Plant Ecology

, Volume 172, Issue 2, pp 159–171 | Cite as

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

  • Wendy K. GramEmail author
  • Elizabeth T. Borer
  • Kathryn L. Cottingham
  • Eric W. Seabloom
  • Virginia L. Boucher
  • Lloyd Goldwasser
  • Fiorenza Micheli
  • Bruce E. Kendall
  • Rebecca S. Burton


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.

Exotic plant species Hummock Invaded communities Spatial refuges 


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  1. Adams T.E., Vaughn C.E. and Sands P.B. 1999. Geographic races may exist among perennial grasses. California Agriculture 53: 33–38.Google Scholar
  2. Armstrong J.K. and Huenneke L.F. 1992. Spatial and temporal variation in species composition in California grasslands: the interaction of drought and substratum. In: Baker A.J.M., Proctor J. and Reeves R.D. (Eds.), The vegetation of ultramafic (serpentine) soils. Intercept Ltd., Andover, England, pp. 213–233.Google Scholar
  3. Baker H.G. 1978. Invasion and replacement in Californian and neotropical grasslands. In: Wilson J.R. (Ed.), Plant relations in pastures. CSIRO, East Melbourne, pp. 368–384.Google Scholar
  4. Brooks R.R. 1987. Serpentine and its vegetation. In: Dudley T.R. (Ed.), Ecology, Phytogeography and Physiology Series. Dioscorides Press, Portland, Oregon.Google Scholar
  5. D'Antonio C.M. and Vitousek P.M. 1992. Biological invasions by exotic grasses, the grass/fire cycle, and global change. Annual Review of Ecology and Systematics 23: 63–87.Google Scholar
  6. Drake J.A., Mooney H.A., di Castri F., Groves R.H., Kruger F.J., Regmanák M. and Williamson M. (Eds.), 1989. Biological invasions: a global perspective. Wiley, Chichester, England.Google Scholar
  7. Galatowitsch S.M. and van der Valk A.G. 1996. The vegetation of restored and natural prairie wetlands. Ecological Applications 6: 102–112.Google Scholar
  8. Harrison S. 1997. How natural habitat patchiness affects the distribution of diversity in Californian serpentine chaparral. Ecology 78: 1898–1906.Google Scholar
  9. Harrison S. 1999a. Local and regional diversity in a patchy landscape: native, alien, and endemic herbs on serpentine. Ecology 80: 70–80.Google Scholar
  10. Harrison S. 1999b. Native and alien species diversity at the local and regional scales in a grazed California grassland. Oecologia 121: 99–106.Google Scholar
  11. Hatch D.A., Bartolome J.W., Fehmi J.S. and Hillyard D.S. 1999. Effects of burning and grazing on a coastal California Grassland. Restoration Ecology 7: 376–381.Google Scholar
  12. Heady H.F. 1956a. Changes in California annual plant community induced by manipulation of natural mulch. Ecology 37: 781–798.Google Scholar
  13. Heady H.F. 1956b. Evaluation and measurement of the California annual type. Journal of Range Management 9: 25–27.Google Scholar
  14. Heady H.F. 1958. Vegetational changes in the California annual type. Ecology 39: 402–415.Google Scholar
  15. Heady H.F. 1977. Valley grassland. In: Barbour M.G. and Major J. (Eds.), Terrestrial vegetation of California. Wiley, New York, pp. 491–514.Google Scholar
  16. Hickman S. 1997. The Jepson manual: higher plants of California. University of California Press, Berkeley.Google Scholar
  17. Hobbs R.J. and Hobbs V.J. 1987. Gophers and grassland a model of vegetation response to patchy soil disturbance. Vegetatio 69: 141–146.Google Scholar
  18. Hobbs R.J. and Humphries S.E. 1995. An integrated approach to the ecology and management of plant invasions. Conservation Biology 9: 761–770.Google Scholar
  19. Hobbs R.J. and Mooney H.A. 1985. Community and population dynamics of serpentine grassland annuals in relation to gopher disturbance. Oecologia (Berlin) 67: 342–351.Google Scholar
  20. Hobbs R.J. and Mooney H.A. 1991. Effects of rainfall variability and gopher disturbance on serpentine annual grassland dynamics. Ecology 72: 59–68.Google Scholar
  21. Hobbs R.J. and Mooney H.A. 1995. Spatial and temporal variability in California annual grassland: Results from a long-term study. Journal of Vegetation Science 6: 43–56.Google Scholar
  22. Huenneke L.F., Hamburg S.P., Koide R., Mooney H.A. and Vitousek P.M. 1990. Effects of soil resources on plant invasion and community structure in Californian serpentine grassland. Ecology 71: 478–491.Google Scholar
  23. Jackson L.E. 1985. Ecological origins of California's Mediterranean grasses. Journal of Biogeography 12: 349–361.Google Scholar
  24. Kruckeberg A.K. 1984. California serpentines: flora, vegetation, geology, soils and management problems. University of California Press, Berkeley.Google Scholar
  25. Lobo A., Moloney K., Chic O. and Chiariello N. 1998. Analysis of fine-scale spatial pattern of a grassland from remotely-sensed imagery and field collected data. Landscape Ecology 13: 111–131.Google Scholar
  26. MacArthur R.H. and Wilson E.O. 1967. The theory of island biogeography. Princeton University Press, Princeton, New Jersey.Google Scholar
  27. Manly B.F.J. 1986. Randomization and regression methods for testing for associations with geographical, environmental and biological distances between populations. Researches in Population Ecology 28: 201–218.Google Scholar
  28. Mantel N. 1967. The detection of disease clustering and a generalized regression approach. Cancer Research 27: 209–220.PubMedGoogle Scholar
  29. McCullagh P. and Nelder J.A. 1989. Generalized Linear Models. Chapman and Hall, New York.Google Scholar
  30. McNaughton S.J. 1968. Structure and function of California grasslands. Ecology 49: 962–972.Google Scholar
  31. Menke J.W. 1992. Grazing and fire management for native perennial grass restoration in California grasslands. Fremontia 20: 22–25.Google Scholar
  32. Meyer M. and Schiffman P. 1999. Fire season and mulch reduction in a California grassland: comparison of restoration strategies. Madroño 46: 25–37.Google Scholar
  33. Moloney K.A. and Levin S.A. 1996. The effects of disturbance architecture on landscape-level population dynamics. Ecology 77: 375–394.Google Scholar
  34. Mooney H.A. and Drake J.A. (Eds.), 1986. Ecology of biological invasions of North America and Hawaii. Springer-Verlag, New York.Google Scholar
  35. Mooney H.A., Hamburg S.P. and Drake J.A. 1986. The invasions of plants and animals into California. In: Mooney H.A. and Drake J.A. (Eds.), Ecology of biological invasions of North America and Hawaii. Springer-Verlag, New York, pp. 250–272.Google Scholar
  36. Murphy D.D. and Ehrlich P.R. 1989. Conservation biology of California's remnant grasslands. In: Huenneke L.F. and Mooney H. (Eds.), Grassland structure and function: California annual grassland. Kluwer Academic Publishers, Dordrecht, pp. 201–211.Google Scholar
  37. Neter J., Wasseman W. and Kutner M.H. 1985. Applied Linear Statistical Models. Richard D. Irwin, Inc., Homewood, Illinois.Google Scholar
  38. Parma A.M., Amarasekare P., Mangel M., Moore J., Murdoch W.W., Noonburg E., Pascual M.A., Possingham H.P., Shea K., Wilcox C. and Yu D. 1999. What can adaptive management do for our fish, forests, food and biodiversity? Integrative Biology 1: 16–26.Google Scholar
  39. Pitt M.D. and Heady H.F. 1978. Responses of annual vegetation to temperature and rainfall patterns in Northern California. Ecology 59: 336–350.Google Scholar
  40. Pollak O. and Kan T. 1998. The use of prescribed fire to control invasive exotic weeds at Jepson Prairie Preserve. In: Witham C.W., Bauder E.T.D., Belk Ferren Jr. W.R. and Ornduff R. (Eds.), Ecology, conservation, and management of vernal pool ecosystems. Proceedings of a 1996 conference. California Native Plant Society, Sacramento, pp. 241–249.Google Scholar
  41. Proctor J. 1971. The plant ecology of serpentine. II. Plant response to serpentine soils. Journal of Ecology 59: 397–410.Google Scholar
  42. Proctor J. and Woodell S.R.J. 1975. The ecology of serpentine soils. Advances in Ecological Research 9: 255–366.Google Scholar
  43. Seabloom E.W., Borer E.T., Boucher V.L., Burton R.S., Cottingham K.L., Goldwasser L., Gram W.K., Kendall B.E. and Micheli F. in press. Competition, seed limitation, disturbance, and reestablishment of California native annual forbs. Ecological Applications.Google Scholar
  44. Shea K., Amarasekare P., Mangel M., Moore J., Murdoch W.W., Noonburg E., Parma A., Pascual M.A., Possingham H.P., Wilcox C. and Yu. D. 1998. Management of populations in conservation, harvesting and control. Trends in Ecology & Evolution 13: 371–375.Google Scholar
  45. Shoulders C.L. 1994. Methods of restoring Nassella pulchra (purple needlegrass) at Jepson Prairie Preserve, Solano County, CA. Thesis. University of Wisconsin, Madison.Google Scholar
  46. Vitousek P.M., D'Antonio C.M., Loope L.L., Rejmanek M. and Westbrooks R. 1997. Introduced species: a significant component of human-caused global change. New Zealand Journal of Ecology 21: 1–16.Google Scholar
  47. Walker R.B. 1954. Factors affecting plant growth on serpentine soils. Ecology 35: 259–266.Google Scholar
  48. Walters C.J. 1986. Adaptive management of renewable resources. Macmillan, New York.Google Scholar

Copyright information

© Kluwer Academic Publishers 2004

Authors and Affiliations

  • Wendy K. Gram
    • 1
    Email author
  • Elizabeth T. Borer
    • 2
  • Kathryn L. Cottingham
    • 1
  • Eric W. Seabloom
    • 1
  • Virginia L. Boucher
    • 2
  • Lloyd Goldwasser
    • 3
  • Fiorenza Micheli
    • 1
  • Bruce E. Kendall
    • 1
  • Rebecca S. Burton
    • 1
  1. 1.National Center for Ecological Analysis and SynthesisSanta Barbara
  2. 2.Department of Ecology, Evolution, and Marine BiologyUniversity of California, Santa BarbaraSanta Barbara
  3. 3.Marine Science InstituteUniversity of California, Santa BarbaraSanta Barbara

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