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Distribution of plants in a California serpentine grassland: are rocky hummocks spatial refuges for native species?

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Abstract

Invasions by non-native taxa can have severe consequences for native species. In the heavily invaded serpentine grasslands of central California, many native species appear to be restricted to isolated outcrops of shallow serpentine soil, or “hummocks,” although the extent to which these hummocks function as refuges for native vegetation has never been quantified. We tested whether native plant species were restricted to hummocks within a serpentine grassland at the University of California Sedgwick Reserve near Santa Barbara, California by sampling species along hummock-grassland gradients. We also examined the influence of soil parameters, hummock area, proximity to other hummocks, and spatial location on species composition across 16 hummocks at this site. Both the hummocks and the surrounding grassland had high Mg, low Ca, and low Ca to Mg ratios typical of serpentine systems. Hummocks appeared to be more stressful environments because of their shallower soils, lower cation exchange capacity, and greater percent sand. Of the 27 most common plant species sampled along hummock-grassland transects, we identified 8 hummock specialists, 7 edge specialists, 8 matrix specialists, and 4 generalists. Importantly, both the hummock and matrix specialist groups included native species. Plant community composition was correlated with spatial positioning of the hummocks and with soil Ca, Na, K, and N. The number of species increased and community composition changed with increasing hummock area. Species composition was most similar among hummocks in close proximity to each other, and decreased with increasing distance between hummocks. Our results suggest that the community structure of serpentine grasslands is spatially complex and an effective management or restoration plan must address this complexity.

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Author notes

  1. Wendy K. Gram

    Present address: Sam Noble Oklahoma Museum of Natural History, University of Oklahoma, Norman, OK, 73072

  2. Kathryn L. Cottingham

    Present address: Department of Biological Sciences, Dartmouth College, Hanover, NH, 03755

  3. Virginia L. Boucher

    Present address: Environmental Science and Policy, University of California, Davis, Davis, CA, 95616

  4. Lloyd Goldwasser

    Present address: Southwest Fisheries Science Center, National Marine Fisheries Service, Tiburon, CA, 94920

  5. Fiorenza Micheli

    Present address: Hopkins Marine Station, Stanford University, Pacific Grove, CA, 93950

  6. Bruce E. Kendall

    Present address: Donald Bren School of Environmental Science and Management, University of California, Santa Barbara, Santa Barbara, CA, 93106

  7. Rebecca S. Burton

    Present address: Department of Biology, Alverno College, Milwaukee, WI, 53234

Authors and Affiliations

  1. National Center for Ecological Analysis and Synthesis, Santa Barbara, CA, 93101

    Wendy K. Gram, Kathryn L. Cottingham, Eric W. Seabloom, Fiorenza Micheli, Bruce E. Kendall & Rebecca S. Burton

  2. Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, Santa Barbara, CA, 93106

    Elizabeth T. Borer & Virginia L. Boucher

  3. Marine Science Institute, University of California, Santa Barbara, Santa Barbara, CA, 93106

    Lloyd Goldwasser

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  1. Wendy K. Gram
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Correspondence to Wendy K. Gram.

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Gram, W.K., Borer, E.T., Cottingham, K.L. et al. Distribution of plants in a California serpentine grassland: are rocky hummocks spatial refuges for native species?. Plant Ecology 172, 159–171 (2004). https://doi.org/10.1023/B:VEGE.0000026332.57007.7b

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  • DOI: https://doi.org/10.1023/B:VEGE.0000026332.57007.7b

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