Ecological and social implications of employing diameter caps at a collaborative forest restoration project near Flagstaff, Arizona, USA

The issue of implementing diameter caps as a means of preserving old-growth trees on forest restoration projects continues to permeate collaborative land management discussions and treatment decisions on public lands in the Southwest and, indeed, throughout the western United States. We examine the ecological and social results of the collaborative Fort Valley Ecosystem Restoration Project on U.S. Forest Service lands near Flagstaff, Arizona. Since this experiment had areas treated with and without a diameter cap, we sought to determine: 1) the ecological consequences of implementing a 16-inch diameter cap, 2) whether the fears and concerns of the environmental groups who proposed the diameter cap were, in fact, warranted, and 3) how the local collaborative responded to implementing the diameter cap. The ecological data revealed that a site's management history played a major role in how a diameter cap would affect the restoration of stand structure in terms of tree density and tree size. The data suggest that, if implemented, diameter caps are best applied in a manner that takes into account both site conditions and stand management history. In general, we found the concerns about the loss of old-growth trees due to thinning treatments were not realized at Fort Valley. Finally, stakeholder surveys indicate that while the discussion of diameter caps caused tension within the collaborative group, the overall goal of forest restoration was not compromised.