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Biotic and Abiotic Changes in Ecosystem Structure over a Shrub-Encroachment Gradient in the Southwestern USA

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Abstract

In this study, we investigate changes in ecosystem structure that occur over a gradient of land-degradation in the southwestern USA, where shrubs are encroaching into native grassland. We evaluate a conceptual model which posits that the development of biotic and abiotic structural connectivity is due to ecogeomorphic feedbacks. Three hypotheses are evaluated: 1. Over the shrub-encroachment gradient, the difference in soil properties under each surface-cover type will change non-linearly, becoming increasingly different; 2. There will be a reduction in vegetation cover and an increase in vegetation-patch size that is concurrent with an increase in the spatial heterogeneity of soil properties over the shrub-encroachment gradient; and 3. Over the shrub-encroachment gradient, the range at which soil properties are autocorrelated will progressively exceed the range at which vegetation is autocorrelated. Field-based monitoring of vegetation and soil properties was carried out over a shrub-encroachment gradient at the Sevilleta National Wildlife Refuge in New Mexico, USA. Results of this study show that vegetation cover decreases over the shrub-encroachment gradient, but vegetation-patch size increases, with a concurrent increase in the spatial heterogeneity of soil properties. Typically, there are significant differences in soil properties between non-vegetated and vegetated surfaces, but for grass and shrub patches, there are only significant differences for the biotic soil properties. Results suggest that it is the development of larger, well-connected, non-vegetated patches that is most important in driving the overall behavior of shrub-dominated sites. Results of this study support the hypothesis that feedbacks of functional connectivity reinforce the development of structural connectivity, which increases the resilience of the shrub-dominated state, and thus makes it harder for grasses to re-establish and reverse the vegetation change.

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Acknowledgments

The authors are grateful for the support from NSF award DEB-0217774 to the University of New Mexico for Long-Term Ecological Research at the Sevilleta National Wildlife Refuge and support form NSF award DEB-0423704 to the Central Arizona – Phoenix Long-Term Ecological Research. Assistance at Sevilleta was generously provided by Scott Collins, Jennifer Johnson, John deWitt, Jim Elliot, and Mike Friggens. We are also grateful to Tony Parsons, The University of Sheffield, The Worshipful Company of Farmers, the Royal Society Dudley Stamp Memorial Fund and Rothamsted Research which is a Biotechnology and Biological Sciences Research Council core funded institute. In particular, we are grateful to two anonymous reviewers and David Huber whose insightful comments have greatly improved this manuscript.

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Authors and Affiliations

  1. Global Institute of Sustainability, Arizona State University, Tempe, Arizona, 85287-5402, USA

    Laura Turnbull

  2. Department of Geography, Sheffield Centre for International Drylands Research, University of Sheffield, Winter Street, Sheffield, S10 2TN, UK

    John Wainwright

  3. Department of Geography, University of Exeter, Amory Building, Rennes Drive, Exeter, EX4 4RJ, UK

    Richard E. Brazier

  4. Biogeochemistry of Soils and Water Group, Sustainable Soils and Grassland Systems Department, Rothamsted Research, North Wyke, Okehampton, EX20 2SB, UK

    Roland Bol

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  1. Laura Turnbull
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  4. Roland Bol
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Correspondence to Laura Turnbull.

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Author Contributions

LT, JW, REB conceived of and designed the study, LT, JW, REB performed research, LT analyzed data, RB contributed new methods, LT wrote the article, and JW, REB and RB contributed to writing the article.

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Turnbull, L., Wainwright, J., Brazier, R.E. et al. Biotic and Abiotic Changes in Ecosystem Structure over a Shrub-Encroachment Gradient in the Southwestern USA. Ecosystems 13, 1239–1255 (2010). https://doi.org/10.1007/s10021-010-9384-8

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