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Influence of pre-commercial thinning of second growth coniferous forests in northwestern British Columbia on porcupine (Erethizon dorsatum) movements, mortality and foraging Zimmerling, Todd

Abstract

To determine if pre-commercial thinning to 1200 stems/ha and 400 stems/ha influences movements, mortality rates and foraging behaviour of porcupines (Erethizon dorsatum), I monitored 33 radio-collared individuals over 4 winters. The study area consisted of three second-growth conifer stands (20 -25 years of age) located in Shames Valley, near Terrace, British Columbia. I found that porcupines selected den locations that had a higher than average density of sitka spruce (Picea sitchensis) within 10 m of the den compared to the rest of the stand. Once a den site was selected, porcupines foraged in a non-random manner, selecting western hemlock (Tsuga heterophylla) > 9.9 cm diameter at breast height. Amabilis fir (Abies amabilis) of all stem diameter sizes were avoided during foraging, as were western hemlock and sitka spruce < 9.9 cm diameter at breast height. Throughout the winter months, a single porcupine damaged an average of 0.54 new forage trees per day. A regression analysis revealed a strong relationship (r2 = 0.92) between midwinter mass of a porcupine and the number of new trees damaged per day. Porcupines chose the type of den they would use (stump, rock, log or pre-excavated den) based on the level of thermal protection provided. Stump dens and rock dens were used most frequently. Although fourteen of the 20 winter months of the study had average minimum temperatures below the lower critical temperature of the porcupines (-4°C), rock and stump dens provided enough thermal protection to maintain den temperatures above -4°C under most conditions. In areas thinned to 400 stems/ha, porcupines increased their foraging radius around the den site compared to porcupines in the control area. Porcupines in thinned areas compensated for the increased cost of travel to reach their forage trees by increasing the amount of vascular tissue removed per forage tree. Porcupines in the thinned areas travelled through deep snow more and in open areas more often compared to control animals, but did not lose more mass or suffer higher predation as a result. Based on the conceptual model developed from this study, in areas where porcupine feeding damage is a risk, forest managers are advised to avoid thinning to low stem densities and thin to > 1200 stems/ha, or avoid pre-commercial thinning completely.

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