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Effects of variable density thinning and burning treatments - JFSP Final Report

Author(s): Jeffrey M. Kane, Alexis Bernal
Year Published: 2019
Description:

Over recent decades, increases in substantial tree mortality events have coincided with severe drought and bark beetle outbreak. This has prompted forest managers to find treatments that enhance resistance to disturbances. Variable density thinning is an alternative management method intended to increase spatial heterogeneity, with the potential to influence levels of bark beetle-associated mortality. However, there is limited research on the ecological consequences related to the structures that characterize variable density thinning. This study evaluated how structures within variable density thinning, that were burned and unburned, affect resistance to bark beetle outbreak, whether the influence of these structures change with scale, and what spatial patterns of bark beetle-associated mortality arise in variable density thinning treatments following a drought. We conducted stand inventories of eight variable density thinning units (four prescribed burned and four unburned) within the Stanislaus-Tuolumne Experimental Forest. We collected data on all trees ≥ 25 cm diameter at breast height and recorded species, diameter at breast height, and mortality agent. We used this data to conduct spatial analysis evaluating the effects of total basal area, intraspecific host proportion, intraspecific host gaps, quadratic mean diameter, and canopy cover on tree mortality at different scales. Our results showed that bark beetle-associated mortality was relatively low in sugar pine (14 %) and equivalent to other levels of reported mortality amongst white fir (24 %) during this most recent drought and insect outbreak in the Sierra Nevada. Prescribed fire treatments did not contribute to differences in tree mortality amounts compared to unburned plots. We found that the proportion of intraspecific hosts was positively associated with white fir mortality at small scales (10 to 20 m), while quadratic mean diameter was negatively associated with mortality at larger scales (30 to 50 m). Increased canopy cover was positively related to bark beetle-associated mortality in sugar pine at 10 m, while proportion of intraspecific hosts had a stronger positive association with mortality at larger scales (20 to 50 m). Despite structural characteristics influencing proportion of bark beetle-associated mortality, spatial analyses revealed that white fir and sugar pine mortality was limited across space, with clustering of mortality occurring at scales less than 14 m. These results suggest that treatments intended to increase spatial heterogeneity and reduce fuel loads may have the positive benefit of mitigating the levels and spatial patterns of bark beetle-associated mortality during severe and prolonged drought.

Citation: Kane JM and Bernal A. 2019. Effects of variable density thinning and burning treatments on the spatial patterns of drought-related tree mortality - Final Report to JFSP for Project 18-1-01-65, 36 p.
Topic(s): Fire & Wildlife, Invertebrates, Mountain pine beetles, Fuels, Fuel Treatments & Effects, Mechanical treatments, Prescribed Fire-use treatments
Ecosystem(s): None
Document Type: Technical Report or White Paper
NRFSN number: 20111
Record updated: Oct 16, 2019