Red Squirrel Use of Dwarf Mistletoe-Induced Witches' Brooms in Douglas-Fir

Abstract

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 II 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.

Nesting habitat of Mexican spotted owls in the Sacramento Mountains, New Mexico

“…In a study of dwarf mistletoe brooms in Douglas‐fir trees in southern Oregon, this minimum size was attained by all type III brooms sampled, by >67% of type II brooms sampled, but by only 33% of type I brooms sampled (Mallams et al ). Thus, use of witches' brooms may be driven by broom size as well as type (Hedwall and Mathiasen , Hedwall et al ). We did not measure broom size, but many of the brooms used in the Sacramento Mountains were large and likely similar to or larger than the minimum size used by northern spotted owls.…”

“…Although these platforms may provide lower quality nest sites than do cavities, they nonetheless provide valuable nesting substrates for owls. Consequently, managers should retain large trees with large type II and III witches' brooms in areas managed as owl nesting habitat (Hedwall et al ).…”

Tree microhabitat structures as indicators of biodiversity in Douglas-fir forests of different stand ages and management histories in the Pacific Northwest, U.S.A.

“…If they contain decayed substrate, these bark pockets with decay and bowls in the bark with fresh residue are also important for invertebrates (Winter and Mö ller, 2008). Small mammals like red tree voles (Arborimus longicaudus) and red squirrels (Tamiasciurus hudsonicus) build their nests in dense possibly misshapen branches throughout the tree canopy of large Douglas-fir trees where needles provide sufficient cover for the young (Hedwall et al, 2006;Sondenaa, 1991). Such brooms, often caused by dwarf mistletoe (Arceuthobium spp.…”

Use of Witches' Brooms by Abert Squirrels in Ponderosa Pine Forests

“…We recommend managers retain ponderosa pine trees !18 m in height having brooms with .7 branches to provide usable caching and foraging sites for Abert squirrels. (WILDLIFE SOCIETY BULLETIN 34(2):467-472;2006)…”

“…Witches' brooms containing bird and mammal nests have been observed in several species of host trees (Hawksworth and Wiens 1996, Mathiasen 1996. Red squirrels (Tamiasciurus hudsonicus) used witches' brooms as caching and nesting sites in Douglas-fir (Pseudotsuga menziesii) in the Pacific Northwest (Parks et al 1999) and Southwest (Hedwall et al 2006). Abert squirrels (Sciurus aberti) were observed nesting in witches' brooms in ponderosa pine in Colorado (Farentinos 1972, Pollock 1981; however, the specific characteristics of nests were not measured.…”