Levels of delayed tree mortality following early and late season burns in ponderosa and Jeffrey pine forests
Since 1999, approximately 800 000 wildland fires have burned an estimated 28 million hectares in the U.S. Generally, fewer wildfires are affecting larger areas at increased intensities and severities than during prior decades. Today, prescribed fire is commonly used to reduce the buildup of hazardous fuels and to restore fire-adapted forest ecosystems. Forest managers are increasingly conducting burns in the spring when atmospheric conditions may be more favorable, but concerns abound about potential increases in the amount of tree mortality, caused both directly from the prescribed fire and indirectly from bark beetle attacks.
A primary objective of using prescribed fire in ponderosa and Jeffrey pine forests is to reduce surface fuel loadings and densities of small trees (e.g., <20.2 cm dbh) that serve as ladder fuels and permit surface fires to become crown fires, while limiting undesirable levels of mortality within the larger diameter classes. In our study, spring and fall burns were equally effective at reducing fine surface fuels and ladder fuels (i.e., small trees that provide vertical continuity between surface and canopy fuels), but fall season burns were more effective at reducing large downed woody fuels (>10-h fuels). Few meaningful differences in levels of indirect tree mortality attributed to bark beetle attack were observed between spring and fall burns. We conclude that fall prescribed burns may be more effective at achieving overall management goals, but that concerns regarding increased levels of indirect tree mortality following spring season burns are unwarranted during the conditions under which this study was executed.