Distinguishing characteristics of Mount Graham red squirrel midden sites

The Mount Graham red squirrel (Tamiasciurus hudsonicus grahamensis) was listed as endangered in 1987 because its population was small and declining, its range was restricted, and its remaining habitat was threatened by human activities. We measured vegetational and other physical features around middens of the Mount Graham red squirrel (n = 215) and at random locations (n = 201) in Engelmann spruce (Picea engelmannii)-cork-bark fir (Abies lasiocarpa var. arizona) and transition-zone forests to identify characteristics that distinguish midden sites from random sites. Foliage volume, canopy closure, basal area, tree density 40 cm in diameter at breast height (dbh) were greater (P < 0.005) at midden sites than at random sites in both forest types. Midden sites in the spruce-fir forest were distributed by aspect in proportions similar to random sites, but in the transition-zone forest midden sites were more common on north-facing slopes and less common on south-facing slopes than expected (P < 0.001). The mean age of dominant trees at midden sites was >212 years in the spruce-fir forest and >183 years in the transition-zone forest. Because regeneration of midden sites will take ≥250 years, management to protect red squirrel middens should focus on preserving existing habitat by limiting activities that remove large trees, snags, or logs, open the forest canopy, or create forest edge.