Effects of uncertainties about stand-replacing natural disturbances on forest-management projections

Designing policies for long-term forest management is difficult, in part because ecological processes that drive forest structure and composition interact strongly, both spatially and temporally, with the many values we want to obtain from the forest. Using the Robson Valley in east-central British Columbia (Canada) as a study area, we developed a spatio-temporal landscape model to assess the effects of uncertainties about stand-replacing natural disturbance regimes on indicators related to the sustainability of forest harvesting and biodiversity. Results show that key timber policy indicators were relatively less sensitive to natural disturbance regime parameters than were the biodiversity indicators of seral stage distribution and tree species composition. The other biodiversity indicator we examined, structural connectivity among old-forest patches, was among those indicators least sensitive to any of the parameters we studied. Other timber supply indicators including non-recoverable losses and volumes and areas disturbed were the most sensitive to both the particular natural disturbance agent chosen and to the parameters describing its behaviour. Projections of a range of scenarios for present and alternative natural disturbance and management regimes for the study area show that most indicators varied from less than 1 up to 93% from the value of the present management/disturbance regime. Generally, three alternative management policies had weak-to-moderate capabilities of reducing effects of natural disturbances. Despite the range of uncertainties explored, the results provided little indication that at the scale of the whole study area, current timber harvesting targets are not sustainable over the long term. However, our findings highlight the lack of knowledge about the future, particularly about changes in climate, resulting in significant uncertainty about the future condition of the forest and about future forest management opportunities.