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Effects of fire on vegetation in the interior douglas-fir zone Hanel, Claudia

Abstract

To investigate the effects of fire on vegetation in dry Douglas-fir (Pseudotsuga menziesii var. glauca) forests, as well as year to year fluctuations in understory vegetation, three studies were conducted. In the first study, the percent cover of different understory plant growth forms was monitored in 1 m2 plots at two sites for three years. Significant (p < 0.05) fluctuations over time were observed for all growth forms, but these depended on site, and were not always similar in adjacent stands with different canopy closure. Only forbs showed a generally increasing trend over the three-year period at all sites. None of these fluctuations was related in a simple way to the temperature or precipitation recorded at nearby weather stations in the same growing season. The second study examined the response of understory vegetation in 1 m2 plots to fire severity and reduction of competition by tliirming of the overstory in moss and herb dominated communities. High fire severity was associated with a lower cover and number of species persisting in the plots, and a higher cover and species richness of invading species, than low fire severity. Existing moss and lichen species were almost elinninated by fire of any severity, and most persisting species were vascular plants. The invaders with the highest cover were pioneer bryophytes colonizing patches of mineral soil. The changes in cover and number of species of all growth forms after removal of overstory competition were less than those after burning, suggesting that fire effects in forest thickets are not just due to reduction of competition. Logistic regression models predicting the probability of mortality of fire injured trees two years after burning in standing forest were developed. Percentage of the crown length scorched was the best predictor of mortality, but species, diameter at breast height and maximum height of bark char were also significant. Lodgepole pines (Pinus contortd) were more likely to die than ponderosa pines (Pinus ponderosa) and Douglas-firs when all other variables were held constant. Six tree mortality models developed in the United States were also tested. Some, but not all of them adequately predicted the tree mortality of the species they were developed for, but the models created in this study were generally superior for the sites that were .studied.

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