When a Tree Dies in the Forest: Scaling Climate-Driven Tree Mortality to Ecosystem Water and Carbon Fluxes

Abstract

Drought-and heat-driven tree mortality, along with associated insect outbreaks, have been observed globally in recent decades and are expected to increase in future climates. Despite its potential to profoundly alter ecosystem carbon and water cycles, how tree mortality scales up to ecosystem functions and fluxes is uncertain. We describe a framework for this scaling where the effects of mortality are a function of the mortality attributes, such as spatial clustering and functional role of the trees killed, and ecosystem properties, such as productivity and diversity. We draw upon remote-sensing data and ecosystem flux data to illustrate this framework and place climate-driven tree mortality in the context of other major disturbances. We find that emerging evidence suggests that climate-driven tree mortality impacts may be relatively small and recovery times are remarkably fast (4 years for net ecosystem production). We review the key processes in ecosystem models necessary to simulate the effects of mortality on ecosystem fluxes and highlight key research gaps in modeling. Overall, our results highlight the key axes of variation needed for better monitoring and modeling of the impacts of tree mortality and provide a founda- 1133tion for including climate-driven tree mortality in a disturbance framework.

Linking drought legacy effects across scales: From leaves to tree rings to ecosystems

“…For example, altered nonstructural carbohydrate (NSC) dynamics (Richardson et al, 2013;Skomarkova et al, 2006), changes in canopy dynamics (Anderegg et al, 2016;Doughty et al, 2015), enhanced autotrophic respiration (Rowland et al, 2018), or increased root allocation (Doughty et al, 2014;Phillips et al, 2016) may be prioritized at the expense of stem growth both during and following drought. For example, altered nonstructural carbohydrate (NSC) dynamics (Richardson et al, 2013;Skomarkova et al, 2006), changes in canopy dynamics (Anderegg et al, 2016;Doughty et al, 2015), enhanced autotrophic respiration (Rowland et al, 2018), or increased root allocation (Doughty et al, 2014;Phillips et al, 2016) may be prioritized at the expense of stem growth both during and following drought.…”

Detecting early warning signals of tree mortality in boreal North America using multiscale satellite data

“…Tree mortality from drought, heat, and pests and pathogens is increasing in many locations globally in response to global change drivers, primarily climate change (Allen et al., ; Brienen et al., ; Carnicer et al., ; Kautz, Meddens, Hall, & Arneth, ; Kharuk, Im, Oskorbin, Petrov, & Ranson, ; van Mantgem et al., ; McDowell et al., ). Tree mortality precipitates a cascade of ecosystem impacts relevant for carbon cycling, energy budgets, nutrient cycling, hydrology, habitat and food webs, and ecosystem services (Adams et al., ; Anderegg, Kane, & Anderegg, ; Anderegg et al., ; Berner, Law, Meddens, & Hicke, ; Breshears, Lopez‐Hoffman, & Graumlich, ; Edburg et al., ; Huang & Anderegg, ). If mortality occurs over large enough areas, it can accelerate shifts in biome distributions (Allen & Breshears, ; Clifford & Booth, ).…”

Simulating the recent impacts of multiple biotic disturbances on forest carbon cycling across the United States

“…Typically, trees are capable of recovering foliage and functionality completely once agent pressure is released. As a consequence, affected forests might function as a C source over few years (Brown et al, 2012) up to decades , with magnitude and duration depending on initial tree mortality fraction and the speed of decomposition and regrowth (Anderegg et al, 2016). The combination of increased heterotrophic respiration and decreased NPP results in a negative NEP immediately following tree mortality, which then typically recovers to levels seen pre-disturbance (Anderegg et al, 2016;Edburg et al, 2012).…”

“…In contrast to non-lethal defoliation, tree mortality results in larger and more sustained negative effects on productivity. In addition to C cycle impacts, BDs have also been shown to alter forest dynamics and composition (Costilow, Knight, & Flower, 2017;Crowley, Lovett, Arthur, & Weathers, 2016;Temperli, Veblen, Hart, Kulakowski, & Tepley, 2015), energy, water, and nitrogen (N) fluxes (Anderegg et al, 2016;Bright, Hicke, & Meddens, 2013;Chen et al, 2015), as well as the emission of biogenic volatile organic compounds to the atmosphere (Berg et al, 2013;Duhl, Gochis, Guenther, Ferrenberg, & Pendall, 2013). As a consequence, affected forests might function as a C source over few years (Brown et al, 2012) up to decades , with magnitude and duration depending on initial tree mortality fraction and the speed of decomposition and regrowth (Anderegg et al, 2016).…”