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Long-term patterns of shrub expansion in a C₄-dominated grassland: fire frequency and the dynamics of shrub cover and abundance¹

²Department of Plant Biology, Arizona State University, Tempe, Arizona, 85287-1601 USA; ³Division of Biology, Kansas State University, Manhattan, Kansas, 66506-4901 USA

Worldwide, grassland ecosystems have experienced a major shift in growth-form dominance as woody plant species have expanded and replaced native grasses. In the C₄-dominated grasslands of central North America, a reduction in fire frequency is the most cited cause of this shift in growth forms as fire both enhances grass productivity and constrains the establishment and expansion of native woody vegetation. Using an 18-yr plant species composition data set, we quantified patterns of change in shrub cover, frequency, and species richness associated with three distinct fire regimes. During the study period (1983–2000), shrub cover increased most dramatically in sites in which the frequency of fire was once every 4 yr (intermediate frequency; 28.6%) followed by sites in which fire occurred only once during the 18-yr period (low frequency; 23.7%). Annual fire effectively prevented the recruitment of new woody species, but even with this high fire frequency, shrub cover increased slightly (3.7%). Comparatively, shrub species richness increased by three and six, respectively, in the intermediate- and low-frequency fire sites. These data indicate that within this grassland, periods without fire are necessary for recruitment of both new individuals and additional shrub species; however, once established, shrub cover will increase regardless of fire frequency and even annual fire will not reduce shrub abundance.

Key Words: C₄ grassland • fire frequency • growth form substitution • shrubs • woody encroachment

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