Interacting Effects of Leaf Water Potential and Biomass on Vegetation Optical Depth

Abstract

Remotely sensed microwave observations of vegetation optical depth (VOD) have been widely used for examining vegetation responses to climate. Nevertheless, the relative impacts of phenological changes in leaf biomass and water stress on VOD have not been explicitly disentangled. In particular, determining whether leaf water potential (ψL) affects VOD may allow these data sets as a constraint for plant hydraulic models. Here we test the sensitivity of VOD to variations in ψL and present a conceptual framework that relates VOD to ψL and total biomass including leaves, whose dynamics are measured through leaf area index, and woody components. We used measurements of ψL from three sites across the US—a mixed deciduous forests in Indiana and Missouri and a piñon‐juniper woodland in New Mexico—to validate the conceptual model. The temporal dynamics of X‐band VOD were similar to those of the VOD signal estimated from the new conceptual model with observed ψL (R2 = 0.6–0.8). At the global scale, accounting for a combination of biomass and estimated ψL (based on satellite surface soil moisture data) increased correlations with VOD by ~ 15% and 30% compared to biomass and water potential, respectively. In wetter regions with denser and taller canopy heights, VOD has a higher correlation with leaf area index than with water stress and vice versa in drier regions. Our results demonstrate that variations in both phenology and ψL must be considered to accurately interpret the dynamics of VOD observations for ecological applications.

Macro to micro: microwave remote sensing of plant water content for physiology and ecology

“…If variations in biomass are assumed small during the peak growing season, this period could also be used to isolate day-to-day variations in plant water status. For example, when Momen et al (2017) compared up-scaled ground-measurements of Ψ l and VOD at three North American forested sites during the growing season, LAI variations played a relatively minor roleaccounting also for LAI instead of Ψ l only led to only a small increase in the explained VOD variance.…”

Implementing Plant Hydraulics in the Community Land Model, Version 5

“…Such trait data have been shown to improve predictions of species vulnerability to drought (Anderegg, Konings, et al, ; Choat et al, ; Giardina et al, ; Mackay et al, ; Powell et al, ). Likewise, vegetation water status observations are now available from remote sensing platforms, at a scale that is directly comparable to model development (Grant et al, ; Konings et al, ) and therefore can be used to validate model results (Konings et al, ; Momen et al, ).…”

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

“…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.…”