Stand conditions alter seasonal microclimate and dead fuel moisture in a Northwestern California oak woodland

Abstract

Fuel moisture exerts considerable influence on fire behavior and effects but varies substantially spatially and temporally. A portion of the spatial variation in dead surface fuel moisture of woodland and forest ecosystems is likely attributed to differences in stand conditions (i.e. stand density and composition). Existing evidence suggests that the role of stand conditions on dead surface fuel moisture is inconsistent across studies and more research examining the effects of stand conditions on seasonal microclimate and fuel moisture is needed. This study examined the effects of different stand conditions within Oregon white oak (Quercus garryana) woodlands and forests, including intact oak-dominated stands without Douglas-fir (Pseudotsuga menziesii), oak stands that have been invaded by Douglas-fir (encroached), and thinned stands with Douglas-fir removed within Oregon white oak ecosystems of northern California on seasonal variation of microclimate and dead surface fuel moisture. Stand condition had a strong effect on microclimate and fuel moisture over time with thinned stands consistently having warmer and drier conditions with lower fuel moisture than encroached stands in most surface fuel components. Differences in fuel moisture among stand conditions were most pronounced in the late spring and early fall. Observed fuel moisture values did not correlate well with estimates from remotely automated weather stations or process-based calculations, especially when observed fuel moisture values exceeded 20%. Collectively, these findings provide strong evidence that stand conditions can modify microclimate that contributes to variation in dead surface fuel moisture within Oregon white oak ecosystems of northern California. These differences are often not considered within commonly used fuel moisture estimates, emphasizing the need to develop better models and tools to allow managers to more accurately anticipate fuel moisture conditions in varying stands to model landscape variation in fire behavior and effects and to meet fuel reduction and restoration objectives.