Differential responses of Pinus ponderosa and Abies concolor foliar characteristics and diameter growth to thinning and prescribed fire treatments

Abstract

Thinning and prescribed fire decrease the threat of catastrophic wildfire in western coniferous forests by reducing forest biomass available as fuel. Such treatments have been shown to alter foliar nitrogen (N) and phosphorus (P) concentrations, which may precede changes in stem diameter growth. However, the timing or duration of change in foliar nutrient concentrations of mature conifers is not well understood. I evaluated whether large-scale thinning and prescribed fire treatments impact nutrient characteristics and diameter growth of mature Pinus ponderosa (pine) and Abies concolor (fir) in a suite of experimental treatments (Size-preference Thin, Pine-preference Thin, Thin+Fire, and Fire) and a Control in the Klamath National Forest, CA. I analyzed pine needles to age 6years and fir needles to age 10years for N and P concentration and content. Vector analysis was used to simultaneously evaluate needle size, nutrient concentration and content to illustrate responses that may not be evident via analysis of nutrient concentration alone. There were no differences in foliar N or P concentration or content for pine or fir for any treatment relative to the Control. Nutrient resorption efficiency and proficiency were not affected by treatment for either species. The length of pine needles produced in 2001 was increased by all treatments that involved thinning, and the mass of needles produced in 2004 was increased by the combination of thinning and prescribed fire. There was no effect of treatment on fir needle length or mass for any cohort of needles. Vector diagrams indicated that pine needle N and P were diluted by treatments as needles increased in mass and total nutrient content while nutrient concentration decreased or remained constant. In contrast, fir needle nutrient concentrations increased or remained constant while dry mass and nutrient content decreased. Vector analysis indicated that the response in foliar characteristics of mature conifers differed between the historically dominant pine and the currently dominant, shade-tolerant fir in this northern California mixed-conifer forest, whereas traditional analysis of individual foliar characteristics generally did not detect differences among treatments. All treatments that involved thinning increased mean annual ring width index between pre- and post-treatment time periods for pine, whereas there was no effect for fir. These results suggest that restoration strategies may have negative or no effects on fir whereas the same treatments may favor pine, thereby facilitating re-establishment of a pine-dominated ecosystem that approximates historical conditions in California’s mixed-conifer forests.