Recent outbreaks of forest insects have been directly linked to climate change-induced warming and drought, but effects of tree stored resources on insects have received less attention. We asked whether tree stored resources changed following mountain pine beetle (Dendroctonus ponderosae Hopkins) attack and whether they affected beetle development. We compared initial concentrations of stored resources in the sapwood of whitebark pine (Pinus albicaulis Engelmann) and lodgepole pine (Pinus contorta Douglas ex. Louden) with resource concentrations one year later, in trees that were naturally attacked by beetles and trees that remained unattacked. Beetles did not select host trees based on sapwood resources—there were no consistent a priori differences between attacked versus unattacked trees—but concentrations of nonstructural carbohydrate (NSC), lipids, and phosphorus declined in attacked trees, relative to initial concentrations and unattacked trees. Whitebark pine experienced greater resource declines than lodgepole pine; however, sapwood resources were not correlated with beetle success in either species. Experimental manipulation confirmed that the negative effect of beetles on sapwood and phloem NSC was not due to girdling. Instead, changes in sapwood resources were related to the percentage of sapwood with fungal blue-stain. Overall, mountain pine beetle attack affected sapwood resources, but sapwood resources did not contribute directly to beetle success; instead, sapwood resources may support colonization by beetle-vectored fungi that potentially accelerate tree mortality. Closer attention to stored resource dynamics will improve our understanding of the interaction between mountain pine beetles, fungi, and host trees, an issue that is relevant to our understanding of insect range expansion under climate change.
Summary Climate change is expected to drive increased tree mortality through drought, heat stress, and insect attacks, with manifold impacts on forest ecosystems. Yet, climate‐induced tree mortality and biotic disturbance agents are largely absent from process‐based ecosystem models. Using data sets from the western USA and associated studies, we present a framework for determining the relative contribution of drought stress, insect attack, and their interactions, which is critical for modeling mortality in future climates. We outline a simple approach that identifies the mechanisms associated with two guilds of insects – bark beetles and defoliators – which are responsible for substantial tree mortality. We then discuss cross‐biome patterns of insect‐driven tree mortality and draw upon available evidence contrasting the prevalence of insect outbreaks in temperate and tropical regions. We conclude with an overview of tools and promising avenues to address major challenges. Ultimately, a multitrophic approach that captures tree physiology, insect populations, and tree–insect interactions will better inform projections of forest ecosystem responses to climate change.
Abstract Widespread drought-induced forest mortality (DIM) is expected to increase with climate change and drought, and is expected to have major impacts on carbon and water cycles. For large-scale assessment and management, it is critical to identify variables that integrate the physiological mechanisms of DIM and signal risk of DIM. We tested whether plant water content, a variable that can be remotely sensed at large scales, is a useful indicator of DIM risk at the population level. We subjected Pinus ponderosa Douglas ex C. Lawson seedlings to experimental drought using a point of no return experimental design. Periodically during the drought, independent sets of seedlings were sampled to measure physiological state (volumetric water content (VWC), percent loss of conductivity (PLC) and non-structural carbohydrates) and to estimate population-level probability of mortality through re-watering. We show that plant VWC is a good predictor of population-level DIM risk and exhibits a threshold-type response that distinguishes plants at no risk from those at increasing risk of mortality. We also show that plant VWC integrates the mechanisms involved in individual tree death: hydraulic failure (PLC), carbon depletion across organs and their interaction. Our results are promising for landscape-level monitoring of DIM risk.
Low-elevation ponderosa pine (Pinus ponderosa Dougl. ex. Laws.) forests of the northern Rocky Mountains historically experienced frequent low-intensity fires that maintained open uneven-aged stands. A century of fire exclusion has contributed to denser ponderosa pine forests with greater competition for resources, higher tree stress and greater risk of insect attack and stand-destroying fire. Active management intended to restore a semblance of the more sustainable historic stand structure and composition includes selective thinning and prescribed fire. However, little is known about the relative effects of these management practices on the physiological performance of ponderosa pine. We measured soil water and nitrogen availability, physiological performance and wood radial increment of second growth ponderosa pine trees at the Lick Creek Experimental Site in the Bitterroot National Forest, Montana, 8 and 9 years after the application of four treatments: thinning only; thinning followed by prescribed fire in the spring; thinning followed by prescribed fire in the fall; and untreated controls. Volumetric soil water content and resin capsule ammonium did not differ among treatments. Resin capsule nitrate in the control treatment was similar to that in all other treatments, although burned treatments had lower nitrate relative to the thinned-only treatment. Trees of similar size and canopy condition in the three thinned treatments (with and without fire) displayed higher leaf-area-based photosynthetic rate, stomatal conductance and mid-morning leaf water potential in June and July, and higher wood radial increment relative to trees in control units. Specific leaf area, mass-based leaf nitrogen content and carbon isotope discrimination did not vary among treatments. Our results suggest that, despite minimal differences in soil resource availability, trees in managed units where basal area was reduced had improved gas exchange and growth compared with trees in unmanaged units. Prescribed fire (either in the spring or in the fall) in addition to thinning, had no measurable effect on the mid-term physiological performance and wood growth of second growth ponderosa pine.
