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Drought has promoted large-scale, insect-induced tree mortality in recent years, with severe consequences for ecosystem function, atmospheric processes, sustainable resources and global biogeochemical cycles. However, the physiological linkages among drought, tree defences, and insect outbreaks are still uncertain, hindering our ability to accurately predict tree mortality under on-going climate change. Here we propose an interdisciplinary research agenda for addressing these crucial knowledge gaps. Our framework includes field manipulations, laboratory experiments, and modelling of insect and vegetation dynamics, and focuses on how drought affects interactions between conifer trees and bark beetles. We build upon existing theory and examine several key assumptions: (1) there is a trade-off in tree carbon investment between primary and secondary metabolites (e.g. growth vs defence); (2) secondary metabolites are one of the main component of tree defence against bark beetles and associated microbes; and (3) implementing conifer-bark beetle interactions in current models improves predictions of forest disturbance in a changing climate. Our framework provides guidance for addressing a major shortcoming in current implementations of large-scale vegetation models, the under-representation of insect-induced tree mortality.

At the beginning of this century, an unprecedented mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreak expanded across western North America. Methods are needed to quantify its impact on forests. Although aerial survey data of infestation are readily available, their translation to tree mortality estimates is challenging because the polygons sketched by aerial surveyors to delimit infested regions often overlap from one year to the next, which produces a lack of independence in yearly estimates of infested landscape. This problem manifests when annual proportions of infested land are summed over years because the sum often exceeds one. Cumulative proportions of infested land larger than one implies overestimation of tree mortality if initial densities of available host trees are multiplied by annual proportions of infested landscape. To address this problem, we developed a probabilistic method for correcting for nonindependence in proportions of infested land recorded using overlapping polygons. We demonstrate our approach in Jasper National Park where the probabilistic correction reduces overestimation bias in regions that were infested multiple years when predictions are compared to validation data. The approach and the spatial raster data sets we generated will be useful to forest managers seeking to quantify the impact of mountain pine beetles. Key words: aerial survey, remote sensing, tree mortality, insect, climate change, pine Au debut de ce siecle, une epidemie sans precedent de dendroctone du pin (Dendroctonusponderosae Hopkins) s'est etendue a l'ouest de l'Amerique du Nord. Il est necessaire de trouver des methodes pour quantifier son impact sur les forets. Meme si les donnees d'enquete aerienne sur l'infestation sont facilement disponibles, leur traduction en estimations de la mortalite des arbres est difficile car les polygones esquisses par les enqueteurs aeriens pour delimiter les regions infestees se chevauchent souvent d'une annee a l'autre, ce qui produit un manque d'independance dans les estimations annuelles du paysage infeste. Ce probleme se manifeste lorsque les proportions annuelles de terrains infestes sont additionnees sur plusieurs annees, car la somme est souvent superieure a un. Les proportions cumulees de terrains infestes superieures a un impliquent une surestimation de la mortalite des arbres si les densites initiales d'arbres hotes disponibles sont multipliees par les proportions annuelles de terrains infestes. Pour aborder ce probleme, nous avons developpe une methode probabiliste pour corriger la non-independance des proportions de terres infestees enregistrees a l'aide de polygones se chevauchant. Nous demontrons notre approche dans le parc national de Jasper oU la correction probabiliste reduit le biais de surestimation dans les regions qui ont ete infestees plusieurs annees lorsque les predictions sont comparees aux donnees de validation. L'approche et les ensembles de donnees matricielles spatiales que nous avons generes seront utiles aux gestionnaires forestiers qui cherchent a quantifier l'impact du dendroctone du pin ponderosa. [Traduit par la Redaction] Mots-cles : releve aerien, teledetection, mortalite des arbres, insecte, changements climatiques, pin, Introduction The mountain pine beetle (Dendroctonus ponderosae Hopkins) is one of the most destructive forest insects in North America (Safranyik and Carroll 2006). The most recent outbreak in western Canada [...]

Nonstructural carbohydrates (NSCs) are essential for maintenance of plant metabolism and may be sensitive to short- and long-term climatic variation. NSC variation in moist tropical forests has rarely been studied, so regulation of NSCs in these systems is poorly understood. We measured foliar and branch NSC content in 23 tree species at three sites located across a large precipitation gradient in Panama during the 2015-2016 El Niño to examine how short- and long-term climatic variation impact carbohydrate dynamics. There was no significant difference in total NSCs as the drought progressed (leaf P = 0.32, branch P = 0.30) nor across the rainfall gradient (leaf P = 0.91, branch P = 0.96). Foliar soluble sugars decreased while starch increased over the duration of the dry period, suggesting greater partitioning of NSCs to storage than metabolism or transport as drought progressed. There was a large variation across species at all sites, but total foliar NSCs were positively correlated with leaf mass per area, whereas branch sugars were positively related to leaf temperature and negatively correlated with daily photosynthesis and wood density. The NSC homoeostasis across a wide range of conditions suggests that NSCs are an allocation priority in moist tropical forests.

The winter mortality of mountain pine beetle (Dendroctonus ponderosae Hopkins) larvae caused by temperatures below survivable thresholds and sudden decreases in temperature is among the most influential factors limiting population growth. Due to the importance of winter mortality in the demise of mountain pine beetle infestations, a widely used winter mortality model was developed at the Canadian Forest Service and the United States Forest Service. It predicts lethal temperatures and survival probabilities given temperature time series over the winter season. We present a rare and possibly the first peer-reviewed validation of this winter mortality model, wherein we independently tested the model at a new region in Canada by comparing model predictions to the observed lower lethal temperature thresholds and cold-associated mortality. Model predictions were biologically reasonable but slightly biased. Bias was exacerbated by the inaccurate translation of air temperature data from weather stations to temperatures under the bark where larvae develop. The spatial prediction of relative mortality observed across the study area in Banff National Park was poor—likely because the mountainous terrain presents a difficult prediction challenge when under-bark temperatures are not directly observed. Our results will help inform users of model constraints and how to optimize the accuracy of model predictions. [ABSTRACT FROM AUTHOR]

Tree mortality rates appear to be increasing in moist tropical forests (MTFs) with significant carbon cycle consequences. Here, we review the state of knowledge regarding MTF tree mortality, create a conceptual framework with testable hypotheses regarding the drivers, mechanisms and interactions that may underlie increasing MTF mortality rates, and identify the next steps for improved understanding and reduced prediction. Increasing mortality rates are associated with rising temperature and vapor pressure deficit, liana abundance, drought, wind events, fire and, possibly, CO2 fertilization-induced increases in stand thinning or acceleration of trees reaching larger, more vulnerable heights. The majority of these mortality drivers may kill trees in part through carbon starvation and hydraulic failure. The relative importance of each driver is unknown. High species diversity may buffer MTFs against large-scale mortality events, but recent and expected trends in mortality drivers give reason for concern regarding increasing mortality within MTFs. Models of tropical tree mortality are advancing the representation of hydraulics, carbon and demography, but require more empirical knowledge regarding the most common drivers and their subsequent mechanisms. We outline critical datasets and model developments required to test hypotheses regarding the underlying causes of increasing MTF mortality rates, and improve prediction of future mortality under climate change.

Quantifying the responses of forest disturbances to climate warming is critical to our understanding of carbon cycles and energy balances of the Earth system. The impact of warming on bark beetle outbreaks is complex as multiple drivers of these events may respond differently to warming. Using a novel model of bark beetle biology and host tree interactions, we assessed how contemporary warming affected western pine beetle (Dendroctonus brevicomis) populations and mortality of its host, ponderosa pine (Pinus ponderosa), during an extreme drought in the Sierra Nevada, California, United States. When compared with the field data, our model captured the western pine beetle flight timing and rates of ponderosa pine mortality observed during the drought. In assessing the influence of temperature on western pine beetles, we found that contemporary warming increased the development rate of the western pine beetle and decreased the overwinter mortality rate of western pine beetle larvae leading to increased population growth during periods of lowered tree defense. We attribute a 29.9% (95% CI: 29.4%–30.2%) increase in ponderosa pine mortality during drought directly to increases in western pine beetle voltinism (i.e., associated with increased development rates of western pine beetle) and, to a much lesser extent, reductions in overwintering mortality. These findings, along with other studies, suggest each degree (°C) increase in temperature may have increased the number of ponderosa pine killed by upwards of 35%–40% °C−1 if the effects of compromised tree defenses (15%–20%) and increased western pine beetle populations (20%) are additive. Due to the warming ability to considerably increase mortality through the mechanism of bark beetle populations, models need to consider climate's influence on both host tree stress and the bark beetle population dynamics when determining future levels of tree mortality. [ABSTRACT FROM AUTHOR]

Phenology models are becoming increasingly important tools to accurately predict how climate change will impact the life histories of organisms. We propose a class of integral projection phenology models derived from stochastic individual-based models of insect development and demography. Our derivation, which is based on the rate summation concept, produces integral projection models that capture the effect of phenotypic rate variability on insect phenology, but which are typically more computationally frugal than equivalent individual-based phenology models. We demonstrate our approach using a temperature-dependent model of the demography of the mountain pine beetle ( Dendroctonus ponderosae Hopkins), an insect that kills mature pine trees. This work illustrates how a wide range of stochastic phenology models can be reformulated as integral projection models. Due to their computational efficiency, these integral projection models are suitable for deployment in large-scale simulations, such as studies of altered pest distributions under climate change. [ABSTRACT FROM AUTHOR]

If carbon ( C) sinks withdraw carbohydrates as they are transported along tree stems, carbohydrate availability may depend on local sink strength and distance from sources. Defenses, including monoterpenes - a major component of resin - limit the invasibility of pines. Since carbohydrate reserves fund monoterpene synthesis, we hypothesized that monoterpene concentrations in pine stems would decrease from the crown to the lower stem, and susceptibility to fungal infection would increase., Here, we measured carbohydrate and monoterpene concentrations along the stems of lodgepole pine trees ( Pinus contorta var. latifolia) before inoculating with a blue-stain fungus at different heights. After 6 wk, we assessed tree responses to fungal infection based on lesion length and carbohydrate mobilization., Concentrations of carbohydrates and monoterpenes in the phloem before inoculation decreased with distance from the crown, whereas lesion lengths after inoculation increased. However, trees mobilized sugars in response to fungal infection such that carbohydrate reserves near lesions were similar at all heights., Despite C mobilization, the lower stem was more vulnerable than the upper stem. Consistent with predictions based on sink-source relationships, vulnerability occurred where carbohydrates were less available, and likely resulted from C withdrawal by sinks higher in the supply chain. [ABSTRACT FROM AUTHOR]