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Title: Differences in plant species composition as evidence of alternative states in sagebrush steppe

Author
item Kachergis, Emily
item FERNANDEZ-GIMENEZ, MARIA - Colorado State University
item ROCCA, MONIQUE - Colorado State University

Submitted to: Rangeland Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/26/2012
Publication Date: 9/15/2012
Citation: Kachergis, E.J., Fernandez-Gimenez, M., Rocca, M.E. 2012. Differences in plant species composition as evidence of alternative states in sagebrush steppe. Rangeland Ecology and Management. 65(5):486-497.

Interpretive Summary: State-and-transition models (STMs), conceptual models of rangeland vegetation change, are increasingly applied as tools for land management decision-making. As STMs are created throughout the US, it is crucial to ensure that they are supported by current evidence. Differences in plant species composition at different locations are commonly used as an indicator of alternate ecosystem states, or characteristic configurations of plant communities and ecological processes. This study aims to create STMs based on plant species composition data for two types of land, also known as ecological sites (Claypan and Mountain Loam), in northwestern Colorado. We sampled 76 plots with different management and natural disturbance histories. We hypothesized that A) differences in species composition between the two different ecological sites would be related to environmental factors and B) differences in species composition within each ecological site would be related to management and disturbance history. Relationships among species composition, site history, and environmental variables were evaluated using multivariate statistics. We found that between land types, species composition was related to differences in soil texture, supporting Hypothesis A and the creation of separate STMs for the two different ecological sites. Within ecological sites, species composition was related to site history and also to environmental variation. This finding partially supports Hypothesis B and the identification of ecosystem states using species composition, but also suggests that these land types are not uniform physical templates upon which rangeland vegetation changes play out. This data-driven, plant species based approach created two objective, credible STMs that are consistent with past research in the sagebrush steppe. Our findings support the division of landscapes into different land types, as currently applied in building STMs. A vegetation measurement approach that acknowledges environmental heterogeneity within ecological sites is necessary to help define finer-resolution ecological sites and highlight cases where specific environmental conditions make vegetation changes more likely.

Technical Abstract: State-and-transition models (STMs), conceptual models of vegetation change based on alternate state theory, are increasingly applied as tools for land management decision-making. As STMs are created throughout the US, it is crucial to ensure that they are supported by ecological evidence. Plant species composition reflects ecosystem processes that are difficult to measure and may be a useful indicator of alternate states. This study aims to create data-driven STMs based on plant species composition for two ecological sites (Claypan and Mountain Loam) in northwestern Colorado. We sampled 76 plots with different management and disturbance histories. Drawing on the hierarchical approach currently taken to build STMs, we hypothesized that A) differences in species composition between the two ecological sites would be related to environmental factors and B) differences in species composition within each ecological site would be related to management and disturbance history. Relationships among species composition, site history, and environmental variables were evaluated using multivariate statistics. We found that between ecological sites, species composition was related to differences in soil texture, supporting Hypothesis A and the creation of separate STMs. Within ecological sites, species composition was related to site history and also to environmental variation. This finding partially supports Hypothesis B and the identification of alternate states using species composition, but also suggests that these ecological sites are not uniform physical templates upon which plant community dynamics play out. This data-driven, plant species based approach created two objective, credible STMs with states and transitions that are consistent with the sagebrush steppe literature. Our findings support the hierarchical view of landscapes currently applied in building STMs. An approach that acknowledges environmental heterogeneity within ecological sites is necessary to help define finer-resolution ecological sites and elucidate cases where specific abiotic conditions make transitions more likely.