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Mountain pine beetle (MPB, Dendroctonus ponderosae) is a significant mortality agent of Pinus, and climate-driven range expansion is occurring. Pinus defenses in recently invaded areas, including high elevations, are predicted to be lower than in areas with longer term MPB presence. MPB was recently observed in high-elevation forests of the Great Basin (GB) region, North America. Defense and susceptibility in two long-lived species, GB bristlecone pine (Pinus longaeva) and foxtail pine (P. balfouriana), are unclear, although they are sympatric with a common MPB host, limber pine (P. flexilis). We surveyed stands with sympatric GB bristlecone–limber pine and foxtail–limber pine to determine relative MPB attack susceptibility and constitutive defenses. MPB-caused mortality was extensive in limber, low in foxtail and absent in GB bristlecone pine. Defense traits, including constitutive monoterpenes, resin ducts and wood density, were higher in GB bristlecone and foxtail than in limber pine. GB bristlecone and foxtail pines have relatively high levels of constitutive defenses which make them less vulnerable to climate-driven MPB range expansion relative to other highelevation pines. Long-term selective herbivore pressure and exaptation of traits for tree longevity are potential explanations, highlighting the complexity of predicting plant–insect interactions under climate change.

Mountain pine beetle (MPB) (Dendroctonus ponderosae Hopkins) is a native bark beetle that reproduces in pine (Pinus) species across western North America. High population levels can result in widespread host tree mortality. Over the past 2 decades, MPB has been responsible for pine mortality across millions of forested hectares in the western United States. Although a majority of the pine species found in the western United States are considered hosts to MPB, the host status of Rocky Mountain (RM) bristlecone pine (Pinus aristata) is unclear. We surveyed stands across the range of RM bristlecone pine in Colorado, USA, and quantified MPB-caused mortality within the past 10 years in stands where RM bristlecone and at least one other pine species co-occurred. We also evaluated in the field whether successful MPB brood production occurred in RM bristlecone pine. Our results confirm that RM bristlecone pine is susceptible to MPB attack and suitable for MPB reproduction. In mixed-species stands, pine species availability influenced MPB attack occurrence. The proportion of trees experiencing fatal beetle attack within a particular Pinus species was best predicted by the basal area proportion of that species in the stand prior to the most recent 10 years of mortality. These results indicate that RM bristlecone pine is vulnerable to ongoing climate change–induced contact with MPB.

Mountain pine beetle (MPB) (Dendroctonus ponderosae Hopkins) is a native bark beetle that reproduces in pine (Pinus) species across western North America. High population levels can result in widespread host tree mortality. Over the past 2 decades, MPB has been responsible for pine mortality across millions of forested hectares in the western United States. Although a majority of the pine species found in the western United States are considered hosts to MPB, the host status of Rocky Mountain (RM) bristlecone pine (Pinus aristata) is unclear. We surveyed stands across the range of RM bristlecone pine in Colorado, USA, and quantified MPB-caused mortality within the past 10 years in stands where RM bristlecone and at least one other pine species co-occurred. We also evaluated in the field whether successful MPB brood production occurred in RM bristlecone pine. Our results confirm that RM bristlecone pine is susceptible to MPB attack and suitable for MPB reproduction. In mixed-species stands, pine species availability influenced MPB attack occurrence. The proportion of trees experiencing fatal beetle attack within a particular Pinus species was best predicted by the basal area proportion of that species in the stand prior to the most recent 10 years of mortality. These results indicate that RM bristlecone pine is vulnerable to ongoing climate change-induced contact with MPB.

Recent bark beetle outbreaks in western North America have led to concerns regarding changes in fuel profiles and associated changes in fire behavior. Data are lacking for a range of infestation severities and time since outbreak, especially for relatively arid cover types. We surveyed fuel loads and simulated fire behavior for ponderosa pine stands of the Colorado Plateau 15–20 years after bark beetle infestation (i.e., “old-stage”). Increasing infestation severity resulted in reduced canopy bulk density, canopy base height, canopy cover, and litter loads whereas woody fuel loads were increased, especially among larger size classes. Using the Fire and Fuels Extension of the Forest Vegetation Simulator, torching index was predicted to decrease with infestation severity whereas crowning index was predicted to increase. Under modeled severe weather conditions, increasing infestation severity was predicted to shift the predominant fire type from surface fire to passive crown fire, whereas the probability of active crown fire was not significantly influenced by old-stage bark beetle-caused tree mortality. We also estimated snagfall rates of infested trees and found that the median time from infestation to snagfall was 9–12 years, with larger trees taking longer to fall.

Although funnel traps are routinely used to manage bark beetles, little is known regarding the relationship between trap captures and tree mortality near the trap. We conducted a 4 year study in Utah to examine the correlation between funnel-trap captures of spruce beetle (Dendroctonus rufipennis Kirby) and mortality of Engelmann spruce (Picea engelmannii Parry ex Engelm.) within a 10 ha block of the trap. Using recursive partitioning tree analyses, rules were developed for predicting spruce mortality and associated levels of beetle population phase (endemic or epidemic), in the current year and subsequent year, for a given level of trap captures. Although model predictions of infested-stem counts had large variances, our results suggest that funnel-trap captures can be reliably used to estimate relative levels of tree mortality, expressed as spruce beetle population phase. Classification-tree analyses indicate that captures of ~842 spruce beetles during a season (late May to mid-August) from a single funnel trap represent a threshold between endemic (<2 mass-attacked stems/ha) and epidemic conditions (≥2 mass-attacked stems/ha) for either the current or the subsequent year relative to deployment of the funnel trap. Likewise, a lack of infested hosts within 10 m of a funnel trap, also known as spillover, was associated with endemic conditions, whereas trees attacked near the trap correlated with epidemic conditions.

El escarabajo del pino de montaña (Dendroctonus ponderosae Hopkins) es un escarabajo nativo de la corteza que se reproduce en especies de pino (Pinus) en el oeste de América del Norte. Los altos niveles de población pueden resultar en una mortalidad generalizada de árboles hospederos. Durante las últimas 2 décadas, el escarabajo del pino de montaña fue responsable de la mortalidad de pinos en millones de hectáreas boscosas en el oeste de los Estados Unidos. Aunque la mayoría de las especies de pino que se encuentran en el oeste de los Estados Unidos se consideran hospederos del escarabajo, el estado de huésped del pino Bristlecone (Pinus aristata) de las Montañas Rocosas no está muy claro. Encuestamos rodales en todo el rango de pino Bristlecone de las Montañas Rocosas en Colorado, Estados Unidos, y cuantificamos la mortalidad causada por el escarabajo de pino en los últimos 10 años en rodales donde coexistieron con el pino Bristlecone y al menos otra especie de pino. También evaluamos en el campo si se produjo una producción exitosa de reproductores de escarabajo de pino de montaña en el pino Bristlecone de las Montañas Rocosas. Nuestros resultados confirman que el pino Bristlecone es susceptible al ataque de los escarabajos y adecuado para su reproducción. En rodales de especies mixtas, la disponibilidad de una especie de pino influyó en la ocurrencia del ataque de escarabajos de pino. La proporción de árboles que experimentaron un ataque mortal de escarabajos dentro de una especie particular de Pinus se predijo mejor por la proporción de área basal de esa especie en el rodal antes de los 10 años más recientes de mortalidad. Estos resultados indican que el pino Bristlecone de las Montañas Rocosas es vulnerable al contacto continúo inducido por el cambio climático con el escarabajo del pino de montaña.