Darcy's law predicts widespread forest mortality under climate warming

McDowell, N. G.; Allen, Craig D.

doi:10.1038/nclimate2641

Cited by 622 publications

(478 citation statements)

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“…Basic principles of plant physiology predict that vulnerability to drought stress increases with tree height 13 ; this theoretical prediction is confirmed by our results. Tall trees have to lift water to a greater height against the effects of gravity and pathlength-associated resistance, and therefore face greater hydraulic challenges [11][12][13][14] . Water-use efficiency increases with tree height globally as a consequence of both increasing hydraulic path length with greater height and higher evaporative demands in the upper canopy 14 .…”

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confidence: 79%

“…Moreover, drought-related mortality increased with tree size in 65% of the droughts examined, especially when community-wide mortality was high or when bark beetles were present. The more pronounced drought sensitivity of larger trees could be underpinned by greater inherent vulnerability to hydraulic stress [11][12][13][14] , the higher radiation and evaporative demand experienced by exposed crowns 4,15 , and the tendency for bark beetles to preferentially attack larger trees 16 . We suggest that future droughts will have a more detrimental impact on the growth and mortality of larger trees, potentially exacerbating feedbacks to climate change.…”

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Larger trees suffer most during drought in forests worldwide

et al. 2015

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The frequency of severe droughts is increasing in many regions around the world as a result of climate change [1][2][3] . Droughts alter the structure and function of forests 4,5 . Site-and region-specific studies suggest that large trees, which play keystone roles in forests 6 and can be disproportionately important to ecosystem carbon storage 7 and hydrology 8 , exhibit greater sensitivity to drought than small trees 4,5,9,10 . Here, we synthesize data on tree growth and mortality collected during 40 drought events in forests worldwide to see whether this size-dependent sensitivity to drought holds more widely. We find that droughts consistently had a more detrimental impact on the growth and mortality rates of larger trees. Moreover, drought-related mortality increased with tree size in 65% of the droughts examined, especially when community-wide mortality was high or when bark beetles were present. The more pronounced drought sensitivity of larger trees could be underpinned by greater inherent vulnerability to hydraulic stress 11-14 , the higher radiation and evaporative demand experienced by exposed crowns 4,15 , and the tendency for bark beetles to preferentially attack larger trees 16 . We suggest that future droughts will have a more detrimental impact on the growth and mortality of larger trees, potentially exacerbating feedbacks to climate change.Climate change has been linked to water deficits in many parts of the world, and future climate projections suggest that droughts are likely to increase in severity because of changes in the timing and magnitude of precipitation and rising temperature 1,2,14,17 . Across a wide range of biomes, drought leads to changes in forest composition, structure, productivity and climate interactions 5,18,19 . Drought has many important consequences for forest communities, as species composition and dominance are shaped by water availability and can change rapidly in response to drought 19,20 . Drought-induced forest decline results in climate feedbacks including reduced CO 2 uptake, reduced carbon stocks, increased albedo and decreased evapotranspiration 21 . The impact of drought on forest structure and function depends on which trees are most adversely affected; greater mortality of small trees may modify future forest succession whereas mortality of large trees causes disproportionate losses of carbon and ecosystem function 5-7 . It has not been clear whether large or small trees would suffer more under drought stress. Several studies have documented a greater impact of drought on large trees at a single site 4,9,10 , and a synthesis of data from the humid lowland tropics revealed a tendency for greater drought-related mortality increases in large trees 5 , but these patterns have never been systematically reviewed for forests worldwide.Here, we perform a meta-analysis of data from 40 drought events at 38 forest locations worldwide, ranging from semi-arid woodlands to tropical rainforests, to address whether trees of different size (seedlings excluded) respond differe...

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Larger trees suffer most during drought in forests worldwide

et al. 2015

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“…3a), which may highlight the importance of plant reliance on stored water or deep rooting systems in taller ecosystems 6 . Plant water use strategy (for example, isohydric or anisohydric) has also been identified as a factor determining the sensitivity of stomatal conductance to VPD 1,27,28 . Accounting for all these sources of variability was outside the scope of this particular study, but should motivate future research.…”

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“…Looking to the future, it will become even more important to separately resolve VPD and soil moisture effects on ecosystem functioning. VPD is highly sensitive to changes in air temperature and is thus expected to rise globally in the future 1,20 . On the other hand, projected changes in precipitation and soil moisture are less certain, more spatially variable, and smaller in relative magnitude 21 .…”

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The increasing importance of atmospheric demand for ecosystem water and carbon fluxes

et al. 2016

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Soil moisture supply and atmospheric demand for water independently limit-and profoundly a ect-vegetation productivity and water use during periods of hydrologic stress [1][2][3][4] . Disentangling the impact of these two drivers on ecosystem carbon and water cycling is di cult because they are often correlated, and experimental tools for manipulating atmospheric demand in the field are lacking. Consequently, the role of atmospheric demand is often not adequately factored into experiments or represented in models 5-7 . Here we show that atmospheric demand limits surface conductance and evapotranspiration to a greater extent than soil moisture in many biomes, including mesic forests that are of particular importance to the terrestrial carbon sink 8,9 . Further, using projections from ten general circulation models, we show that climate change will increase the importance of atmospheric constraints to carbon and water fluxes in all ecosystems. Consequently, atmospheric demand will become increasingly important for vegetation function, accounting for >70% of growing season limitation to surface conductance in mesic temperate forests. Our results suggest that failure to consider the limiting role of atmospheric demand in experimental designs, simulation models and land management strategies will lead to incorrect projections of ecosystem responses to future climate conditions. Ecosystem moisture stress is often characterized by changes in soil water availability 10,11 . Declining soil moisture impedes the movement of water to evaporating sites at the soil or leaf surface 12 , reducing the surface conductance to water vapour (G S )-a key determinant of carbon and water cycling-and thereby evapotranspiration (ET). However, atmospheric demand for water, which is directly related to the atmospheric vapour pressure deficit (VPD), also affects G S and ET. Plants close their stomata to prevent excessive water loss when VPD is high [13][14][15][16] and thus, increases in VPD during periods of hydrologic stress represent an independent constraint on plant carbon uptake and water use in ecosystems.While the plant physiological community has long recognized the critical role of VPD in determining plant functioning, VPD is often overlooked in many fields of hydrologic and climate science. For example, precipitation manipulation experiments are frequently used to draw conclusions about ecosystem response to drought stress, even though VPD is unaffected by precipitation manipulation 10 . Some terrestrial ecosystem and ecohydrological models do not permit stomatal conductance to vary with atmospheric demand 5,11 . Many models designed to capture these impacts rely on empirical parameterizations for soil moisture and VPD stress that promote compensating effects and model equifinality 5 , and/or use relative humidity instead of VPD as the primary driver, with significant consequences for projections of

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Increasing net ecosystem biomass production of Canada's boreal and temperate forests despite decline in dry climates

Hember

Kurz

Coops

2017

Global Biogeochemical Cycles

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Repeated measurements of tree biomass at field plots describe recovery from disturbances, sampling artifacts, and potential effects of environmental change on forest ecosystems. Challenges in differentiating between intrinsic and extrinsic sources of variation, both in theory and in practice, continue to confound claims of an anthropogenic carbon sink in forest biomass. Here we analyzed observations at 10,307 plots across southern ecozones of Canada to investigate temporal trends in stand-level biomass growth (G), biomass loss due to mortality (M), and net ecosystem biomass production (NEBP) of intact stands. Net extrinsic forcing (F ex ) was expressed by the collective dependence of biomass fluxes on climate anomalies, nitrogen deposition (N), and atmospheric carbon dioxide concentration (C). Inferences drawn directly from linear mixed-effects model coefficients only reflect the static behavior of the model specifically at field plot locations. We, therefore, defined a dynamic landscape-scale net extrinsic forcing (F ex 0 ), which additionally accounted for potential negative feedback responses to anthropogenic growth enhancement. Simulations were performed over 1501-2012 to estimate F ex 0 . Overall, F ex 0 was positive, suggesting that environmental changes drove a 90% increase in NEBP. The increase in NEBP was confined to wet regions, while the biomass sink in dry regions decreased, suggesting that large expanses of northern forests, historically located near the boundary between wet and dry climates, may be at high risk of decline under continued increases in evaporative demand. These results have important implications for the greenhouse gas balance of Canada's forest sector.

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Darcy's law predicts widespread forest mortality under climate warming

Cited by 622 publications

References 25 publications

Larger trees suffer most during drought in forests worldwide

Larger trees suffer most during drought in forests worldwide

The increasing importance of atmospheric demand for ecosystem water and carbon fluxes

Increasing net ecosystem biomass production of Canada's boreal and temperate forests despite decline in dry climates

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