Fire and Root Disease Interactions in Coniferous Forests of the Inland West: Development of Decision Tools and Guidance for Supporting Healthy-Forest Management

Mee-Sook Kim, Ned B. Klopfenstein, Raini C. Rippy, Jane E. Stewart, Paul J. Zambino, Joanne M. Tirocke, and Walter G. Thies

USDA Forest Service – Rocky Mountain Research Station (1-6) and Pacific Northwest Research Station (7)

Long-term fire exclusion, introduced pests, selective timber harvesting, and selective planting have caused dramatic changes in forest-stand composition in much of the western USA. These changes have also contributed to heavy accumulations of fuels and ample fuel ladders that increase threats of severe wildfires. Consequently, many forest managers are practicing fuels treatments to lower the risk of severe wildfire. However, fuels treatments can affect many complex interactions within forest ecosystems, where even subtle disturbances can trigger unforeseen responses due to associated organisms.

Root diseases are among the most understudied and underestimated sources of mortality that cause buildup of wildland fuels. Historically, root diseases apparently played a predominantly beneficial role in many forests by favoring ecological processes of succession, decomposition, and stand renewal through fire. In most natural stands, native root diseases do not typically cause irreparable, large-scale damage to ecosystems or threaten the loss of any host species. Recently, shifts in stand composition resulting from natural and human-caused disturbances have contributed to increased damage from root-rot diseases. At sites with increased risk of root rot disease, changes in root-disease dynamics have the potential to influence forest growth and succession for centuries. Thus, it is prudent to critically consider the potential impacts from root disease before selecting management activities, including fuels treatments.

Various fuels treatments are available, but treatments routinely involve mechanical treatments and/or prescribed fire. Mechanical treatments, such as thinning, can remove fuels as forest products, including biomass for energy. Thinning can be selective for species and size class. Other mechanical treatments may involve removing fuels ladders, placing fuels on the forest floor, and/or reducing the size of downed fuels to accelerate decomposition. Prescribed fire is also commonly used to reduce fire risk; however, its effects on resulting stand structure are less predictable than mechanical treatments. Long-term success of fuels treatments in lowering the risk of severe wildfire depends in part on the long-term impacts on root diseases that contribute to subsequent fuels accumulation. Thus, fuels treatment should be selected to match the overall objectives for a stand, while addressing potential impacts of root disease dynamics.

Fuels management requires an ecosystem-level approach, because root disease (and other causes of tree mortality) result from complex ecological interactions. For example, tree thinning can increase the inoculum potential of root-rot pathogens that colonize stumps, residual root systems, and/or slash. New opportunities for root disease can also be created by thinning-associated wounds on remaining trees. Also, thinning treatments that select against seral tree species (e.g., pine or larch) may result in stands dominated by tree species that are more susceptible to root rots (e.g., true firs or Douglas-fir). Fuels treatments that cause physiological stress to host trees may also increase the likelihood of root-rot problems. Although slash burning may vary in its effects on root disease caused by different pathogens, superficial burning appears to have little long-term impact on the physical distribution of some pathogens (e.g., Armillaria species, Phellinus weirii, and Heterobasidion annosum) that occupy protected areas within structural roots. However, some measures may reduce root-rot risk by limiting or removing substrate that would be available for colonization by root pathogens. Thus, appropriate fuels treatments should be selected to reduce current and future levels of fuels, while avoiding undesirable increases in root disease. Also, some pathogens are specific to certain tree species.  For these reasons, an essential step in selecting appropriate fuels treatments is an on-site evaluation for the presence and distribution of each root disease likely to be present in an area proposed for fuels treatment. Root disease surveys include observing signs and symptoms on living trees, with additional examinations of snags, stumps, and overturned trees.

Currently, fuels reduction treatments are a prominent national issue; however, relatively few research studies have specifically addressed fuels-treatment effects on root diseases. This is of particular concern because the consequences of fuels treatments on root disease may not be readily apparent until decades after treatment. Long-term studies are critically needed to provide more precise information about the complex interrelationships among specific fuels treatment and root diseases in diverse environments. This paper explores concepts, technologies, and research needs to provide a better understanding of interactions among root pathogens, fuels treatments, and fire. A general synthesis of this information is essential for forest managers. Such information helps provide decision tools and guidance for developing fuels treatments prescriptions and root-rot management. In addition, the recent development of a web-based decision tool for evaluating fuels treatments on root disease will be presented and discussed.   

Native Pests Session - Wednesday Afternoon

corresponding author:

Mee-Sook Kim
USDA Forest Service
Rocky Mountain Research Station
1221 South Main Street
Moscow, ID 83843
208-883-2362
mkim@fs.fed.us