Schmidt, K.N. and D.P. Christian. 1988. Porcupine-eastern hemlock interactions at Hemlock Ravine Scientific and Natural Area. Final report submitted to the Nongame Wildlife Program, Minnesota Department of Natural Resources. 25 pp.
The present study was conducted to assess damage by porcupines to the hemlocks in Hemlock Ravine Scientific and Natural Area and to determine feeding patterns, selectivity for hemlocks and other tree species, and other aspects of the winter feeding ecology of porcupines in this area.
Porcupine transect data were collected weekly from 31 January 1988 to 1 May 1988. On 22 April 1988, a survey of the ravine adjacent ot HRSNA was done to assess use of feed trees there relative to HRSNA. To calculate preference ratios and electivity indices, a survey of vegetation in the ravine and adjacent top slopes was conducted.
In the ravine and top slopes porcupines utilized sugar maple (Acer saccharum) and white pine (Pinus strobus) with greatest frequency. The electivity values and preference ratios for these 2 species indicate weak selection by porcupines for sugar maple and very strong selection for white pine. Even though feeding appeared slight relative to that of white pine and sugar maple, porcupines selected hemlock as a food source with a frequency far exceeding its proportional occurrence in the area. Average size classes of feed trees with only trunk-bark removal were also above the average for the area. There appeared to be a seasonal shift by porcupines toward use of hemlock coinciding with milder spring temperatures. Five den sites were discovered in HRSNA. A density of one porcupine per 6.75 hectares (ha) was calculated for the area. Porcupines were observed roosting in nine different trees, all of which were utilized as feed trees.
Not only are porcupines specialists in terms of preferred species in an area but, for a particular tree species, they are selective about the individuals they feed upon. Porcupines at HRSNA used the same individual trees year after year as was evidenced by the presence of old scars on the majority of feed trees and by the fact that this year's feeding activity had commenced on last year's feed trees. It appeared that porcupines traveled about the area "testing" different trees for palatability as there were many occurrences of trees with only small bits of bark removed. It is not clear whether porcupines return to the same feed trees year after year because of high nutritional value of those trees or simply because the trees occur in the vicinity of a highly desirable den site. It seems intuitive that den sites influence in some way the occurrence of porcupines in an area. The 3 active dens in HRSNA are spaced approximately equidistant from one another along the slope of the ravine. Interestingly, all three dens also are located at about the same contour on the slope. The porcupines at HRSNA appeared to specifically seek out hemlocks. The relatively high electivity values and preference ratios support the hypothesis that porcupines recognize hemlocks in HRSNA and preferentially use them as feed and roost trees above what their abundance in the area would predict. However, the impact of porcupine feeding on hemlock is minimal in HRSNA. It is apparent that porcupines have much greater impact on sugar maple and white pine. The winter feeding activity of porcupines at HRSNA is quite similar to that observed elsewhere in the eastern portion of the range. The hemlocks at HRSNA do not appear to be attracting large numbers of porcupines to the area or radically influencing their behavior. It is possible that numbers of porcupines observed at HRSNA are near capacity for the area and that densities are limited by the number of suitable den sites and station tree areas in the ravine. There appeared to be a shift by porcupines into the hemlocks when maximum weekly temperatures rose to about 32 deg F.