Radial thinning ineffective at increasing large sugar pine survival

Abstract

White pines (Pinus, subgenus Strobus) in North America are highly vulnerable to the native mountain pine beetle (Dendroctonus ponderosae), invasive white pine blister rust (Cronartium ribicola), and consequences of altered disturbance regimes, with sugar pine (Pinus lambertiana) being among the most in peril. Radial thinning treatments reflect an effort to balance the needs of late seral, high forest cover for some wildlife species with the desire to also promote habitat for species requiring higher light and lower competition, as well as to create heterogenous forest structure that can be more resilient to wildfire and drought. We examined 16-year effects of three radial thinning treatments that cleared approximately 0.017 ha to 0.05 ha around focal large sugar pines on reducing tree mortality and increasing sugar pine recruitment relative to untreated, control trees. Thinning treatments by order of increasing intensity included a compressed radius treatment which removed all trees and shrubs to 3 m from the crown edge, an extended radius with large tree retention treatment which removed trees <64 cm DBH and shrubs to 7.6 m from the crown edge, and an extended radius treatment which removed all trees and shrubs to 7.6 m outside of the crown edge. None of the three radial thinning treatments tested reduced mortality relative to the control, and mountain pine beetle caused the most mortality across the four treatments. However, mortality trended lower in the extended radius with retention treatment (19% compared to 30–37%) and was the only treatment where no mortality from wind or insects other than mountain pine beetle occurred. For trees that survived through the 16-year monitoring period, only trees in the extended radius treatment experienced increased growth above pre-treatment values and higher presence of sugar pine seedlings. Our results show no clear benefit was derived from the radial thinning treatments tested. While radial thinning may be a useful tool when used in conjunction with other silvicultural prescriptions that create heterogeneous stands and landscapes with a variety of forest structures, it seems unlikely that employing only radial thinning will meet restoration efforts that aim to decrease large tree mortality. This study highlights the value of long-term monitoring in the continual effort to develop new treatments that are effective at increasing forest resilience.