Changing fire regimes and the avifauna of California oak woodlands

Abstract

Abstract. Natural and anthropogenic fi re once played an important role in oak woodlands of California. Although lightning-ignited fi res were infrequent, the California Indians used fi re to modify oak woodland vegetation for at least 3,000 yr. These high-frequency, low-intensity fi res likely resulted in little mortality of mature oaks, low but continuous tree recruitment, an open understory, and a fi ne-grained mosaic of vegetation patches. Following settlement by Europeans in the mid-1800s, ranchers burned to reduce shrub cover and to increase grassland area and forage production; surface fi res were common with average fi re-return intervals of 8–15 yr. Fire suppression, begun in the 1940s to 1950s, led to increases in surface and crown fuels, invasion of woody vegetation in the understory, and increased tree density. In the absence of demonstrated fi re effects on oak woodland birds, we used changes in vegetation structure expected to result from fi re and fi re suppression to predict the response of oak woodland birds to fi re and fi re suppression based on nesting habitat of 17 common oak woodland species breeding at the San Joaquin Experimental Range, Madera County, California. Our results suggest that populations of Western Kingbirds (Tyrannus verticalis), Western Bluebirds (Sialia mexicana), and Violet-green Swallows (Tachycineta thalassina), would increase in abundance following fi re, because they consistently nested in habitat similar to that expected to result from frequent, low-intensity fi re. The species predicted to respond negatively to changes resulting from fi re differed among the variables examined. If fi re produces a mosaic of habitat patches rather than a homogeneous landscape, we expect that the differing habitat needs of most species will be provided for. As with fi re, the most obvious change resulting from excluding livestock was an increase in shrub cover. The question naturally arises to what extent livestock grazing creates habitat similar to that created by historical fi re, but this question remains unstudied. More fi re-history research is needed to understand past fi re regimes of oak woodlands and the effects of fi re, including prescribed fi re, on the vegetation and the bird community. The effects of grazing and the extent to which grazing mimics fi re clearly require more study. We encourage others to test our hypotheses regarding responses of birds to variables expected to be altered by fi re: shrub cover, tree density, and numbers of snags, saplings, and logs. Finally, we need to test our working hypothesis that a mosaic of habitat patches will provide the habitat conditions needed to sustain the high avian diversity characteristic of oak woodlands.

Citation

Purcell, Kathryn L.; Stephens, Scott L. 2005. Changing fire regimes and the avifauna of California oak woodlands. Studies in Avian Biology No. 30:33–45