UBC Theses and Dissertations
Characteristics of historical forest fires in complex mixed-conifer forests of southeastern British Columbia Cochrane, Jared Douglas
Forest fires in southeastern British Columbia are considered the dominant natural disturbance to have shaped forest structure. In the mixed conifer montane forests of the Dry Cool Montane Spruce (MSdk) biogeoclimatic subzone, we have limited understanding of the characteristics of fire which have resulted in current forest structure. A better understanding of fire in these forests is needed to improve forest management and ecosystem restoration strategies which seek to emulate natural disturbance. To assess the historic role of fire in mixed conifer forests and to test the null hypothesis that fires do not vary spatially or temporally, this study uses cambial fire scars to analyze the historic frequency of fire in 20 stands that are statistically representative of complex, mixed-conifer forests in the southern Rocky Mountain Trench of British Columbia. Determining the date of cambial injury on a tree is an important objective of ecological research that determines the timing of disturbances such as fire, tree falls, or human modification of trees. Methods to determine scar dates require either a full stem cross-section or a partial cross-section of the wounded area. The latter method is less destructive however it is rarely used in British Columbia due to potential violations of established standard of care procedures regarding wildlife and danger trees. I developed procedures and protocols that provide a standard of care that was reviewed by WorkSafeBC and found to meet the intent of the Occupational Health and Safety Regulations ensuring the health and safety of workers. These procedures allow large, old trees of interest to researchers to contribute to the ecosystem over the long term and prevent the creation of dangerous trees that may threaten the safety of other forest users, making a valuable contribution to future research using cambial injuries in British Columbia. Spatial variation in fire is an important driver of forest heterogeneity at the stand and landscape scale. Using cambial fie scars on trees sampled at 20 study sites, I determined that fire frequency varied considerably between and among studied plots. I expected fires would be more frequent in plots with southern aspects than northern aspect plots. Instead, using logistic regression, I found fires to be more frequent on plots with northern aspects plots than southern aspect plots. Plot elevation, slope angle and solar radiation significantly influenced fire frequency, while plot aspect and latitude did not. Differences in season of fires were predominantly the result of differences in phenology between the two most commonly sampled tree species, Douglas-fir (Pseudostuga menzeisii var glauca (Beissn.) Franco) and western larch (Larix occidentalis Nutt.). Temporally, fire was most frequent during the time period of documented European settlement and the least frequent during the modern fire suppression period. My research results have important implications for forest management strategies which emulate natural disturbance to promote ecological resilience. Historically, fire occurred in the complex montane forests at much higher frequency than is currently classified using the Natural Disturbance Types (NDT). As a result, forest management strategies are likely preventing the occurrence of low- to moderate-severity disturbances in these forests, reducing their ecological resilience. Future management strategies should incorporate the variability observed in this study at the stand and landscape scales and return low- to moderate severity disturbances to these stands more frequently. Ecosystem restoration in these forests should be prioritized on stands that have deviated the most from the historic frequency of fires; particularly in stands which are at lower elevations and closer to valley bottom of the Rocky Mountain Trench.
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