-linear trajectories through state variable space with measurement periods forming distinct clusters; PCA axes 1 and 2 combined to explain 80.7% of the variation during both 2002 and 2003. 4 During both years, bunchgrass species interacted with soil type such that there was a reduction in ecosystem functional resistance in plots planted with the non-native bunchgrass species on the fine-textured clay geomorphic surface. 5 System-level hysteresis, emerging as a result of independent responses of photosynthesis, respiration and evapotranspiration to precipitation, indicated the potential for alternative functional states. 6 Quantifying the frequency and duration of ecosystem alternative functional states in response to individual precipitation events within a season will provide insights into the controls of species, soils and climate on ecosystem carbon and water cycles.