Old-growth ponderosa pine (Pinus ponderosa) forests, formerly widespread across much of the western United States, are of critical conservation concern. Over 85% of pre-settlement stands have been logged, and those remaining have been compromised by fire suppression and grazing, increasing their susceptibility to damage from insects, pathogens, and stand-replacing fires. Forest managers are increasingly using prescribed fire to restore historic stand structures and fire regimes; federal agencies alone have been burning nearly 600,000 ha per year, at an annual cost of over 51 million dollars. Although the efficacy of prescribed fire for reducing fuels has been documented, little attention has been devoted to potential risks associated with its application, despite evidence that large ponderosa pines may experience up to 40% mortality in treated stands. Adequate evaluation of the effects of prescribed fire on pine mortality and vigor has been hampered by the local and largely descriptive nature of previous studies, as well as inadequate synthesis of research findings. We are exploring the consequences of prescribed fire for old-growth ponderosa pine through two studies: a comparison of mortality, vigor, and resistance to bark beetles among 60 widely distributed stands with different burn histories (spring-burned, fall-burned, and unburned) in the inland Northwest, and a meta-analysis of experimental results from multiple independent investigations.
Preliminary analyses suggest that old-growth trees subjected to prescribed fire treatments may produce less oleoresin than those that have not been burned (Figure 1). Oleoresin is critical for trees to withstand attack by bark beetles and, therefore, trees with low oleoresin volumes may be at greater risk of mortality from insect attacks. Interestingly, young trees did not show similar declines in resin production in response to burning.
Figure 1. Volume of resin produced by younger trees (left panel) and old-growth trees (right panel) in a control (unburned) and adjacent burned stand. The burned stand had been subjected to a single prescribed fire, 6 years prior to sampling. P values are given for significant (P ≤ 0.05) differences between unburned and burned trees (based on a 2 sample t-test).
Fire-adapted forests across the interior Northwest are increasingly susceptible to damage from insects, pathogens, and stand-replacing fires following decades of fire suppression. As a result, managers are employing thinning and prescribed burning treatments to reduce fuel loadings and to restore the stand structure, species, and processes that historically characterized these forests. However, the consequences of these activities on understory plant communities are not well understood. We are examining the effects of thinning and prescribed fire on plant composition and diversity in ponderosa pine forests. Data on the abundance and richness of native and non-native plants were collected in 70 stands in the Colville, Okanogan, and Wenatchee National Forests in eastern Washington. Stands represented one of three types of fuel-hazard reduction treatments (or a control) conducted 3-20 yr prior to sampling: thinning, burning, thinning plus burning, or no treatment.
Multivariate analysis revealed no significant effect of thinning or burning on understory plant composition. Similarly, there were no significant differences among treatments in the cover or richness of native plants. In contrast, non-native plants showed small, but highly significant, increases in both cover and richness in response to thinning and/or burning (Figure 1). The combined treatment (thinning plus burning) yielded the greatest abundance and richness of non-native plants, although cover rarely exceeded 2% in any treatment. Analysis of temporal trends suggests slight increases in the abundance of non-native species with time in only burned treatments. Although thinning and burning may promote invasion of exotic plants in these forests, our data suggest that increases in their abundance and diversity are limited on most sites.
Figure 1. Cover and richness of non-native plants and of non-native graminoids and forbs/subshrubs for stands with different thinning and burning histories. Non-native low or tall shrubs were not found on study sites. Values are means (+ 1 SE). Closed symbols represent thinned stands; open symbols represent un-thinned stands. P values are reported for significant relationships (alpha level of 0.05) based on 2-factor ANOVAs; asterisks denote 0.1>P>0.05.