Sciurids in Pacific Northwest Managed and Old‐Growth Forests

An understanding of the factors governing sciurid abundance in the Pacific Northwest is essential for prescribing forest management practices for second‐growth forests where recovery of Spotted Owl (Strix occidentalis) populations and enhancement of biodiversity are objectives. We compared results of companion studies of sciurids in western Washington and Oregon and examined patterns of abundance in relation to habitat elements on the Olympic Peninsula to elucidate governing factors and make recommendations for forest management. Regional contrasts show that Glaucomys sabrinus and Tamias townsendii in Douglas‐fir forests in Oregon are 4 times more abundant than in western hemlock forests in Washington, and dietaries of Glaucomys, and the fungal communities that provide its food, are more diverse in Oregon than in Washington. Glaucomys sabrinus in old forests are 2 times more abundant than in young, managed forests without old‐forest legacies (large live trees, large snags and large, decaying fallen trees); populations in young forests with old‐forest legacies and with understory development may equal those in old growth. On the Olympic Peninsula, Glaucomys sabrinus abundance can be predicted by density of large snags and abundance of ericaceous shrubs. At least seven large snags/ha and well‐distributed patches of dense shrubs (cover within patches >24% and patches covering 40% of the total area) are necessary for high densities of Glaucomys sabrinus. Abundance of Tamias townsendii reflects size of dominant tree and well‐developed understories. Abundance of Tamiasciurus douglasii seems to reflect territoriality in concordance with food supply and was greatest where Glaucomys and Tamias were low in abundance. Patterns of abundance of the sciurids in old‐ and managed forests suggests that silvicultural manipulation of vegetation and creative snag or den‐tree management could be used in a management strategy to accelerate the development of Spotted Owl habitat in areas where old growth is lacking.