Limber Pine in the Central and Southern Rocky Mountains: Stand Conditions and Interactions with Blister Rust, Mistletoe, and Bark Beetles

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Forest Ecology and Management

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Combined effects of the recent mountain pine beetle (Dendroctonus ponderosae) outbreak and white pine blister rust (caused by Cronartium ribicola) (WPBR) are causing extensive crown dieback and mortality in limber pine (Pinus flexilis) in the central and southern Rocky Mountains. Additional stressors such as climate change and limber pine dwarf mistletoe (Aceuthobium cyanocarpum) may significantly alter stand structure and biodiversity of these ecosystems. The objectives of this study were to: (1) assess site, stand, and tree health conditions in limber pine stands of northern Colorado, Wyoming, and southeastern Montana, (2) quantify impacts of WPBR, bark beetles, and dwarf mistletoe, and (3) determine biotic and abiotic factors that influence the occurrence and incidence of these damage agents.

We assessed 22,700 limber pines on 508 plots in limber pine-dominated stands in twenty-five study areas in northern Colorado, Wyoming, and southeastern Montana. Average live limber pine density was 311 stems ha−1. Overall, 50% of limber pines were classified as healthy with over 50% of limber pine mortality attributed to bark beetle attacks. White pine blister rust damage was evident on over 60% of declining or dying limber pines. Blister rust was the primary damage agent, occurring on 73% of the plots and 26% of the trees. Bark beetle-caused mortality was found on 75% of plots and 18% of trees. Limber pine dwarf mistletoe was present on 29% of plots and 9% of trees. In study areas that were previously monitored, incidence of WPBR increased over 8–9 years by 6%, bark beetle-caused mortality by 17%, while dwarf mistletoe incidence remained the same. We used statistical modeling to determine meteorological, macro and micro site factors, and stand factors that influenced the occurrence and incidence of WPBR, bark beetles, and dwarf mistletoe on limber pines.

Declines in health and rise in mortality due to bark beetles and WPBR has left some study areas with low limber pine basal area and stand density. These stands may be at risk for localized extirpation if the minimum viable population thresholds are not maintained and are identified as areas of concern for restoration and conservation efforts. Current condition status and long-term monitoring of limber pine is needed for land manager decision making and facilitation of restoration goals.