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

Abstract

Abert squirrels (Sciurus aberti) are ecologically dependent on ponderosa pine forests. Southwestern dwarf mistletoe (Arceuthobium vaginatum) is a common parasite of ponderosa pine (Pinus ponderosa) that can result in the formation of an abnormal growth structure or branching pattern in its host known as a witches' broom. Management of this parasite has generally included intensive removal of infected trees. Previous studies have identified witches' brooms as important for wildlife; however, little is known about the relationship between it and Abert squirrels. We examined the contents of 226 dwarf mistletoe-induced witches' brooms in 144 ponderosa pine trees for evidence of Abert squirrel use between September 2000 and November 2001 and documented use in 39 brooms (31 caching and foraging sites and 8 nest sites). We compared the physical characteristics of brooms with evidence of use to those with no evidence of use to predict the probability of a broom being used as a caching and foraging site by Abert squirrels. As number of branches within a broom and tree height increased so did the probability of Abert squirrel use. 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.