Bat activity across the vertical gradient of an old-growth Sequoia sempervirens forest

Sampling in forests across the world has revealed clear variation in bat use across the height gradient from ground level, up through the canopy, and above the tallest trees (Francis, 1994; Bradshaw, 1996; Kalcounis et al., 1999; Hayes and Gruver, 2000; Bernard, 2001; Henry et al., 2004; Menzel et al., 2005; Pereira et al., 2010). The ways bats distribute their activities along this vertical gradient differs among forests (Kalcounis et al., 1999). Diver sity of foraging strategies and flight specializations among bat species confer advantages to differentially exploit forest canopy structures. Oldgrowth forests have higher structural complexity than younger forests and consequently exhibit higher levels of bat activity and species diversity (Hayes and Gruver, 2000). Ground-based capture and acoustic techniques have insufficient range to sample forest canopies and will miss or underrepresent high-flying open-air foraging bats that exploit the canopy and above-canopy airspace. In tall forests, ground-based methods will also underrepresent bats using the understory and lower canopy. The western North American coniferous forest canopy has only received limited sampling of bat use. Bats in the tallest of these forests, which are dominated by Sequoia sempervirens (hereafter redwood), have been studied exclusively from ground level, revealing that bats routinely use basal crevices and fire cavities of redwood trees for night and day roosts throughout the year (Gellman and Zielinski, 1996; Zielinski and Gellman, 1999; Purdy, 2002; Mazurek and Zielinski, 2004; Roberts, 2008). Several species use these cavities as maternity roosts, including a State of California species of special concern and known cave roosting species, Townsend’s big-eared bat (Corynorhinus townsendii) (Mazurek, 2004). Terrestrial based acoustic, netting, and roost surveys indicate a high degree of overall bat activity associated with redwoods, but the can opy remains virtually unexplored for bats. Rich with suitable roosting habitat and a temperate climate (S. C. Sillett and R. Van Pelt, personal Acta Chiropterologica, 16(1): 53–63, 2014 PL ISSN 1508-1109 © Museum and Institute of Zoology PAS doi: 10.3161/150811014X683264