This report synthesizes the literature and current state of knowledge pertaining to reintroducing fire in stands where it has been excluded for long periods and the impact of these introductory fires on overstory tree injury and mortality.
Only forested ecosystems in the United States that are adapted to survive frequent fire are included. Treatment options that minimize large-diameter and old tree injury and mortality in areas with deep duff and methods to manage and reduce duff accumulations are discussed. Pertinent background information on tree physiology, properties of duff, and historical versus current disturbance regimes are also discussed.
Sharon Hood, Ecologist
GOALS AND OBJECTIVES
This synthesis is organized into seven sections: (1) Fire Impacts on Trees and Causes of Tree Death; (2) Properties of Soil and Duff Related to Fire; (3) Historical and Current Fire Regimes and Stand Structures; (4) Improving Resilience and Physiological Capacity of Old Trees; (5) Management Options; (6) Monitoring; and (7) Knowledge Gaps. The first two sections provide a background for fire-related tree injury and ground fuels in fire-excluded stands. The third section contrasts historical and current stand conditions and disturbance regimes for historically fire-frequent forest types. The fourth and fifth sections provide information on treatment options at various scales based on pertinent studies and makes general treatment recommendations by forest type. Management Options also discusses defining treatment objectives, treatment prioritization, no action, and monitoring techniques. Differences between stand and individual tree monitoring, what variables to monitor, and appropriate monitoring time lengths are discussed in the Monitoring section. The last section identifies gaps in the scientific literature and recommends topics for future research.
Basal duff less than 2 inches (5 cm) deep the southeastern United States and less than 5 inches (13 cm) deep in the western United States at the base of mature trees are generally not considered hazardous. In these cases, prescribed burns can be conducted in accordance with locally accepted methods, objectives, and prescriptions.
Management options exist for areas with deep duff, but are limited. The provided decision key highlights the best available treatment options based on project scale. Options are: (1) burning when the basal duff layer is very moist during the dormant season and (2) reducing basal duff around individual trees by physical removal. The available burning window is narrow for prescribed burning when basal duff is very moist. It is also much more difficult to predict actual duff consumption, making it harder to achieve burn objectives. More overstory tree mortality should be expected if no individual tree treatments are implemented before broadcast burning. All individual tree treatments are labor intensive and require extra time to implement before broadcast burning. However, individual tree treatments widen the broadcast burn prescription window and can be completed years before broadcast burning. They should be considered additional tools in the manager’s toolbox when concerns exist that standard prescribed burning techniques will not meet objectives.
Thinning from below, followed by activity fuel treatments, will also reduce competition and increase water and nutrient resources available to old trees. Mechanical thinning can quickly manipulate stand structures to more closely resemble historical stand conditions and to be more resilient to future disturbances.
These efforts must be couched in the larger perspective of the importance of maintaining and perpetuating old trees on the landscape and with the realization that no action will likely result in significant tree mortality in forests that historically burned frequently. Acceptable levels of old tree mortality will vary by location and species. Places where little mortality is acceptable will warrant more intensive treatments. The high value of old trees on certain landscapes, especially at historically significant and high-use recreation sites, and the length of time required to produce large and old trees merit strong consideration of using the unconventional burning and individual tree treatments previously described. Some may hesitate to use a novel strategy to reduce overstory tree mortality when reintroducing fire to long-unburned areas because of the increased treatment costs or logistical difficulties. But remember that an individual tree treatment need only occur once to initially reduce the deep duff layer, and then regularly scheduled maintenance burning can be conducted without supplemental treatment.
Figure 1. Decision key of treatment options when reintroducing fire to long-unburned forests to reduce overstory tree mortality. Treatment options apply to forests that historically burned frequently. See RMRS-GTR-238 for detailed treatment descriptions.
Joint Fire Science Program #07-S-09
Hood, S. M. 2010. Mitigating old tree mortality in long-unburned, fire-dependent forests: a synthesis. Gen. Tech. Rep. RMRS-GTR-238. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. 71 p.