Detecting forest response to droughts with global observations of vegetation water content

Konings, Alexandra G.; Saatchi, Sassan; Frankenberg, Christian; Keller, Michael; Leshyk, Victor O.; Anderegg, William R. L.; Humphrey, Vincent; Matheny, Ashley M.; Trugman, Anna T.; Sack, Lawren; Agee, Elizabeth; Barnes, Mallory L.; Binks, Oliver; Cawse‐Nicholson, Kerry; Christoffersen, Bradley; Entekhabi, Dara; Gentine, Pierre; Holtzman, Nataniel M.; Katul, Gabriel G.; Liu, Yanlan; Longo, Marcos; Martínez‐Vilalta, Jordi; McDowell, Nate G.; Meir, Patrick; Mencuccini, Maurizio; Mrad, Assaad; Novick, Kimberly A.; Oliveira, Rafael S.; Siqueira, Paul; Steele‐Dunne, Susan; Thompson, David R.; Wang, Yujie; Wehr, Richard; Wood, J. D.; Xu, Xiangtao; Zuidema, Pieter A.

doi:10.1111/gcb.15872

Cited by 105 publications

(65 citation statements)

References 160 publications

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“…Diurnal differences in backscatter during the dry season are dominated by transpiration losses. Long-term monitoring of these diurnal differences could provide insight into moisture availability and its influence on transpiration and vegetation functioning (Konings et al, 2021). Consistent with previous studies on the effect of drought on the backscatter signal over the Amazon forests (Frolking et al, 2011;Saatchi et al, 2013), a negative anomaly in backscatter was observed during the 2010 and 2015 drought; although this was minor for the moist forests, strong anomalies were observed in the Cerrado.…”

Section: Discussionsupporting

confidence: 87%

The influence of vegetation water dynamics on the ASCAT backscatter–incidence angle relationship in the Amazon

Petchiappan

Steele‐Dunne

Vreugdenhil³

et al. 2022

Hydrol. Earth Syst. Sci.

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Abstract. Microwave observations are sensitive to plant water content and could therefore provide essential information on biomass and plant water status in ecological and agricultural applications. The combined data record of the C-band scatterometers on the European Remote-Sensing Satellites (ERS)-1/2, the Metop (Meteorological Operational satellite) series, and the planned Metop Second Generation satellites will span over 40 years, which would provide a long-term perspective on the role of vegetation in the climate system. Recent research has indicated that the unique viewing geometry of the Advanced SCATterometer (ASCAT) could be exploited to observe vegetation water dynamics. The incidence angle dependence of backscatter can be described with a second order polynomial, the slope and curvature of which are related to vegetation. In a study limited to grasslands, seasonal cycles, spatial patterns, and interannual variability in the slope and curvature were found to vary among grassland types and were attributed to differences in moisture availability, growing season length and phenological changes. To exploit ASCAT slope and curvature for global vegetation monitoring, their dynamics over a wider range of vegetation types needs to be quantified and explained in terms of vegetation water dynamics. Here, we compare ASCAT data with meteorological data and GRACE equivalent water thickness (EWT) to explain the dynamics of ASCAT backscatter, slope, and curvature in terms of moisture availability and demand. We consider differences in the seasonal cycle, diurnal differences, and the response to the 2010 and 2015 droughts across ecoregions in the Amazon basin and surroundings. Results show that spatial and temporal patterns in backscatter reflect moisture availability indicated by GRACE EWT. Slope and curvature dynamics vary considerably among the ecoregions. The evergreen forests, often used as a calibration target, exhibit very stable behavior, even under drought conditions. The limited seasonal variation follows changes in the radiation cycle and may indicate phenological changes such as litterfall. In contrast, the diversity of land cover types within the Cerrado region results in considerable heterogeneity in terms of the seasonal cycle and the influence of drought on both slope and curvature. Seasonal flooding in forest and savanna areas also produced a distinctive signature in terms of the backscatter as a function of incidence angle. This improved understanding of the incidence angle behavior of backscatter increases our ability to interpret and make optimal use of the ASCAT data record and vegetation optical depth products for vegetation monitoring.

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Section: Discussionsupporting

confidence: 87%

The influence of vegetation water dynamics on the ASCAT backscatter–incidence angle relationship in the Amazon

Petchiappan

Steele‐Dunne

Vreugdenhil³

et al. 2022

Hydrol. Earth Syst. Sci.

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“…Alternatively, Konings & Gentine (2017) demonstrated that spatial variations in isohydricity could be derived from microwave estimates of vegetation water content. Conceivably these data may offer a pathway to better capture inter-and intraspecific variation in drought responses at the ecosystem scale, directly relevant to global models (please refer to also Konings et al (2021) for a review). Nevertheless, microwave estimates are coarse (> 25 km), hampering comparison with field-based hydraulic traits; therefore, if used to parameterise models, we could run the risk of conflating within-species vs among-species drought risk.…”

Section: Future Directionsmentioning

confidence: 99%

Towards species‐level forecasts of drought‐induced tree mortality risk

et al. 2022

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Predicting species-level responses to drought at the landscape scale is critical to reducing uncertainty in future terrestrial carbon and water cycle projections.We embedded a stomatal optimisation model in the Community Atmosphere Biosphere Land Exchange (CABLE) land surface model and parameterised the model for 15 canopy dominant eucalypt tree species across South-Eastern Australia (mean annual precipitation range: 344-1424 mm yr −1 ). We conducted three experiments: applying CABLE to the 2017-2019 drought; a 20% drier drought; and a 20% drier drought with a doubling of atmospheric carbon dioxide (CO 2 ).The severity of the drought was highlighted as for at least 25% of their distribution ranges, 60% of species experienced leaf water potentials beyond the water potential at which 50% of hydraulic conductivity is lost due to embolism. We identified areas of severe hydraulic stress within-species' ranges, but we also pinpointed resilience in species found in predominantly semiarid areas. The importance of the role of CO 2 in ameliorating drought stress was consistent across species.Our results represent an important advance in our capacity to forecast the resilience of individual tree species, providing an evidence base for decision-making around the resilience of restoration plantings or net-zero emission strategies.

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“…In many cases, substantial increases in resolution are possible with drone‐mounted instruments (e.g., SIF, Mohammed et al, 2019). The information provided by these platforms is sensitive to limitations and biases, which are well‐reviewed elsewhere (Fisher et al, 2020; Konings et al, 2021; Magney et al, 2020), and will continue to benefit from validation with flux tower data (e.g. Fisher et al, 2020; Sun et al, 2017), including novel strategies for fusing data across different scales of observation (see Section 3.3).…”

Section: Informing Nbcss With a Full Set Of Tools And Approachesmentioning

confidence: 99%

“…These next‐generation platforms include (a) solar‐induced fluorescence (SIF), which is physiologically related to the rate of photosynthesis (Magney et al, 2020), (b) column‐averaged atmospheric CO 2 which can be used for “inverse” estimates of land carbon fluxes (Wang et al, 2019), and (c) instruments for sensing ecosystem water stress (e.g., ECOSTRESS, Fisher et al, 2020, and microwave data on canopy water content, Konings et al, 2021). While the spatial resolution of these satellite products can be coarse, some are now available at scales that match those of individual farms (e.g., ECOSTRESS, Fisher et al, 2020), and the need for finer‐scale versions of other products has been clearly articulated (Konings et al, 2021). In many cases, substantial increases in resolution are possible with drone‐mounted instruments (e.g., SIF, Mohammed et al, 2019).…”

Section: Informing Nbcss With a Full Set Of Tools And Approachesmentioning

confidence: 99%

Informing Nature‐based Climate Solutions for the United States with the best‐available science

Novick

Metzger

Anderegg

et al. 2022

Global Change Biology

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Nature‐based Climate Solutions (NbCS) are managed alterations to ecosystems designed to increase carbon sequestration or reduce greenhouse gas emissions. While they have growing public and private support, the realizable benefits and unintended consequences of NbCS are not well understood. At regional scales where policy decisions are often made, NbCS benefits are estimated from soil and tree survey data that can miss important carbon sources and sinks within an ecosystem, and do not reveal the biophysical impacts of NbCS for local water and energy cycles. The only direct observations of ecosystem‐scale carbon fluxes, for example, by eddy covariance flux towers, have not yet been systematically assessed for what they can tell us about NbCS potentials, and state‐of‐the‐art remote sensing products and land‐surface models are not yet being widely used to inform NbCS policymaking or implementation. As a result, there is a critical mismatch between the point‐ and tree‐scale data most often used to assess NbCS benefits and impacts, the ecosystem and landscape scales where NbCS projects are implemented, and the regional to continental scales most relevant to policymaking. Here, we propose a research agenda to confront these gaps using data and tools that have long been used to understand the mechanisms driving ecosystem carbon and energy cycling, but have not yet been widely applied to NbCS. We outline steps for creating robust NbCS assessments at both local to regional scales that are informed by ecosystem‐scale observations, and which consider concurrent biophysical impacts, future climate feedbacks, and the need for equitable and inclusive NbCS implementation strategies. We contend that these research goals can largely be accomplished by shifting the scales at which pre‐existing tools are applied and blended together, although we also highlight some opportunities for more radical shifts in approach.

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Detecting forest response to droughts with global observations of vegetation water content

Abstract: This is an open access article under the terms of the Creat ive Commo ns Attri bution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Cited by 105 publications

References 160 publications

The influence of vegetation water dynamics on the ASCAT backscatter–incidence angle relationship in the Amazon

The influence of vegetation water dynamics on the ASCAT backscatter–incidence angle relationship in the Amazon

Towards species‐level forecasts of drought‐induced tree mortality risk

Informing Nature‐based Climate Solutions for the United States with the best‐available science

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