Confronting model predictions of carbon fluxes with measurements of Amazon forests subjected to experimental drought

Powell, Thomas L.; Galbraith, David; Christoffersen, Bradley; Harper, Anna; Imbuzeiro, Hewlley Maria Acioli; Rowland, Lucy; Almeida, Samuel; Brando, Paulo M.; Costa, Antonio; Costa, Marcos Heil; Levine, Naomi M.; Malhi, Yadvinder; Saleska, S. R.; Sotta, Eleneide Doff; Williams, Mathew; Meir, Patrick; Moorcroft, P. R.

doi:10.1111/nph.12390

Cited by 270 publications

(383 citation statements)

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“…This formulation is known as the Priestley and Taylor equation, and usually a value of PT=1.26 is adopted, as estimated by Priestley and Taylor (1972) in their original experiments. In this study, we will also include a vegetation-specific value.…”

Section: Methods Based On Radiation and Temperaturementioning

confidence: 99%

“…This sort of stress functions are used by evaporation retrieval models based on remote sensing or weather station data at regional or global scales (e.g. Fisher et al, 2008;Miralles et al, 2011b;Martens et al, 2017;Mu et al, 2007), and can be considered analogous to the  factor used in most land surface models to incorporate the effect of soil moisture in the estimation of surface turbulent fluxes (Powell et al, 2013). This definition of Ep based on the 35 actual ecosystem as reference system is also preferred for global streamflow and runoff studies based on the Budyko approach.…”

Section: Introductionmentioning

confidence: 99%

“…In addition, considering such an extensive area also avoids problems with advection and entrainment flows. Consequently, such an extensive reference 10 system was used to define Ep by many authors (Penman, 1963;Priestley and Taylor, 1972;Brutsaert, 1982;Shuttleworth, 1993). Others opined that the meteorological conditions should not depend on the extent of the reference system, by making this reference system infinitesimally small (Morton, 1983;Gentine et al, 2011b;Pettijohn and Salvucci, 2009).…”

Section: Introductionmentioning

confidence: 99%

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Potential evaporation at eddy-covariance sites across the globe

Maes¹,

Gentine²,

Verhoest³

et al. 2018

Preprint

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Abstract. Potential evaporation (Ep) is a crucial variable for hydrological forecast and in drought monitoring systems. However, multiple interpretations of Ep exist, and these reflect a diverse range of methods to calculateEp. As such, a comparison of the performance of these methods against field observations in different global ecosystems is badly needed. In this study, we used eddy-covariance measurements from 107 sites of the FLUXNET2015 database, covering 11 different biomes, to parameterize and compare the main Ep methods and 10 uncover their relative performance. For each site, we extracted the days for which ecosystems are unstressed based on both an energy balance approach and on a soil water content approach. The evaporation measurements during these days were used as reference to validate the different methods to estimate Ep. Our results indicate that a simple radiation-driven method calibrated per biome consistently performed best, with a mean correlation of 0.93, an unbiased RMSE of 0.56 mm day -1, and a bias of -0.02 mm day -1 against in situ measurements of unstressed 15 evaporation. A Priestley and Taylor method, calibrated per biome, performed just slightly worse, yet substantially and consistently better than more complex Penman, Penman-Monteith-based or temperature-based approaches.We show that the poor performance of Penman-Monteith based approaches relates largely to the fact that the unstressed stomatal conductance was assumed constant. Further analysis showed that the biome-specific parameters required for the simple radiation-driven methods are relatively constant per biome. This makes this 20 simple radiation-driven method calibrated per biome a robust method that can be incorporated into models for improving our understanding of the impact of global warming on future global water use and demand, drought severity and ecosystem productivity.

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Section: Methods Based On Radiation and Temperaturementioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Potential evaporation at eddy-covariance sites across the globe

Maes¹,

Gentine²,

Verhoest³

et al. 2018

Preprint

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show abstract

“…Other modeling approaches explicitly simulate the hydraulic gradient throughout the soil-plant-atmosphere continuum [Baldocchi and Meyers, 1998;Williams et al, 2001;Manoli et al, 2014] and these approaches have recently been shown to provide substantial benefits in land models [Hickler et al, 2006;Bonan et al, 2014]. A limitation of this approach to date is that it is based on setting a minimum canopy xylem pressure, and so it can underpredict drought responses when scaling up to larger areas [Powell et al, 2013]. An alternative is to model the soil-plant hydraulic continuum using both the hydraulic properties of soil and cavitation in plant xylem represented with s-shaped curves representing the distinct changes in root, stem, and leaf hydraulic conductances with declining xylem pressure [Sperry et al, 1998].…”

mentioning

confidence: 99%

Improving the representation of hydrologic processes in Earth System Models

Clark

Fan

Lawrence

et al. 2015

Water Resources Research

412

396

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Many of the scientific and societal challenges in understanding and preparing for global environmental change rest upon our ability to understand and predict the water cycle change at large river basin, continent, and global scales. However, current large-scale land models (as a component of Earth System Models, or ESMs) do not yet reflect the best hydrologic process understanding or utilize the large amount of hydrologic observations for model testing. This paper discusses the opportunities and key challenges to improve hydrologic process representations and benchmarking in ESM land models, suggesting that (1) land model development can benefit from recent advances in hydrology, both through incorporating key processes (e.g., groundwater-surface water interactions) and new approaches to describe multiscale spatial variability and hydrologic connectivity; (2) accelerating model advances requires comprehensive hydrologic benchmarking in order to systematically evaluate competing alternatives, understand model weaknesses, and prioritize model development needs, and (3) stronger collaboration is needed between the hydrology and ESM modeling communities, both through greater engagement of hydrologists in ESM land model development, and through rigorous evaluation of ESM hydrology performance in research watersheds or Critical Zone Observatories. Such coordinated efforts in advancing hydrology in ESMs have the potential to substantially impact energy, carbon, and nutrient cycle prediction capabilities through the fundamental role hydrologic processes play in regulating these cycles.

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“…An increase in mortality rate can be as important as a change in productivity for carbon sinks 15 . Application of DGVMs to known severe drought stress in controlled rainfall exclusion experiments reveals that they do not accurately capture drought-induced forest dieback 5 . Thus, there is a compelling need for an approach to simulate spatial and temporal patterns of tree mortality and to test model predictions against regional mortality data sets, such as remote-sensing estimates 16 .…”

mentioning

confidence: 99%

Tree mortality predicted from drought-induced vascular damage

et al. 2015

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The projected responses of forest ecosystems to warming and drying associated with twenty-first-century climate change vary widely from resiliency to widespread tree mortality 1-3 . Current vegetation models lack the ability to account for mortality of overstorey trees during extreme drought owing to uncertainties in mechanisms and thresholds causing mortality 4,5 . Here we assess the causes of tree mortality, using field measurements of branch hydraulic conductivity during ongoing mortality in Populus tremuloides in the southwestern United States and a detailed plant hydraulics model. We identify a lethal plant water stress threshold that corresponds with a loss of vascular transport capacity from air entry into the xylem. We then use this hydraulic-based threshold to simulate forest dieback during historical drought, and compare predictions against three independent mortality data sets. The hydraulic threshold predicted with 75% accuracy regional patterns of tree mortality as found in field plots and mortality maps derived from Landsat imagery. In a high-emissions scenario, climate models project that drought stress will exceed the observed mortality threshold in the southwestern United States by the 2050s. Our approach provides a powerful and tractable way of incorporating tree mortality into vegetation models to resolve uncertainty over the fate of forest ecosystems in a changing climate.Forests play a central role in global water, energy and biogeochemical cycles and provide substantial ecosystem services to societies around the globe 6 . Yet the fate of forest ecosystems in a changing climate is highly uncertain. Rising atmospheric CO 2 concentrations may benefit trees, particularly through increasing water-use efficiency 7 , but concomitant increases in temperature and drought stress could potentially overwhelm these benefits, leading to widespread forest dieback in many ecosystems globally 8 . Although precipitation projections under climate scenarios are more variable and uncertain, general circulation models project consistent increases in air temperature and thus evaporation over much of the world and resulting decreases in soil moisture in many regions, leading to more intense and frequent droughts 9 . Recent studies have indicated resilience in forest biomes in response to early twenty-first-century droughts through inter-annual modulations in water-use efficiency 10 and long-term increases in forest water-use efficiency 7 . In contrast, severe regional droughts have strongly decreased the carbon sink of key forest ecosystems [11][12][13] and widespread, climate-induced tree mortality has been observed around the globe 8,14 .

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Confronting model predictions of carbon fluxes with measurements of Amazon forests subjected to experimental drought

Cited by 270 publications

References 53 publications

Potential evaporation at eddy-covariance sites across the globe

Potential evaporation at eddy-covariance sites across the globe

Improving the representation of hydrologic processes in Earth System Models

Tree mortality predicted from drought-induced vascular damage

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