Natural forest floor clearings around trees in Jeffrey pine forests reduce tree damage and mortality following wildfire

Abstract

In the western United States, trees in pine-dominated forests historically characterized by frequent, low severity wildfires often support deep litter and duff accumulations due to the fire exclusion policies of the last century. These accumulations at the bases of trees can smolder for long periods of time after the passage of fire, which can result in high temperatures, cambial injury, and subsequent tree mortality. As a result, prescribed fire prescriptions often call for the manual removal of litter and duff from the bases of large trees by raking or other means before fire is applied. Here we report on the presence of naturally formed, circular clearings in the soil surface litter layer that occur around the bases of tree boles in seasonally dry forests dominated by Pinus jeffreyi (Jeffrey pine) in eastern California and Baja California (along the eastern edge of the North American Mediterranean-climate zone). We show that such clearings significantly reduce fire severity and tree mortality in surface fires. In the Lake Tahoe Basin (LTB) study area, Jeffrey pine individuals surrounded by clearings experienced much lower bole char heights, and were 1/3 as likely to support crown torching as individuals lacking the clearings; mortality of Jeffrey pine lacking clearings was more than five times higher than individuals surrounded by clearings. Clearings also reduced mortality in incense cedar and white fir. Data from both the LTB and Inyo National Forest (INF) study areas show that clearings are larger and much more common in forests that have recently burned at low severity. In both locations, natural clearings around trees were about four times more common, and existing clearings about three times wider, in recently burned versus long unburned (>60years) sites. Logistic regression models show that clearing size, specifically the percentage of tree bole surrounded by a clearing, is an important predictor of Jeffrey pine survivorship in low to moderate severity fires, but clearing measures have never been included in previously published assessments of tree survival. We suggest that these previously unreported clearings are important features of Jeffrey pine and related semiarid forests and we recommend that more in-depth studies be made of their occurrence, formation, and ecological relationships.