Understanding Fire Skips in Juvenile Pine Plantations: Exploring the Relationship Between Fuel Moisture Content and Burn Severity

Abstract

Wildfires are increasing in size and severity due to climate change, and pose a serious threat to humans, infrastructure, and industry. In western North America, wildfires often leave unburned blocks of forest known as fire skips, fire islands, or fire refugia. Research elsewhere has suggested that young forests burn more severely, while mature forest is often found unburned as wildfire refugia. However, preliminary observations in central British Columbia suggest that some planted juvenile stands burn less severely than the surrounding mature forests. The existence of these fire skips could be due to differences in fuel moisture content (FMC), stand structure characteristics that influence fire spread, or both. To identify the role of FMC in fire skips, this study develops relationships between 6116 individual ground-based FMC measurements and remote sensing data collected in 2021 and 2022. Remotely sensed estimates of pre-fire FMC were then used to analyse burn severities in different stand types encountered in three major wildfires in central British Columbia in 2017 and 2018. Juvenile plantations, mostly dominated by lodgepole pine (Pinus contorta var. latifolia), had lower burn severity than mature and recent cutblocks (open sites) in three case study areas. No significant relationships were found between pre-fire satellite-estimated FMC and burn severity in mature forest and juvenile plantations. However, at open stands, moderate correlations between estimated duff FMC and difference normalized burn ratio (dNBR) were found in two case study areas. This work provides tools for assessing pre-fire FMC at large spatial scales and suggests that the limited fire severity in juvenile stands may be a function of stand characteristics other than pre-fire FMC.