Changes in forest structure and composition associated with unique land use histories: Implications for restoration

Many contemporary semi-arid forests of western North America are denser and have a greater proportion of shade tolerant species relative to preEuro-American settlement. While many causes have been invoked to explain these changes, the active suppression of fire since the early 1900s has been the most widely studied and cited. However, widespread logging in western North American forests has often predated effective fire suppression and has affected a majority of semi-arid forests. The extent to which historical logging has contributed to uncharacteristically high densities and other changes in contemporary forests have never been adequately quantified. Therefore, true elucidation of the causes of departures of contemporary forests relative to historical conditions may be incomplete. I studied ponderosa pine/Douglas-fir forests of the Northern Rockies to address four main questions: 1) has historical logging exacerbated the effects of fire exclusion on forest density, structure and species composition?, 2) What is the magnitude of this change relative to that due to fire exclusion alone?, 3) in the absence of fire, which structural components in unlogged vs. historically logged stands are mostly responsible for deviations from reference ranges of variability?, and 4) what is the magnitude of such deviation in logged vs. unlogged forests? Based on a paired design (n=23 pairs) of logged, fire excluded stands with unlogged, fire excluded stands I found that fire excluded, logged stands were twice as dense as fire excluded, unlogged stands, and had higher numbers of small living and dead trees. While unlogged fire excluded forests generally experienced minimal to no departures relative to the range of stand densities observed in reference, fire-maintained stands, most logged fire excluded forests experienced substantial departures. Responses to the interaction of logging and fire exclusion varied by habitat type, with significant departures in Douglasfir but not in ponderosa pine habitat types. The magnitude of the response was proportional to the intensity of historical logging. We suggest that unique restoration approaches are warranted for unlogged and logged, fire excluded forests and caution that fuel reduction and restoration policies which do not account for the legacy of logging may be ineffective in accomplishing their desired goals.