Protecting old trees is an important part of restoration treatments. Due to logging and other ecological changes, old trees have grown scarce in most southwestern ponderosa pine forests. Many of those that remain are under increased stress due to competition from overcrowded younger trees, as well as from fire and insects. One of the primary aims of forest restoration is to preserve old trees – and to reduce competition around younger trees so that they grow large more quickly.

Values of Large, Old Trees in Ponderosa Pine Forests

• The increased structural diversity provided by large, old trees improves habitats for birds, insects, and other animals. For example, bats can roost under loose slabs of bark on older trees.
• Large, old trees often become long-lasting snags when they die, benefiting birds and other wildlife.
• Old trees, when not surrounded by large amounts of ground fuels or “ladder fuels” that can carry fire into their canopy, are largely resistant to low-intensity surface fires.
• Old trees have greater genetic diversity than even-aged groups of young trees, and provide forests a better chance of adapting to changing climatic conditions and other environmental factors.
• The presence of old trees, especially “yellow pines,” enhances the aesthetics and overall success of restoration projects.

Not by Fire Alone

Frequent low-intensity fires are an essential part of restoration treatments in southwestern ponderosa pine forests. They cycle nutrients, reduce woody fuel loads, maintain an open forest structure, and promote the growth of numerous understory plant species. Such fires produce a cascade of positive effects up the food chain, including insects, birds, and mammals that feed on the plants.

While prescribed fire by itself can reduce the risk of wildfire damage because it reduces fuel surface loads under certain circumstances, it can also cause high mortality of remaining old trees. This may not be quickly apparent, as foliage can remain green for a long time because of moisture stored in the stem. Evidence of harm to old trees from prescribed burns has begun to show on several study plots north of Flagstaff, where experiments have been ongoing since the 1970s. Without previous manipulation of forest fuel loads (such as raking duff away from trunks), up to 60 percent of old trees died within 20 years following controlled burns (Sackett et al. 1996). As a result, burning often needs to be combined with other treatments to maximally benefit ponderosa pine forests – and to prevent widespread fire-caused tree mortality.

Specific Ways to Prevent Fire-Caused Tree Mortality

• Burn when trees are dormant. Under a natural fire regime, most fires would occur in late spring and early summer. Burning the heavy fuel loads that follow restoration thinning treatments after long periods of fire exclusion, though, is more safely done when trees are dormant, in the colder times of the year. Trees survive scorch better when they are dormant than when they are allocating resources to growth. Root sensitivity to heat may be highest early in the growing season, leaving trees more vulnerable to mortality in early-season burns (Swezy and Agee 1991).
• Burn when forests are moist. Warm and dry weather can produce undesirable fire behavior that results in unplanned damage to large trees. In addition, researchers do not advise prescribed fire during periods of severe drought when trees are already stressed by lack of water. Burning at night can aid in keeping fire intensities low.
• Utilize fire spread types and ignition techniques that result in low-intensity fire. Backing fires, short run strip headfires, and spot ignition techniques assist in reducing tree mortality.
• Avoid burning on lava and very thin soils. Fulé et al. (2002)documented unexplained high tree mortality on a prescribed burn site atop cinder soils near Mount Trumbull in northern Arizona. They noted that some of the dying trees bore scars from natural fires much earlier in their lives, indicating they would normally be able to withstand the effects of fire. The researchers recommend that burning on cinders should be discontinued pending further investigation of tree mortality, and that thinning-only treatments should be considered for cinder soils.
• Rake fuels away from old trees to increase their survival rate. Thick piles of needles and bark flakes that have accumulated under large pines through a century or more of fire exclusion may cause higher soil and root temperatures during fires than those experienced by trees subjected to frequent, low-intensity fires. Glowing combustion of this material under large trees provides enough heat to penetrate even thick bark and soil layers, often resulting in tree mortality (Sackett et al. 1996). Raking fuels at least 2 feet away from the trunks of desirable trees can aid their survival.
• Manually clear out live and dead and downed fuel from beneath the canopy of trees to be protected. Large, woody fuels were once sparsely distributed in southwestern ponderosa pine forests due to frequent fires. Now stumps, downed trees, and fallen branches often exceed the average of 8 tons per acre (Sackett 1979). These materials may burn at high temperatures and for long durations. Manual removal or scattering of these fuels may be necessary before prescribed burning in order to maintain low fire intensities. Thinning, lopping, and scattering small-diameter ladder fuel trees within 50 feet of old trees can prevent mortality caused by canopy scorch or ignition. Crushing forest floor fuels and slash with a small bulldozer to reduce flame height has also been shown to be effective in some cases.
• Burn frequently to maintain healthy forests. Once fires are established as part of treatment in a restored site, they should be repeated frequently to prevent the buildup of needles, bark flakes, and other fuels, as well as the growth of seedling thickets. Burn intervals will vary according to local needs, conditions, and restrictions. Historical fire frequencies that often ranged from 2 to 12 years (Covington and Moore 1994) can guide burning prescriptions, but even longer intervals may help promote open forest conditions. Ultimately, fires set by managers or by lightning may again burn in late spring and early summer, as was historically the case—and when smoke disperses more readily.

This is a modified version of ERI's publication Working Paper 03: Protecting Old Trees from Prescribed Fire.

References

• Covington, W.W., M.M. Moore. 1994. Southwestern Ponderosa Forest Structure: Changes since Euro-American settlement. Journal of Forestry 92(1):39-47.
• Fule, P.Z., G. Verkamp, A.E.M. Waltz, W.W. Covington. 2002. Burning Under Old-Growth Ponderosa Pines on Lava Soils. Fire Management Today 62(3):47-49.
• Sackett, S.S. 1979. Natural Fuel Loadings in Ponderosa Pine and Mixed Conifer Forests of the Southwest. Research Paper RM-213. Fort Collins, Colorado: USDA Forest Service.
• Sackett, S.S., S.M. Haase, M.G. Harrington. 1996. Lessons Learned from Fire Use for Restoring Southwestern Ponderosa Pine Ecosystems. Pp. 54-61 in Conference on Adaptive Ecosystem Restoration and Management: Restoration of Cordillesan Conifer Landscapes of North America, tech. coords. W. Covington and P.K. Wagner. General technical report RM-278. Fort Collins, Colorado: USDA Forest Service.
• Swezy, D.M. and J.K. Agee. 1991. Prescribed-fire Effects on Fine-root and Tree Mortality in Old-growth Ponderosa Pine. Canadian Journal of Forest Research 21:626-634.