COS 237-3 - Physical characteristics and fine root density within duff mounds of old-growth sugar and Jeffrey pines in a fire-excluded Sierran mixed-conifer forest

Abstract: Fire exclusion has profoundly impacted frequent-fire forests in western North America, disrupting fundamental ecological processes and 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 organic and 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, we 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. We specifically asked: 1) Do duff mound physical characteristics and fine root density differ between tree species, and tree size (diameter at breast height)? 2) Does fine root density differ between organic and mineral soil horizons? 3) Are topographic attributes and tree crown metrics important drivers of duff mound physical characteristics and fine root biomass for either tree species?

We found strong relationships between tree size and total mound volume and depth, with consistently greater variance explained for sugar versus Jeffrey pine. Fine root density was significantly greater in the upper mineral soil than the organic horizon for both species. The relationship between tree size and fine root density was weakly positive in the organic layer, while species dependent in the upper mineral soil: weakly positive for Jeffrey pine versus weakly negative for sugar pine. Slope, elevation, and tree size best explained total duff mound volume and maximum mound depth. Topographic position index (100 m) and topographic wetness index were also important predictors for maximum depth. Elevation was the most important predictor of fine root density in the upper mineral soil horizon, whereas topographic position index (30 m) and leaf area density best explained organic horizon fine root density. These findings suggest that efforts to mitigate large pine mortality prior to prescribed burning should prioritize the largest individuals in topographic settings most associated with larger duff mounds and greater root density.