Diameter and height release of small whitebark pine trees ten years after two daylighting treatments
Whitebark pine forests have been declining throughout its range in North America due to increasing populations and spread of mountain pine beetle and the exotic white pine blister rust. The current decline and reduction of whitebark pine from the landscape will have broad reaching consequences for ecosystem diversity as well as snow pack and soil retention in the high elevation sites it inhabits. Efforts are underway to restore these iconic forests using a wide variety of mechanical and prescribed burning techniques to reduce competition around living whitebark pine trees to improve tree vigor and promote regeneration. One experimental mechanical method is “daylighting”, which removes competition in a small circle around individual trees. In this study, we evaluated the increase in diameter and height over ten years after two types of daylighting treatments (removing competition in a 2 m and 4 m radius around selected trees) along the Idaho-Montana, USA divide. We found that, while there were increases in diameter and height growth over the 10 years, daylighting around trees using a 2 m radius produced no significant increases in either height or diameter growth for individual whitebark pine trees when compared across two re-measurement periods to control trees (trees that were not daylighted). There were significant differences in growth for trees in the 4 m radius treatment compared to the control, but the increases were all small. While individual trees chosen for the study had no blister rust cankers in 2008, we found a significant increases in blister rust presence and rust-caused mortality from 2013 to 2018. However, the increases were the same across the treated trees and the controls. The lack of a significant diameter or height response may be partially due to the rust infections, but we feel that the daylighting treatments were ineffectual because the areas treated were insufficiently large enough to effectively reduce competition.
