Burn severity and whitebark pine (Pinus albicaulis) regeneration in the North Cascades

Whitebark pine (Pinus albicaulis, Engelm.) is a long-lived and slow-growing high elevation tree and a key part of subalpine communities in the North Cascades, Washington State. Whitebark pine populations in Washington are declining because of an exotic fungus, white pine blister rust (Cronartium ribicola), and successional replacement due to fire exclusion. An increase in whitebark pine seedling density could help restore populations and accelerate the process of natural selection towards rust resistance. Where whitebark pine is firedependent, fire exclusion has impeded whitebark pine regeneration. The relationship between whitebark pine regeneration and burn severity was studied in the subalpine and timberline ecotone in the North Cascades in 2005. Whitebark pine regeneration data were collected eleven years after two 1994 fires, the Boulder Creek Fire in the North Cascades National Park and in the Tyee Complex Fire in the Wenatchee National Forest. A comprehensive model of the ecological factors related to post-fire whitebark pine seedling presence and density was created showing how whitebark pine regeneration may be related to many characteristics of the environment, pre-fire forest, burn severity, and post-fire condition. Whitebark pine seedling presence and density models were compared using Akaike information criterion. Leading models for explaining variability in seedling presence on the Boulder Creek site included total overstory cover and burn severity factors, where areas with greater canopy cover and moderate burn severity had the greatest probability of seedling presence. At the Tyee Mountain site, the top models for explaining variability in seedling presence were overstory whitebark pine cover, char depth and soil organic matter. The probability of seedling presence increased as char depth increased and soil organic matter decreased. The variability of whitebark pine seedling densities was best explained by the distance to the edge of the burn, with seedling densities increasing with greater distance into the core burn area. Allowing moderate severity fires to burn in large areas may create more opportunities for natural whitebark pine regeneration.