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Plant Ecology

, Volume 217, Issue 7, pp 825–841 | Cite as

The impacts of isolation, canopy size, and environmental conditions on patterns of understory species richness in an oak savanna

  • Karen A. StahlheberEmail author
Article
  • 295 Downloads

Abstract

Fragmentation and habitat loss have transformative effects on landscapes. Like many savanna communities worldwide, California oak savannas (Quercus spp.) have experienced extensive anthropogenic clearing, increasing the isolation of individual trees. Isolation and tree size may have consequences for populations of understory plants, but this has never been evaluated in a temperate savanna community. I address two questions: (1) How does the presence of oak trees affect site-level species richness? (2) What landscape and/or local attributes are associated with trees that contribute many species to a site? Vegetation and environmental attributes in the understory of isolated oaks and in surrounding grassland were surveyed at four sites in California, USA. I counted the number of species occurring in plots underneath and at the edge of each tree that were absent from surrounding plots in open grassland (‘oak-associated species’). I used species-area curves to evaluate the contribution of oaks to site-level species richness. The importance of crown area, isolation, environmental heterogeneity, and understory soil fertility on species occurrences was assessed using linear mixed models. At two of the four sites, oaks increased site species richness by 23–57 %. At the southernmost site, understory soil fertility and isolation were negatively related to oak-associated species. Across all four sites, trees with larger crown area had more oak-associated species. Savanna oaks are important structuring forces for landscape plant species richness, especially in dry locations. Large trees may be especially important for conserving diverse understories. These positive effects, however, are not always negatively influenced by isolation.

Keywords

Quercus Connectivity Crown area Species-area curve Environmental heterogeneity Island biogeography theory California Stress-gradient hypothesis 

Notes

Acknowledgments

This work was funded by a University of California Natural Reserve System Mildred E. Mathias Graduate Student Research Grant, a California Native Plant Society Educational Grant, and a National Science Foundation Dissertation Improvement Grant (DEB-1110569). The directors of the UC Reserves provided much needed logistical support and site knowledge: K. McCurdy, M. Stromberg, M. Hamilton, P. Aigner, and C. Koehler. Comments from C.M. D’Antonio, B. Mahall, J.M. Levine, and two anonymous reviewers on earlier versions of this manuscript were helpful, as well as suggestions from E. Damschen on data analysis.

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© Springer Science+Business Media Dordrecht 2016

Authors and Affiliations

  1. 1.Department of Ecology, Evolution and Marine BiologyUniversity of California Santa BarbaraSanta BarbaraUSA
  2. 2.W. K. Kellogg Biological StationMichigan State UniversityHickory CornersUSA

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