Long-term impacts of prescribed fire on stand structure, growth, mortality, and individual tree vigor in Pinus resinosa forests

Prescribed fire is increasingly being viewed as a valuable tool for mitigating the ecological consequences of long-term fire suppression within fire-adapted forest ecosystems. While the use of burning treatments in northern temperate conifer forests has at times received considerable attention, the long-term (>10 years) effects on forest structure and development have not been quantified. We describe the persistence of prescribed fire effects in a mature red pine (Pinus resinosa Ait.)-dominated forest in northern Minnesota, USA over a ~50 year period, as well as the relative roles of fire season and frequency in affecting individual tree and stand-level structural responses. Burning treatments were applied on 0.4 ha compartments arranged in a randomized block design with four blocks. Burning treatments crossed fire season (dormant, summer) and frequency (annual, biennial, and periodic), and include an unburned control for comparison. Treatments were applied from 1960 to 1970, with no further management interventions occurring since. Data were collected periodically from 1960 to 2014.Forest structural development trajectories were significantly altered by the application of fire treatments. Burning treatments led to lower overstory densities, lower stand basal area, and larger tree diameters when compared to the unburned control over the study period. Differences between burning treatments were less apparent suggesting that the application of burning itself rather than a particular season and/or frequency of burning drives this long-term response. Overstory tree mortality and stand growth showed little or no response to burning treatments. In addition, we detected no impact of burning on long-term overstory tree growth efficiency (based on assessments >40 years post burning) suggesting these treatments had little cumulative effect on tree vigor. Our results indicate that the effects of burning treatments on structural dynamics are not ephemeral, but rather alter stand development trajectories in the long-term. The persistent nature of these effects highlights their potential as a tool for long-lasting structural alterations in treated stands without compromising overstory tree growth and vigor. The lack of red pine recruitment throughout the duration of the study suggests that prescribed fire alone cannot regenerate this species, and that further alteration to overstory and seedbed conditions would be needed to secure new cohorts of this species.