Leaf surface water, not plant water stress, drives diurnal variation in tropical forest canopy water content

Xiangtao Xu; Alexandra G Konings; Marcos Longo; Andrew Feldman; Liang Xu; Sassan Saatchi; Donghai Wu; Jin Wu; Paul Moorcroft

doi:10.1111/nph.17254

Leaf surface water, not plant water stress, drives diurnal variation in tropical forest canopy water content

New Phytol. 2021 Jul;231(1):122-136. doi: 10.1111/nph.17254. Epub 2021 Mar 13.

Authors

Xiangtao Xu^{1

2}, Alexandra G Konings³, Marcos Longo⁴, Andrew Feldman⁵, Liang Xu⁴, Sassan Saatchi^{4

6}, Donghai Wu², Jin Wu⁷, Paul Moorcroft¹

Affiliations

¹ Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA, 02138, USA.
² Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY, 14850, USA.
³ Department of Earth System Science, Stanford University, Stanford, CA, 94305, USA.
⁴ Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, 91109, USA.
⁵ Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA.
⁶ Institute of Environment and Sustainability, University of California, Los Angeles, CA, 90024, USA.
⁷ School of Biological Sciences, University of Hong Kong, Hong Kong, China.

PMID: 33539544
DOI: 10.1111/nph.17254

Abstract

Variation in canopy water content (CWC) that can be detected from microwave remote sensing of vegetation optical depth (VOD) has been proposed as an important measure of vegetation water stress. However, the contribution of leaf surface water (LW_s ), arising from dew formation and rainfall interception, to CWC is largely unknown, particularly in tropical forests and other high-humidity ecosystems. We compared VOD data from the Advanced Microwave Scanning Radiometer for the Earth Observing System (AMSR-E) and CWC predicted by a plant hydrodynamics model at four tropical sites in Brazil spanning a rainfall gradient. We assessed how LW_s influenced the relationship between VOD and CWC. The analysis indicates that while CWC is strongly correlated with VOD (R² = 0.62 across all sites), LW_s accounts for 61-76% of the diurnal variation in CWC despite being < 10% of CWC. Ignoring LW_s weakens the near-linear relationship between CWC and VOD and reduces the consistency in diurnal variation. The contribution of LW_s to CWC variation, however, decreases at longer, seasonal to inter-annual, time scales. Our results demonstrate that diurnal patterns of dew formation and rainfall interception can be an important driver of diurnal variation in CWC and VOD over tropical ecosystems and therefore should be accounted for when inferring plant diurnal water stress from VOD measurements.

Keywords: ED2; X-band; canopy water content (CWC); ecosystem modeling; leaf surface water; vegetation optical depth (VOD).

Publication types

Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

Brazil
Dehydration
Ecosystem*
Forests
Plant Leaves
Seasons
Trees
Water*

Substances

Water