Satellite Observations of the Contrasting Response of Trees and Grasses to Variations in Water Availability

• Published in: Geophysical research letters Vol. 46; no. 3; pp. 1429 - 1440
• Main Authors: Walther, Sophia, Duveiller, Gregory, Jung, Martin, Guanter, Luis, Cescatti, Alessandro, Camps‐Valls, Gustau
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16-02-2019
• Subjects: Annual variations; Anomalies; Availability; Boreal forests; Carbon; Carbon sequestration; Carbon uptake; Chlorophyll; Chlorophylls; Detection; Ecological monitoring; Ecosystems; Fluorescence; forest; Forest ecosystems; Forests; GPP; Indicators; Interannual variations; Learning algorithms; Light; Machine learning; Photosynthesis; Pigments; Primary production; Productivity; Remote monitoring; Remote sensing; satellite; Satellite observation; Satellites; SIF; Soil; Soil moisture; Temperature; Trees; Uptake; Vegetation; Water; Water availability; water effects; Water shortages
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2018GL080535

Interannual variations in ecosystem primary productivity are dominated by water availability. Until recently, characterizing the photosynthetic response of different ecosystems to soil moisture anomalies was hampered by observational limitations. Here, we use a number of satellite‐based proxies for productivity, including spectral indices, sun‐induced chlorophyll fluorescence, and data‐driven estimates of gross primary production, to reevaluate the relationship between terrestrial photosynthesis and water. In contrast to nonwoody vegetation, we find a resilience of forested ecosystems to reduced soil moisture. Sun‐induced chlorophyll fluorescence and data‐driven gross primary production indicate an increase in photosynthesis as a result of the accompanying higher amounts of light and temperature despite lowered light‐use‐efficiency. Conversely, remote sensing indicators of greenness reach their detection limit and largely remain stable. Our study thus highlights the differential responses of ecosystems along a tree cover gradient and illustrates the importance of differentiating photosynthesis indicators from those of greenness for the monitoring and understanding of ecosystems.
Plain Language Summary
The capacity of vegetation to thrive and to sequester carbon depends on how much water they can have access to. In this work, we evaluate how different types of satellite observations can describe the response of vegetation to changes in soil moisture over the entire planet. The first source of observation measures only the greenness of the land surface, the second measures light that is emitted by pigments in plants which are photosynthetically active (chlorophyll fluorescence), and the third are simulations of gross carbon uptake derived from machine learning techniques. For periods of water shortage all three indicate a reduction of growth in ecosystems with few trees. However, in cold boreal forests, when soil moisture is particularly low, we still detect an increase in photosynthesis due to higher light and temperature conditions, but this is not reflected in the greenness indicator. This work illustrates how lack of water is not necessarily harmful for catching carbon through photosynthesis, but to monitor this effect, we need remote sensing indicators that measure more than just how green the plants are, and fluorescence is likely a good candidate.
Key Points
Satellite‐based plant greenness, chlorophyll fluorescence, and gross photosynthesis are evaluated with respect to soil moisture fluctuations
Ecosystem responses to water availability change sign along a tree cover gradient
We show spaceborne observations of light‐ and temperature‐enhanced forest photosynthesis in times of reduced soil moisture at constant greenness