Master of Science degree with a major in Natural Resources, option Forestry, Watershed, & Wildland Sciences
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Fire exclusion has profoundly impacted frequent fire forests in western North America, disrupting fundamental ecological processes while leaving large, old pine trees vulnerable to drought, insects and disease, and fire. Forest managers want to increase the pace and scale of prescribed burning, yet heavy accumulations of organic material (duff mounds) at the bases of large pines can smolder for prolonged periods, damaging the cambium or consuming fine roots occupying the O horizon and/or upper mineral soil horizons. Increased duff mound depth is associated with greater mortality risk during prescribed fire, yet the biotic and abiotic drivers of duff mound accumulation and fine root density in large old pines is poorly understood. To understand the relative importance of factors influencing duff mound physical characteristics and fine root density, I combined field-collected duff mound and tree data with lidar-derived tree crown and topographic metrics for 324 large, old sugar (Pinus lambertiana) and Jeffrey pine (Pinus jeffreyii) at Teakettle Experimental Forest, a mixed-conifer forest in the southern Sierra Nevada, CA, USA which last experienced wildfire in 1865. I specifically asked: 1) Do duff mound physical characteristics differ between tree species, and tree size (diameter at breast height)? 2) Does fine root density differ between tree species, tree size, and between the O horizon and upper mineral soil horizon? 3) Are topographic attributes and tree crown metrics important drivers of duff mound physical characteristics and fine root density for either tree species?
I found strong, positive relationships between tree size and total duff mound depth and volume for sugar and Jeffrey pine. Fine root density was greater in the upper mineral soil than the O horizon for both species. Fine root density increased with tree size in the O horizon, but was species dependent in the upper mineral soil: with a weak negative relationship for sugar pine and no relationship for Jeffrey pine. In terms of lidar-derived metrics, leaf area density, slope, and topographic position index (100 m) best explained total duff mound volume and maximum mound depth. Topographic wetness index (TWI) was the most important predictor of fine root density in the upper mineral soil horizon, whereas leaf area density, topographic position index (30 m) and TWI best explained O horizon fine root density. Overall low variance explained in models is likely due to uncertainty in tree age and time since the last fire consumed duff material for individual trees. These findings suggest efforts to mitigate large pine mortality in preparation for prescribed fire treatments should prioritize the largest individuals, while accounting for heightened risk associated with specific topographic features.
Bartley, Jules, "Physical characteristics and fine roots within duff mounds of old-growth sugar and Jeffrey pine in a fire-excluded Sierran mixed-conifer forest" (2023). Cal Poly Humboldt theses and projects. 680.