Lack of Fire Has Limited Physiological Impact on Old-Growth Ponderosa Pine in Dry Montane Forests of North-Central Idaho

Reduced frequency of fire in historically fire-adapted ecosystems may have adverse effects on ecosystem structure, function, and resilience. Lack of fire increases stand density and promotes successional replacement of seral dominant trees by late-successional, more shade-tolerant species. These changes are thought to increase competition for limited resources among trees and to increase physiological stress of dominant, fire-adapted species. However, there has been little effort to directly investigate effects of lack of fire on the physiological status of old trees, especially in unlogged, protected forests. At four remote sites in the Selway-Bitterroot region of Idaho, we tested whether the physiological status of dominant old-growth ponderosa pine trees in repeatedly burned stands (three to four 20thcentury wildfires at roughly historical fire frequency) differs from trees in paired stands not burned for at least 70 years. We hypothesized that trees in relatively unburned stands would exhibit signs of physiological stress due to increased competition for resources in higherdensity stands. Needle chemistry and morphological variables, fine root production, mycorrhizal infection rates, depth of soil water resources, and recent basal area growth rates were measured as indictors of competition-induced stress. Contrary to predictions, needle carbon isotopic ratio (dC) and fine root production, variables related to water stress, were slightly higher in repeatedly burned stands driven by site-specific responses, and there were no significant biological differences between trees in repeatedly burned stands vs. stands unburned for at least 70 years in the remaining variables. Our results raise the possibility that dominant ponderosa pine trees in uneven-aged forests may be more resilient to increased stand density associated with the lack of fire than previously thought. If so, our results have implications for the management of uneven-aged, old-growth forests.