Small mammal response to experimental canopy gaps in the southern Washington Cascades

Abstract

General patterns of small mammal species occurrence across major successional gradients have been studied extensively in western Washington forests, but the effects of smaller-scale disturbances are less clear. Understanding such effects will help to evaluate alternatives to traditional clear-cut logging. We compared relative abundances, population characteristics, and species occurrence of small mammals in experimentally created canopy gaps and adjacent forests in the southern Washington Cascade Range. Contrary to our predictions, species characteristic of closed-canopy conditions dominated captures in all gap sites. No significant declines in abundance between forests and large gap patches were observed for any closed-canopy species. Measures of persistence, reproductive status, sex ratios, and weight distributions did not indicate that gaps were sinks or lower quality habitat for Trowbridge’s shrew ( Sorex trowbridgii), Keen’s mouse ( Peromyscus keeni), or the southern red-backed vole ( Clethrionomys gapperi). The creeping vole ( Microtus oregoni) was the only early-successional species to respond strongly to the openings. It occurred frequently in large (0.1–0.2 ha) gaps but rarely in smaller gaps and forested patches. The large gaps targeted in this study are at the upper end of the range of natural gap sizes reported for the western Cascades, but may be too small to provide habitat for most early-successional species. Our results indicated that the main direct effect of disturbance-created small gaps in these areas was to increase resources for forest species rather than to facilitate recruitment of early-successional small mammals.