Porcupine Population Fluctuations in Past Centuries Revealed by Dendrochronology

Abstract

In northern New England porcupines tend to concentrate in the winter in the none-tooplentiful rock outcrops and ledges, leaving faecal deposits several feet thick as evidence of repeated use. Hardwood trees, maple, beech and yellow birch in front of such dens have been suppressed to straggly shrubs. Near the perimeter of the feeding area near-normal trees bear many feeding scars (Plate 4c). At the 1942 Annual Meeting of the American Society of Mammalogists, the writer proposed that the history of porcupine occupancy of a given ledge area could be traced by dating the feeding scars in the nearby timber stands. An opportunity to delve into population fluctuations in past centuries did not present itself until 1946. At that time a serious eruption of porcupines in Mesa Verde National Park culminated in a degree of tree destruction that could not be ignored, even on an area dedicated to the conservation of wildlife. However reductional control of pest species is, at best, a temporary solution, and such mammal populations can be properly managed only when the dynamics of fluctuations in their numbers are clearly understood. No other location in the Western Hemisphere could have been selected that was better adapted to the initiation of a dendrochronologic study of mammal populations of past centuries. Dr A. E. Douglass of the University of Arizona and his associate, the late Dr Edmund Schulman, had worked extensively in south-western Colorado dating archaeological ruins and developing the dendroclimatic indices. A master dendrochronological key, based on Douglas fir (Pseudotsuga taxifolia) had been developed for the Park area (Schulman 1946, 1947). Dr Schulman visited Mesa Verde shortly after the first crosssections of porcupine feeding scars had been obtained; at that time his instructions on dating the injuries and his continued guidance over the years made possible the perfection of techniques here presented. During the winter in south-western Colorado, the porcupine (Erethizon epixanthum) feeds almost exclusively on the phloem layer of pinon pine (Pinus edulis) and western yellow pine (P. ponderosa). The outer corky layers of bark are chipped away and discarded. In the skilful removal of the phloem and cambium layers, the xylem ring laid down the previous growing season is unbroken. If the wound left by this feeding completely encircles the trunk or limb, the distal portion of the tree dies. If the wound is merely a 'cat face' it will remain discernible as long as the pinon lives. Western yellow pine and many other tree species close and obscure such a wound in a relatively few years by the in-rolling of new wood from the perimeter. The fact that pinon pine is a major component of the Mesa Verde vegetative cover was particularly favourable to the present study. Located in extreme south-western Colorado, the mesa rises precipitously some 2000 ft from the agricultural plains that surround it (from 6500 to 8500 ft altitude at Park Point on the northern escarpment). The northern escarpment is a series of rugged finger-like points where cliff, talus and a scattered stand of pinon pine make a favoured winter habitat for porcupines. The mesa, while having the appearance of a huge tableland, slopes