The influence of fire and windthrow dynamics on a coastal spruce–hemlock forest in Oregon, USA, based on aerial photographs spanning 40 years

To gain understanding of patterns in forest structure and their causes, we mapped the distribution of three canopy cover classes and measured change in one of them over 40 years using aerial photographs for the 500 ha Neskowin Crest Research Natural Area (Lincoln and Tillamook Counties, Oregon). One class (fine texture, trees of uniform crown diameter and height) covered about half the area; it was identified as second growth originating after a large regional fire in 1845. The other major class (coarse texture, trees of variable crown diameter and height), occupying about 35% of the area, was unburned or partially burned in 1845. The third class (openings with down stems visible on the ground) was blowdown patches. The blowdown patches were very small in 1953; they grew incrementally, and by 1994 had coalesced into a large patch occupying about 15% of the area. A long-term windstorm susceptibility model developed for southeast Alaska identified the region where the blowdown patch occurred as being very susceptible to maritime windstorm disturbance. This correspondence between predicted susceptibility to damage and actual blowdown supports the hypothesis that windstorm effects may be strongly constrained by topography. The results also suggest that blowdown in storm-susceptible topographic settings can be the result of multiple windstorm events over time, rather than a single event. The resulting forest is a mosaic of large multi-aged chronic-disturbance patches embedded in a matrix structured by fine-scale patch processes. A consequence of a constraint on blowdown is that at the scale of hundreds of hectares biomass may not fluctuate strongly over time unless stand-destroying fires occur.