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Long-term data reveal complex dynamics in grassland in relation to climate and disturbance

Hobbs, R.J., Yates, S. and Mooney, H.A. (2007) Long-term data reveal complex dynamics in grassland in relation to climate and disturbance. Ecological Monographs, 77 (4). pp. 545-568.

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We conducted a long-term experiment in grassland with the aim of elucidating grassland dynamics in relation to variations in rainfall amount and spatial and temporal variation in disturbance. In particular we aimed to increase our understanding of ecosystem dynamics and function, species redundancy, invasion biology, and other related topics. We studied the dynamics of serpentine annual grassland in northern California over the period 1983-2002 in a replicated series of experimental plots comprising controls, gopher exclosures, and aboveground herbivore exclosures. Annual rainfall amount varied greatly during the study period, which included two major El Niño events and a period of prolonged below-average rainfall. Gopher disturbance was highly variable both spatially and temporally but was positively correlated with soil depth. Disturbance was reduced but not eliminated from the gopher exclosures and was significantly increased in the aboveground herbivore exclosures. Grassland dynamics were driven by rainfall amounts and distributions that had the most pronounced effects on the dominant plant species, while gopher disturbance had additional effects on the rarer species. Effects of excluding aboveground herbivores were swamped by a large increase in gopher disturbance within aboveground exclosures. Overall species numbers were reduced during a period of below-average rainfall but recovered in subsequent years. There was a large array of different responses of individual plant species to both rainfall and disturbance. Our results provide support for the "insurance" hypothesis, which suggests that biodiversity buffers ecosystem processes against environmental changes because different species (or phenotypes) respond differently to these changes, leading to functional compensations among species. Here, a species that was at very low abundance levels at the start of the study (Microseris douglasii) temporarily increased in abundance to become one of the dominant species in the grassland following a period of prolonged below-average rainfall. We also observed the repeated invasion of the serpentine grassland by the nonnative grass Bromus hordeaceus, which increased greatly in abundance following both of the major El Niño events. The results emphasize the importance of long-term observations in providing a context for shorter-term studies and allowing analysis of plant community responses to climate variation and disturbance, particularly in the face of ongoing global change.

Item Type: Journal Article
Murdoch Affiliation(s): School of Environmental Science
Publisher: Ecological Society of America
Copyright: © 2007 by the Ecological Society of America
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