Spatial and temporal dynamics of fire and vegetation change in Thunder Creek watershed, North Cascades National Park, Washington
Little is known about the history of fire and vegetation in the North Cascade Range. I conducted two studies in the lower Thunder Creek watershed, North Cascades National Park to evaluate past fire and forest vegetation dynamics. The first study evaluated forest development following a series of fires c. 150 years ago. Tree species, age and size data were sampled along altitudinal transects. Tree species distributions and relationships with environmental factors were analyzed using multivariate and regression techniques, and forest development was evaluated from tree age and size frequency distributions. The 7.5 km2 study area supports an unusually high number of conifer species (12). A combination of steep environmental gradients, slow rates of forest succession, frequent fire, and other disturbances maintain a diversity of species assemblages and structures. The second study reconstructed a Holocene fire and vegetation history at a montane site from lake sediment charcoal, macrofossil and pollen records. During the early Holocene (>10,500 to c. 7700 cal year BP) forests were likely open grown and dominated by Pinus contorta. During the mid Holocene (c. 7700 to c. 5200 cal yr BP) Pinus contorta became uncommon and Pseudotsuga menziesii and Pinus monticola were dominant. Shade tolerant species including Thuja plicata and Tsuga heterophylla first appeared at the beginning of the late Holocene (5200 cal yr BP to present). Chamaecyparis nootkatensis appeared most recently at 2000 years BP. Fire frequency does not significantly change over millennial time scales, but fire return intervals are highly variable. Of the 62 detected fires, 39 have a quantifiable response in the macrofossil record. Overall, peaks in charcoal accumulation rates are associated with an initial peak followed by a drop in macrofossil accumulation rates. Douglas-fir may be favored by fire with a strong peak in accumulation rates following fire events whereas western hemlock and subalpine fir decline following fire events. Together, these studies demonstrate the spatial and temporal variability of fire and vegetation in mountain forests. Climate is a major driver of vegetation change over millennial time scales, and fire and other disturbances are important agents of change at decadal to century time scales.
- Forestry