Prescribed Burning to Protect Large Diameter Pine Trees From Wildfire - Can We Do It Without Killing the Trees We Are Trying to Save?

PRINCIPAL INVESTIGATOR

Sharon Hood, Forester

CO-INVESTIGATORS

Jim Reardon, Sheri Smith, Danny Cluck

INTRODUCTION

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. Current management guidelines in Region 5, under the Sierra Nevada Framework, call for the use of fire as the primary management tool to deal with high surface fuel loads and dense conifer understories. The window of opportunity for carrying out a prescribed burn is limited by weather, fuel conditions, air quality concerns, and potential mortality of large diameter trees. Attempts to burn stands have often resulted in increased mortality of these trees. Even with mechanical thinning to reduce ladder fuels and the probability of crown damage, the problem of deep duff mounds and below-ground injury still exists. Increased mortality of large diameter and old pine following fire has been reported in other areas as well and there is increased concern about maintaining large-diameter trees on the landscape (Kolb et al. 2007).

Accumulation of litter and duff around large diameter trees has reached unprecedented levels in the California eastside pine type as a result of 100+ years of fire exclusion. The unprecedented litter and duff accumulations observed in most western forests are well documented and described by various authors including Sackett et al. (1996), Covington et al. (1997), Sackett and Haase (1998), and Haase and Sackett (1998). Because duff smoldering does not cause intense fire behavior, its consequences are often overlooked.

Several studies have attributed large diameter tree mortality to basal injury caused by duff mound smoldering and bark beetle attacks. Long-term smoldering can cause soil heating above 60o C, the temperature required to kill living tree tissue. Hartford and Frandsen (1992) reported soil temperatures under smoldering duff mounds of 400oC, with temperatures in duff above 100oC for over 16 hours, compared to soil temperatures of less than 80oC and duff temperatures above 100oC for 1 hour under burning slash. Temperatures in smoldering duff mounds were above 300oC for 2-4 hours during a prescribed burn in Glacier National Park, resulting in the mortality of 45% of the cambium samples (Ryan and Frandsen 1991). Bradley and Tueller (2001) stated that a burned tree was 24.81 times more likely to be attacked by a bark beetle than an unburned tree, and that trees with deep soil charring were 9.81 times more likely to be attacked than all other trees combined.

Although some work has been done on the effect of duff and litter removal, little is known about the factors that determine its success or failure as a practical management tool. Sample sizes in the existing studies were very small and there were no controls or raking-only treatments. Our study examines the feasibility of removing deep duff mounds around ponderosa and Jeffrey pine trees by raking prior to prescribed burning. By removing duff, managers could burn under a wider range of weather conditions and different seasons, leading to more acres treated with potentially fewer bark beetle attacks and less large-diameter tree mortality. It includes a duff removal-only treatment, in order to determine if the impact of raking alone causes tree mortality.

Three planned burned units were chosen that contained large (>63.5 cm (> 25 inches) DBH) ponderosa and Jeffrey pine. These areas had not burned in over 100 years (Taylor 2000). Two sites were established in the Grays Flat area on the Eagle Lake District of the Lassen National Forest (LNF) and a third site in the Prospect Peak burn unit in Lassen Volcanic National Park (LVNP). Ponderosa and Jeffrey pine trees were chosen randomly throughout the units. All sample trees in each unit were paired based on species and similar size, vigor class, and close proximity to each other. One tree in each pair was then randomly selected to receive the raking treatment. The LVNP study area of the Prospect Peak unit was prescribed burned June 14-15, 2005. The LNF Grays Flat thinned unit was prescribed burned October 21, 2005 and the masticated unit was prescribed burned October 22, 2005. We measured woody fuel and duff consumption, tree injuries, and bark beetle attacks after the burn. Trees are monitored annually for additional mortality and attacks.

GOALS AND OBJECTIVES

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
3. Develop relationships between duff characteristics (depth, moisture content, mineral content) and duff consumption.

KEY RESULTS

• Raking the duff mounds away from tree boles did not cause tree mortality. Three out of 189 raked trees (<2%) in this study died and one of those was due to a fire scar igniting and burning out the center of the tree. The trees in this study have been raked for 4 years on the LNF sites and 3 years on the LVNP site.
• Raking reduced the probability of red turpentine beetles attacks in the burned units. We found significantly more red turpentine beetle attacks on the unraked, burned trees than the raked, burned trees. Only 1 tree was attacked by RTB in the unburned units. While the number of trees attacked by western pine beetle or Jeffrey pine beetle was low, most of the attacked trees had previously been heavily attacked by RTB. This seems to indicate the burned trees with numerous RTB attacks are susceptible to attacks by primary bark beetles. It is unclear if it was the charring of the tree bole or cambium injury that attracted the RTB.
• Raking decreased cambium injury by limiting heating at the base of the trees in the burned units. However, to date very few trees have died in the burned units, whether or not they were raked. We believe that it is still too early in our study to expect much tree mortality. If mortality is going to occur, it will probably take several years. Based on the results to d ate, 2 and 3 years post-fire, the decision to rake should be based on the management objectives for large trees in the prescribed fire area, current bark beetle activity, amount of duff around the large trees and the burning conditions. Given conditions similar to those in this study, raking may not be worth the time or effort involved. However, it is preliminary to conclude from our study whether raking will reduce tree mortality.
• 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 duff suggests that smoldering cannot be sustained above moistures of 65-85% for ponderosa 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. 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.

REFERENCES

Bradley, T.; Tueller, P. 2001. Effects of fire on bark beetle presence on Jeffrey pine in the Lake Tahoe basin. Forest Ecology and Management. 142: 205-214.

Covington, W. W.; Fule, P. Z.; Moore, M. M.; Hart, S. C.; Kolb, T. E.; Mast, J. N.; Sackett, S. S.; Wagner, M. R. 1997. Restoring ecosystem health in ponderosa pine forests of the Southwest. Journal of Forestry. 95: 23-29.

Haase, S. M.; Sackett, S. S. 1998. Effects of prescribed fire in giant sequoia-mixed conifer stands in Sequoia and Kings Canyon National Parks. In: Fire in ecosystem management: shifting the paradigm from suppression to prescription, proceedings of the 20th Tall Timbers fire ecology conference, 1996 7-10 May, Boise, ID. Tallahassee, FL: Tall Timbers Research Station: 236-243.

Hartford, R. A.; Frandsen, W. H. 1992. When it's hot, it's hot- or maybe it's not! (Surface flaming may not portend extensive soil heating). International Journal of Wildland Fire. 2: 139-144.

Kolb, T. E.; Agee, J. K.; Fulé, P. Z.; McDowell, N. G.; Pearson, K.; Sala, A.; Waring, R. H. 2007. Perpetuating old ponderosa pine. Forest Ecology and Management. 249: 141-157.

Ryan, K. C.; Frandsen, W. H. 1991. Basal injury from smoldering fires in mature Pinus ponderosa Laws. International Journal of Wildland Fire. 1: 107-118.

Sackett, S. S.; Haase, S. M. 1998. Two case histories for using prescribed fire to restore ponderosa pine ecosystems in northern Arizona. In: Fire in ecosystem management: shifting the paradigm from suppression to prescription, proceedings of the 20th Tall Timbers fire ecology conference, 1996 7-10 May, Boise, ID. Tallahassee, FL: Tall Timbers Research Station: 380-389.

Sackett, S. S.; Haase, S. M.; Harrington, M. G. 1996. Lessons learned from fire use for restoring southwestern ponderosa pine ecosystems. In: Conference on adaptive ecosystem restoration and management: restoration of Cordilleran conifer landscapes of North America, 1996 June 6-8, Flagstaff, Arizona. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range: 54-61.

Taylor, A. H. 2000. Fire Regimes and Forest Changes in Mid and Upper Montane Forests of the Southern Cascades, Lassen Volcanic National Park, California, U.S.A. Journal of Biogeography. 27: 87-104.

FUNDING ORGANIZATION

We acknowledge funding from the Joint Fire Science Program under Project # 03-3-2-04.

PRODUCTS

JFSP final report

The masticated treatment resulted in the development and testing of a new method to estimate fuel loadings in masticated fuels. This method was presented at the 1st Fire Behavior and Fuels Conference in 2006 and is described in:

Hood, S. M. and Wu, R. 2006. Estimating fuel bed loadings in masticated areas. In: P. L. Andrews and B. Butler, eds. Fuels Management-How to measure success: Conference Proceedings. 28-30 March 2006. Portland, OR. Proceedings RMRS-P-41. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fort Collins, CO: 333-340.

Preliminary results were presented at the 3rd International Fire Ecology and Management Congress in 2006.