Tree carbon allocation explains forest drought‐kill and recovery patterns

Trugman, Anna T.; Detto, Matteo; Bartlett, Megan K.; Medvigy, D.; Anderegg, William R. L.; Schwalm, Christopher R.; Schaffer, Benjamin E.; Pacala, Stephen W.

doi:10.1111/ele.13136

Cited by 230 publications

(233 citation statements)

References 53 publications

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“…In temperate forests, trees may persist at slower growth rates for years following a drought before they actually die from the stress (Trugman et al. ), whereas, here, trees died rapidly. We observed the highest mortality rates and reductions in tree densities during our first measurement interval, during or immediately following the drought—and we may have missed even greater early losses because this was an opportunistic study that started after the drought had already begun.…”

Section: Discussionmentioning

confidence: 94%

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Severe drought limits trees in a semi‐arid savanna

Case

Coetsee

Nzima

et al. 2019

Ecology

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Increasingly frequent and severe droughts under climate change are expected to have major impacts on vegetation worldwide. However, research to date has focused on tree vulnerability to drought in forests. Less is known about trees and drought in savannas, where a sparse tree layer coexists with grass. These tree–grass interactions (often mediated by fire and herbivory) shape savanna tree ecology, and confound predictions of how strongly drought might affect trees. On the one hand, drought is physiologically stressful, which could harm trees and be exacerbated by herbivore impacts; on the other hand, trees adapted to semiarid savannas might be relatively drought tolerant, and the considerable impacts of drought on grass could even benefit trees via reduced grass competition and fire risk, especially in the year following a drought. Here, we sought to understand the net effects of severe drought on the savanna tree layer, and how fire and herbivory mediate these effects. We monitored tree growth, mortality, and community structure for 2 yr within existing long‐term fire and herbivory experiments across a drought‐severity contrast, following a major drought in Kruger National Park, South Africa. Overall, severe drought was a major stressor for trees. Tree mortality rates in most species increased by an order of magnitude in the year following drought, and slower growth rates for some persisted for 2 yr. At the community level, this translated into substantial decreases in tree densities. Herbivory and fire did little either to mitigate or exacerbate drought effects on trees, and overall, drought swamped effects of herbivory and fire that have otherwise been observed. However, species differed in their responses to drought, with some dominant encroaching species especially vulnerable. We suggest that increasing drought frequency and severity could drastically alter savanna vegetation by repeatedly killing off trees.

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Section: Discussionmentioning

confidence: 94%

“…, Trugman et al. ). Lags in growth rates in surviving trees in the herbivore experiment (with some species suffering drastically reduced growth rates despite a return to normal rainfall in the 2016–2017 growth year) also match widely observed patterns from temperate forest tree‐ring records (Anderegg et al.…”

Section: Discussionmentioning

confidence: 99%

Severe drought limits trees in a semi‐arid savanna

Case

Coetsee

Nzima

et al. 2019

Ecology

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“…Consistent with interpretation that drought legacies result from physiological impairment caused by drought‐induced water stress (Anderegg, Schwalm, et al, ; Peltier et al, ), compounded drought resulted in additional decreases in average ring width and changes in growth–climate sensitivities in all three P. ponderosa regions (Figure b). Underlying causes of drought legacies are potentially numerous, but major mechanisms likely include induced hydraulic dysfunction (cavitation) and reduced NSC stores, which combine to limit post‐drought growth due to slow recovery of water transport capacity and associated carbon gain (Trugman et al, ). Drought during the recovery period likely further reduces NSC stores if photosynthetic carbon gain is also impaired (West, Hultine, Jackson, & Ehleringer, ; Williams & Ehleringer, ), and may further reduce stem conductance if trees experience additional xylem cavitation or production of new sapwood is limited (Plaut et al, ; Resco et al, ); any of these mechanisms would lead to reduced post‐drought growth and longer recovery times.…”

Section: Discussionmentioning

confidence: 99%

Legacies of more frequent drought in ponderosa pine across the western United States

Peltier

Ogle

2019

Global Change Biology

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Despite widespread interest in drought legacies—multiyear impacts of drought on tree growth—the key implication of reported drought legacies remains unaddressed: as impaired growth and slow recovery associated with drought legacies are pervasive across forest ecosystems, what is the impact of more frequent drought conditions? We investigated the assumption that either multiple drought years occurring during a short period (multiyear droughts), or droughts occurring during the recovery period from previous drought (compounded droughts), are detrimental to subsequent growth. There is evidence that drought responses may vary among populations of widespread species, leading us to examine regional differences in responses of the conifer Pinus ponderosa to historic drought frequency in the western United States. More frequent drought conditions incurred additional growth declines and shifts in growth–climate sensitivities in the years following drought relative to single‐drought events, with ‘triple‐droughts' being worse than ‘double‐droughts'. Notably, prediction skill was not strongly reduced when ignoring compounded droughts, a consequence of the temporally comprehensive formulation of our stochastic antecedent model that accounts for the climatic memory of tree growth. We argue that incorporating drought‐induced temporal variability in tree growth sensitivities can aid inference gained from statistical models, where more simplistic models could overestimate the severity of drought legacies. We also found regional differences in response to repeated drought, and suggest plastic post‐drought sensitivities and climatic memory may represent beneficial physiological adjustments in interior regions. Within‐species variability may thus mediate forest responses to increasing drought frequency under future climate change, but experimental approaches using more species are necessary to improve our understanding of the mechanisms that underlie drought legacy effects on tree growth.

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“…In addition to impacting C uptake and growth during drought, the physiological changes trees undergo during water stress can also hinder recovery (Anderegg et al, 2013;Rowland et al, 2015;Trugman et al, 2018). In addition to impacting C uptake and growth during drought, the physiological changes trees undergo during water stress can also hinder recovery (Anderegg et al, 2013;Rowland et al, 2015;Trugman et al, 2018).…”

Section: Introductionmentioning

confidence: 99%

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

Kannenberg

Novick

Alexánder³

et al. 2019

Global Change Biology

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135

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Severe drought can cause lagged effects on tree physiology that negatively impact forest functioning for years. These “drought legacy effects” have been widely documented in tree‐ring records and could have important implications for our understanding of broader scale forest carbon cycling. However, legacy effects in tree‐ring increments may be decoupled from ecosystem fluxes due to (a) postdrought alterations in carbon allocation patterns; (b) temporal asynchrony between radial growth and carbon uptake; and (c) dendrochronological sampling biases. In order to link legacy effects from tree rings to whole forests, we leveraged a rich dataset from a Midwestern US forest that was severely impacted by a drought in 2012. At this site, we compiled tree‐ring records, leaf‐level gas exchange, eddy flux measurements, dendrometer band data, and satellite remote sensing estimates of greenness and leaf area before, during, and after the 2012 drought. After accounting for the relative abundance of tree species in the stand, we estimate that legacy effects led to ~10% reductions in tree‐ring width increments in the year following the severe drought. Despite this stand‐scale reduction in radial growth, we found that leaf‐level photosynthesis, gross primary productivity (GPP), and vegetation greenness were not suppressed in the year following the 2012 drought. Neither temporal asynchrony between radial growth and carbon uptake nor sampling biases could explain our observations of legacy effects in tree rings but not in GPP. Instead, elevated leaf‐level photosynthesis co‐occurred with reduced leaf area in early 2013, indicating that resources may have been allocated away from radial growth in conjunction with postdrought upregulation of photosynthesis and repair of canopy damage. Collectively, our results indicate that tree‐ring legacy effects were not observed in other canopy processes, and that postdrought canopy allocation could be an important mechanism that decouples tree‐ring signals from GPP.

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Tree carbon allocation explains forest drought‐kill and recovery patterns

Cited by 230 publications

References 53 publications

Severe drought limits trees in a semi‐arid savanna

Severe drought limits trees in a semi‐arid savanna

Legacies of more frequent drought in ponderosa pine across the western United States

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

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