Abstract
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.
Highlights
We provide a brief overview of general aspects of carbon metabolism, including allocation of nonstructural carbohydrates (NSC) to the biosynthesis of SMs, and how this process may be influenced by drought
In this Viewpoint we propose an ambitious research agenda bridging carbon partitioning, defence functioning and vegetation modelling, which will provide substantial progress toward projecting future tree mortality from bark beetle outbreaks
Our synthesis strongly suggests that much knowledge necessary to improve vegetation models can be achieved through modification of existing research protocols and by capitalizing on the wealth of data and samples already collected from field manipulations
Summary
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. Future in situ drought studies should – where possible – apply isotope labelling (e.g. 13CO2) to trace the flow of metabolites within the trees and from trees to insects, which can help identify the key physiological processes This includes assessing the potential of phloem failure during drought to inhibit carbon transport to tissues attacked by bark beetles and pathogens (Sevanto, 2018); partitioning the relative contribution of newly-assimilated vs old stored carbon to the production of SM (Huang et al, 2018), and unravelling the role of terpenoids in anatomical and chemical components of tree defence. Additional data on insect populations are needed for calibration of insect component for host–bark beetle interactions
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