Density, disease, and fuels treatment effects on large tree growth and mortality

Background/Question/Methods Mechanical thinning and prescribed fire are widely used in many fire-suppressed western forests to reduce fuels and restore ecosystem structure and function. These treatments should reduce competition for resources, increasing growth of the large leave trees and the treated stand's potential future carbon storage. Several studies, however, have reported significant increases in post-treatment large tree mortality and different growth responses between leave trees in the same stand. We examined potential factors affecting growth and mortality on all large (> 75 cm dbh) trees in 18 four ha plots treated with 3 levels of thinning crossed with two levels of prescribed fire seven years after treatment. We made pre- and post-treatment measurements of local stem density, pest and pathogen presence, mechanical and burn injury, and diameter growth on more than 3000 mixed-conifer trees. To accurately measure annual growth response, we also collected two increment cores on a subset of 200 trees stratified by species and treatment. Results/Conclusions Although prescribed fire treatments were low intensity, large tree mortality was significantly higher in all burn treatments. On average, radial growth increased with thinning intensity however individual responses varied by species, local density, and pest incidence. We examined different pathways by which these factors might influence growth and mortality using structural equation modeling. Results suggest higher pre-treatment stem density is associated with higher post-burn rates of bark beetle damage and mortality in sugar and Jeffrey pine, when char occurs on > 180o of the bole. The greatest increases in pine growth rates were associated with low post-treatment density in the thin-only treatments and low pre-treatment density and bole char in the burn treatments. Mortality of white fir was only associated with bole char in burn treatments, while white fir and incense cedar growth were associated with reductions in local stem density. Large pine mortality resulted in continued dominance of fire-sensitive, shade-tolerant species. Further research is needed to examine whether large pine survival rates would improve with increasing lag periods (> 1 year) between thinning and burning treatments.