A method for characterizing and mimicking forest canopy gaps caused by different disturbances

Abstract

Forest canopy gaps are measurable indicators of past small-scale disturbances. These disturbances can impact forest stand dynamics in ways that help maintain and enhance many ecosystem values. Impacts are commonly assessed using timber production metrics based on average stand values, but metrics based on smaller scales are needed to assess impacts on nontimber resources. In this study, we developed a multivariate method of quantitatively characterizing individual canopy gaps in Douglas-fir/grand fir stands in the Blue Mountains of eastern Oregon. We then established a reference or target range of variability by profiling gaps according to numbers of snags, downed logs, gap size and shape, and recolonizing vegetation. Cluster analysis, multidimensional scaling, and classification "trees" were used with these profiles to quantify similarity among individual gaps. Gaps fell into four groups, which could be interpreted as three ecologically functional groups on the basis of their role in stand development. Gaps caused by the same agents commonly fell within the same functional group: (1) flooding, logging, spruce budworm, and windfall cause stand level impacts that reset the stand development sequence; (2) bark beetles, tree fall, and root diseases cause within stand impacts that altered the rate of stand development; and (3) stem decay and dwarf mistletoe, and, under some circumstances, all disturbance agents, could kill trees yet have no impact on stand development except create space into which neighboring trees expand. We present a simple test to monitor how gaps change with silvicultural manipulations. Results indicate that this method offers (1) a way to systematically guide silvicultural manipulations aimed at mimicking natural disturbances at the canopy gap scale, and (2) a way to monitor the effectiveness of such silvicultural treatments and management actions using gap-based silviculture prescriptions.