Differentiating mature and old-growth forests in the Upper Foothills and Subalpine Subregions of west-central Alberta

Abstract

Weyerhaeuser Company Ltd. is developing harvest strategies that will maintain appropriate levels of late to very late seral stages ("old growth") in its Drayton Valley Forest Management Area. This management area encompasses 490 570 ha in the Foothills and Rocky Mountain Natural Regions of west-central Alberta. In planning for future forest landscapes, Weyerhaeuser intends to maintain a range of age structures consistent with the ecological processes characteristic of each natural region and subregion. The absence of a discrete point separating mature forest from old growth means that the age at which a stand is currently identified as "old growth" and subject to special management practices is arbitrary. In a research study initiated in the summer of 2000, we seek to understand the differences in structure and composition between forests of various ages and topographic site conditions (elevation, aspect, and slope angle). Using 95 sampling plots in a 123-km2 study area in the Upper Foothills and Subalpine Natural Subregions, we quantified vegetation structure and composition for stands ranging in age from 70 to 300 years. Variables measured and analysed included live-tree height and diameter, snag density, diameter and decay class, downed woody material volume, diameter and decay class, vascular plant species richness, sapling and regeneration density, and duff depth. An old-growth index was developed for each sampled stand that took into account multiple attributes. Preliminary results indicate that specific attributes (snag basal area and density, decay stage and density of downed woody material, variation in live-tree age, and variation in live-tree height and age) separate a younger forest from a more mature one and hence may describe "old-growth" conditions. The age of onset of these old-growth attributes is variable but appears to occur between 160 and 180 years. Key factors other than stand age that contribute to or modify the development of old-growth attributes (as measured by the old-growth index) are elevation and moisture regime (as modified by site position). Further investigation is required to more accurately assess the effect of site factors on old-growth attributes. These results are now used by Weyerhaeuser to address retention of late seral stages in long-term forest planning. Key words: old growth, mature forests, old growth protection, forest management, Alberta, Weyerhaeuser, Rocky Mountains foothills