Red squirrel use of dwarf mistletoe-induced witches' brooms in Douglas-fir

Douglas-fir dwarf mistletoe (Arceuthobium douglasii) is a prevalent species in southwestern mixed-conifer forests, affecting host trees by reducing growth and seed production and increasing mortality. Dwarf mistletoe infections can also form witches' brooms, which are profusely branched, dense masses of distorted host branches in the crowns of infected trees. Despite their impact on trees, brooms provide nesting, roosting, and foraging habitat for many wildlife species. To determine whether red squirrels (Tamiasciurus hudsonicus) selected brooms for nesting, foraging, or caching sites, we compared use of brooms in Douglas-fir (Pseudotsuga menziesii) in mixed-conifer forests in northern Arizona and New Mexico, USA, in 1998 and 1999. Douglas-fir brooms are classified into 3 distinct forms based on structure and point of origin on the tree. Type I brooms form from an infection near the terminal end of a branch. Type III brooms form from an infection ≤1 m from the tree trunk (bole); the supporting branch usually grows vertically from the point of infection, parallel to the bole. Type III brooms result from an infection on the bole, creating a dense profusion of branches and often forming a platform. We selected trees of 3 size classes (10-25-, 26-40-, >40-cm dbh) from stands and examined them for signs of red squirrel use (e.g., nesting, food storage). Red squirrels selected Type II and III brooms located closer to the bole and with larger platform size for nesting. They selected Type II and III over Type I brooms for caching and foraging use. Retaining Type II and III brooms that have platforms >700 cm², volumes >15 m², and are located 4.5-10 m above ground will retain this habitat element for red squirrels.