Prescribed Burning: Can We Do It Without Killing the Trees We Are Trying to Save?
The use of prescribed fire has become a major tool for restoring fire-dependent ecosystem health throughout the west and use will likely increase in the future. Prescribed burning to reduce fuel in some areas with large diameter and old-growth trees may cause significant mortality of these high-value trees even with low intensity fires. A probable cause is the extended burning of large duff accumulations resulting from 100 years of fire exclusion. Burning when duff moistures are low can lead to root mortality and basal girdling from consumption of the duff mounds, which may then lead to tree mortality. Our project objectives were: 1) determine if removal of the litter and duff by raking around the base of large-diameter pine trees will increase their survivability when exposed to prescribed fire, 2) estimate time required to complete raking treatment, and 3) develop relationships between duff characteristics (depth, moisture content, mineral content) and duff consumption. We found no significant differences in tree mortality between raked and unraked trees 2 years after the prescribed burns on the Lassen National Forest and 3 years after the burn on the Lassen Volcanic National Park. Raking reduced cambium injury and red turpentine beetle attacks in the burn units. The average time to rake duff around the first 60 cm (2 ft) to mineral soil was 16 minutes/person. Raking time depended on the depth of the duff mound. Laboratory tests suggests that sustained smoldering of Jeffrey pine duff occurs above 40-50 moisture content and 65-85% for ponderosa pine.
Raking allows managers to burn under a wider range of duff moisture scenarios without worry that the raking treatment alone will cause tree death. It is difficult to predict the percent of duff consumption in duff mounds based on pre-fire duff moisture to determine when to burn. We found that FOFEM does not accurately predict duff mound consumption, and should not be used for this purpose. Laboratory burning of ponderosa pine duff suggests that smoldering cannot be sustained above moistures of 65-85% and 40-50% for Jeffrey pine. However, these results were based on a small sample size and warrant future research.
In areas of deep duff, where the potential for basal cambium injury is high, raking minimizes injury to the tree bole near groundline from long-term duff smoldering. By reducing the residence time of the fire, the chance of cambium injury and bole char is reduced. In our study, we reduced the duff to mineral soil; however, this is probably not necessary. Raking the majority of the duff will prevent long residence times and the time required to rake. However, a large factor in burning large-diameter or old-growth is existing fire scars. If fire scarred trees are in the unit, raking to mineral soil and complete removal of duff in the scar is important. Two of the 3 dead trees to date in our burned units had fire scars that ignited and burned through.
Our study found that raking is a viable option when there is concern that burning will cause large-diameter, old ponderosa and Jeffrey pine mortality. Crews of 2-3 can clear duff away from a tree bole in approximately 6 minutes per tree. Raking alone did not reduce tree growth rates or affect mortality (Noonan-Wright et al. 2010). While raking may not be appropriate for every prescribed burn in old stands of ponderosa and Jeffrey pine, it should be considered a tool managers can use when trying to limit tree mortality from fire.