Effects of biotic feedback on the pattern and rate of subalpine forest advancement.

摘要

Although many biogeographical and ecological studies have investigated how processes create patterns, the reciprocal effect of pattern on processes has not been well studied. In this research, I examine the potential for pattern to influence processes through spatial, biotic feedbacks at the alpine treeline ecotone in Glacier National Park, Montana. I use a combination of computer simulation models and dendrochronology to examine the effects of feedback on post-Little Ice Age advancement of subalpine forest in linear, finger-like strips that interdigitate with tundra. I first use a combination of physiologically mechanistic models to produce a measure of site quality, representing changing growing conditions related to climate. Feedback is then incorporated into a forest-stand simulation model as an increase in site quality due to habitat amelioration by upwind neighbors, primarily through snow deposition. Finally, I compare the model output with detailed, cross dated dendrochronological reconstructions of the pattern and rate of forest advancement.; The oldest trees sampled established in the early 1700s, but forest advancement was slow until the 1800s. Most establishment before the 1900s occurred within several meters of existing trees and/or boulders, suggesting that feedback was important to the survival of new trees. Toward the end of the 1800s, however, the rate and extent of establishment increased considerably, suggesting that continually improving climatic conditions began to override the importance of feedback. The model was able to produce the observed patterns and rates of advancement, and also predicted that these fingers of upright trees actually migrate upslope, as krummholz islands do. The model did not, however, adequately predict stand basal area or density, which is probably due to unrealistic mortality rates and competition for light. The results suggest that feedback is an important process controlling vegetation patterns and dynamics at treeline ecotones, which may affect their sensitivity to climate change. In addition, they suggest that models may need to be altered when used to simulate environments other than those for which they were originally developed.