Tree defence and bark beetles in a drying world: carbon partitioning, functioning and modelling

Abstract

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. 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.

Understanding the uncertainty in global forest carbon turnover

“…In addition, as for leaves, scaling observations across large areas needs to take account of relative species abundances, assuming turnover rates are related to species. Assuming a turnover time of circa 1 year, fine-root production has been estimated to total a third of NPP (Jackson et al, 1997), a larger value than simulated by most of the TBMs included in this study.…”

Plant carbon allocation in a changing world – challenges and progress: introduction to a Virtual Issue on carbon allocation

“…Recent approaches presented in this collection combine biomass measurements, flux assessments and estimates of GPP to produce a more holistic budgeting of C allocation (e.g. Klein & Hoch, 2015;Huang et al, 2019a) and consideration of additional functional pools, like secondary metabolites and their synthesis pathways, may further complement our understanding of plant allocation responses to environmental change (Huang et al, 2019b). The studies compiled in this Virtual Issue highlight some important advances in our understanding of the C allocation in plants and ecosystems, including the following:…”

Tree growth sensitivity to climate is temporally variable

“…Certain single disturbances such as fire and stochastic treefall are already incorporated into DGVMs (Thonicke et al 2001;Sitch et al 2003;Longo et al 2019), but disturbances that alter the carbon status of individuals (e.g. bark beetles) are infrequently incorporated (Medvigy et al 2012;Huang et al 2019). Extreme climate years could be conceptualised in mechanistic models as disturbances leading to changes in carbon allocation, but could also be modelled as temporary changes in the sensitivity of cambial growth or carbon supply to climate variables, using average responses from empirical drought legacy studies (both in magnitude and duration).…”