Resource selection by porcupines: Winter den site location and forage tree choices

Abstract

We examined the importance of tree species composition and tree size-class distribution in determining the selection of winter den sites and forage trees by porcupines (Erethizon dorsatum). Porcupines selected winter den sites with a higher composition of sitka spruce, in all diameter size classes. However, statistically significant selection was found only for the 15.0-19.9 cm dbh size classes of sitka spruce. Porcupines displayed a statistically significant avoidance of 5.0-9.9 cm dbh and 10.0-14.9 cm dbh western hemlock trees when selecting winter den sites. When foraging around the den sites, porcupines showed a statistical avoidance of amabilis fir in all size classes and selected western hemlock, except the smallest size class (5.0-9.9 cm dbh). The results of this study indicate that porcupines do not choose winter den site locations based on the availability of preferred forage resources. Porcupines appear to choose den sites based on within-stand, patch variation in the proportion of sitka spruce and small dbh western hemlock. Forage trees are then selected, nonrandomly from specific resource categories around the den site. Porcupines in this study were found to feed on a mean of 0.54 new forage trees/day. Regression analysis revealed a positive linear relationship between new forage trees used/day and porcupine midwinter weight (y = O.15x-0.54; r2 = 0.73). The results indicate that larger porcupines may produce a larger proportion of observed conifer tree feeding damage in second growth stands, as compared to smaller porcupines.