Estimating Global Ecosystem Isohydry/Anisohydry Using Active and Passive Microwave Satellite Data

Abstract

The concept of isohydry/anisohydry describes the degree to which plants regulate their water status, operating from isohydric with strict regulation to anisohydric with less regulation. Though some species level measures of isohydry/anisohydry exist at a few locations, ecosystem‐scale information is still largely unavailable. In this study, we use diurnal observations from active (Ku‐Band backscatter from QuikSCAT) and passive (X‐band vegetation optical depth (VOD) from Advanced Microwave Scanning Radiometer on EOS Aqua) microwave satellite data to estimate global ecosystem isohydry/anisohydry. Here diurnal observations from both satellites approximate predawn and midday plant canopy water contents, which are used to estimate isohydry/anisohydry. The two independent estimates from radar backscatter and VOD show reasonable agreement at low and middle latitudes but diverge at high latitudes. Grasslands, croplands, wetlands, and open shrublands are more anisohydric, whereas evergreen broadleaf and deciduous broadleaf forests are more isohydric. The direct validation with upscaled in situ species isohydry/anisohydry estimates indicates that the VOD‐based estimates have much better agreement than the backscatter‐based estimates. The indirect validation with prior knowledge suggests that both estimates are generally consistent in that vegetation water status of anisohydric ecosystems more closely tracks environmental fluctuations of water availability and demand than their isohydric counterparts. However, uncertainties still exist in the isohydry/anisohydry estimate, primarily arising from the remote sensing data and, to a lesser extent, from the methodology. The comprehensive assessment in this study can help us better understand the robustness, limitation, and uncertainties of the satellite‐derived isohydry/anisohydry estimates. The ecosystem isohydry/anisohydry has the potential to reveal new insights into spatiotemporal ecosystem response to droughts.

Beyond soil water potential: An expanded view on isohydricity including land–atmosphere interactions and phenology

“…It is also notable that our results suggest isohydricity is a reasonable predictor of cross‐site dynamics of GEP (Figure ). This result supports the integration of remotely sensed isohydricity indexes (Hwang et al, ; Konings & Gentine, ; Li et al, ) with remotely sensed proxies for photosynthetic functioning.…”

“…In some cases, the theory has been extended to characterize how plant photosynthesis, growth, and mortality respond to climate variability (Gu, Pallardy, Hosman, & Sun, ; Konings et al, ; Kumagai & Porporato, ; McDowell et al, ; Vose & Elliott, ). Recent advances in diagnosing ∂Ψ L /∂Ψ S from remotely sensed data (Konings & Gentine, ; Li et al, ; Momen et al, ) seemingly open the door for linking isohydricity to broad spatial patterns in climate and plant function.…”

Beyond isohydricity: The role of environmental variability in determining plant drought responses

“…Even as (an)isohydricity metrics have come under more scrutiny within the plant ecophysiology community (Hochberg et al, ; Martínez‐Vilalta & Garcia‐Forner, ), their adoption in other scientific fields, for example, hydrology and water resources, forest management, remote sensing, and earth systems modelling, is proliferating. Prompted by the urgency to characterize plant water relations at the biome level, the increasing availability of trait datasets, and advances in remote sensing, many of these new applications (e.g., Giardina et al, ; Konings & Gentine, ; Li et al, ) diagnose (an)isohydricity from variations in plant water potential at 100‐ to 1,000‐km scales, thereby encompassing an enormous range of variability. Such studies have creatively leveraged novel remote sensing products and placed important empirical constraints on predictions of variations in plant water potentials at large scales.…”

“…Both response-based and trait-based have come under more scrutiny within the plant ecophysiology community (Hochberg et al, 2018;Martínez-Vilalta & Garcia-Forner, 2016), their adoption in other scientific fields, for example, hydrology and water resources, forest management, remote sensing, and earth systems modelling, is proliferating. Prompted by the urgency to characterize plant water relations at the biome level, the increasing availability of trait datasets, and advances in remote sensing, many of these new applications (e.g., Giardina et al, 2018;Konings & Gentine, 2016;Li et al, 2017)…”

Multicomponent Satellite Assessment of Drought Severity in the Contiguous United States From 2002 to 2017 Using AMSR‐E and AMSR2

“…The LPDR data are posted to a consistent 25-km global Equal-Area Scalable Earth (EASE) Grid projection format (Armstrong & Brodzik, 1995). The current AMSR LPDR (version 2) has been used for a diverse set of science application studies, including quantifying the influence of vegetation and soil moisture conditions on SMAP FW retrievals (Du, Kimball, Galantowicz, et al, 2018), studying ecosystem water storage and phenology (Brandt et al, 2018;Tian et al, 2018), detecting vegetation isohydry/anisohydry behavior and resilience to drought (Anderegg et al, 2018;Li et al, 2017), and mapping paddy rice planting areas (Song et al, 2018).…”