Tamm review: Current and recommended management practices for the restoration of whitebark pine (Pinus albicaulis Engelm.), an imperiled high-elevation Western North American forest tree

Whitebark pine (Pinus albicaulis Engelm.) is an ecologically important subalpine and treeline forest tree of the western U.S. and Canada. It is categorized as endangered by the IUCN and by Canada under the Species at Risk Act and was recently proposed for listing in the U.S. as threatened under the Endangered Species Act. Whitebark pine populations are declining nearly rangewide primarily from the spread and intensification of Cronartium ribicola J.C. Fisch., the exotic, invasive pathogen that causes white pine blister rust (WPBR); recent, large-scale outbreaks of mountain pine beetles (MPB) (Dendroctonus ponderosae Hopkins); altered fire regimes; and, multiple impacts from climate change. For more than two decades, researchers and managers within the U.S. Forest Service and Canadian forestry agencies have been developing restoration and conservation tools and techniques to help mitigate these threats. Four conservation and restoration principles for whitebark pine were previously emphasized: (1) conserve genetic diversity, (2) promote WPBR resistance, (3) protect seed sources, and (4) deploy restoration treatments, while mitigating for climate change. These principles are served by ten additional management or conservation actions that form the basis of a restoration and adaptive management plan but apply primarily to regions with moderate to high levels of WPBR and MPB outbreaks. Where the pathogen and MPB are absent or present at low levels, managers can implement proactive management to build resilience to prevent the future loss of ecological function. Here, we review the key management actions currently used for whitebark pine conservation and restoration in the U.S. and Canada, which include gene conservation, increasing natural genetic resistance to C. ribicola, cone collections, growing and planting seedlings or directly sowing seeds, protecting seed sources, prescribed fire and silvicultural thinning to reduce competition in late seral communities, proactive intervention, stand health surveys and monitoring, and monitoring the impacts of restoration for adaptive management. This review is the outcome of an experts’ workshop held in association with the development of the National Whitebark Pine Restoration Plan (NWPRP), a collaborative U.S. multi-agency and tribal effort initiated in 2017 in consultation with the U.S. Forest Service and facilitated by the non-profit organizations, the Whitebark Pine Ecosystem Foundation and American Forests.