Agroecology 4 min

Satellite observations reveal secrets of dry tropical forest greening

PRESS RELEASE - In dry tropical forests, vegetation takes up water at the end of the wet season and stores it during the driest season of the year. This large amount of stored water enables trees to flush new leaves about one month before the next rainy season. This surprising phenomenon has been revealed for the first time using satellite observations, mainly in the African region of Miombo (around four times the surface area of France), in a study coordinated by the University of Copenhagen and INRA in collaboration with the CEA, CNRS, CNES and Bordeaux Science Agro. These findings, published on 13 August 2018 in Nature Ecology and Evolution, will help researchers improve current Earth system models (which do not fully account for plant hydraulic mechanisms) and future climate change and water cycle projections in these regions of the world.

Published on 13 August 2018

illustration Satellite observations reveal secrets of dry tropical forest greening
© INRAE, C. Ryan

What are the relationships between plant water storage and leaf development? Are both variables closely related in time and space across the Earth’s surface?  These are critical questions to improve vegetation-atmosphere feedback in Earth system models and predict ecosystem responses to climate change.

A discovery in the African tropical forest of Miombo


Using satellite observations, the study conducted by the University of Copenhagen1and INRA2, in collaboration with the CEA, CNRS, CNES and Bordeaux Science Agro3, demonstrated that seasonal variations in plant water storage and leaf development are highly synchronous in boreal and temperate regions. However, more surprisingly, the researchers showed that these variations are highly asynchronous in dry tropical forests, where an increase in plant water storage precedes vegetation greening by 25 to 180 days. The study focused on the Miombo woodlands, which cover an immense surface area of more than 2.7 million square kilometres to the south of the African rainforests. Satellite observations of this region clearly show that the leaf area index (LAI) begins to increase several weeks before the rainy season begins, a clear sign of “pre-rain” green up that has already been documented in numerous studies. The mechanisms behind this phenomenon are not yet fully understood but likely involve large construction costs to the plants, which must invest in their rooting system to access deep ground water and in their woody stems to increase their water storage capacity.

The novelty comes from observations of the L-band vegetation optical depth (L-VOD) index (a crucial indicator of the plant water content dynamic) from the European Space Agency (ESA)-CNES SMOS satellite that show that vegetation in Miombo takes up water at the end of the rainy season (when transpiration losses fall) and stores it in woody tissues during most of the dry season until the emergence of new leaves a few weeks before rain starts. This early leaf flushing has physiological and ecological advantages, reducing the time lag between the onset of the rainy season and that of photosynthetic activity. This intriguing hydraulic behaviour had previously been seen in in situ experiments of a few trees in dry tropical forests, particularly in Costa Rica. However, this new study is the first demonstrating that this is a large-scale phenomenon, visible over forested areas as large as the Miombo woodlands, as well as in the northern African woodlands and the Brazilian Cerrado.

Moreover, these physiological and hydrological processes are still not included in Earth system models. The new L-VOD data set will be key for improving the next generation of Earth system models, leading to more robust projections of the future climate and water cycle in these regions of the world.

1 Department of Geosciences and Natural Resource Management, University of Copenhagen, Copenhagen, Denmark
2 Interaction Sol Plante Atmosphère joint research unit (INRA, Bordeaux Sciences Agro), INRA Nouvelle-Aquitaine – Bordeaux
3 The other French laboratories involved in this research: Laboratoire des sciences du climat et de l’environnement (CEA-CNRS-UVSQ), Évolution et diversité biologique (CNRS-IRD-UT3 Paul Sabatier-ENFA), Centre d’études spatiales de la biosphère (CNES-CNRS-IRD-UT3 Paul Sabatier), Bordeaux Science Agro (National School of Agricultural Engineering of Bordeaux)

A large set of satellite observations

This study was based on a large set of satellite observations that aim to characterise the time variations in key hydrological and vegetation parameters at the ecosystem scale. The research benefited from the new SMOS-IC data set of the vegetation index referred to as L-band vegetation optical depth, or L-VOD, retrieved from space-borne observations of the ESA-CNES SMOS satellite. This index is closely related to the vegetation water content (VWC, kg/m2) of the whole canopy layer. More specifically, along with the L-VOD (a proxy of vegetation water storage), the other variables considered in the study include leaf area index (LAI) retrieved from optical satellite observations and used to parameterise foliar phenology, terrestrial groundwater storage anomalies (TWS) retrieved from GRACE satellites, surface soil moisture, rainfall and transpiration. Surface soil moisture observations considered here were retrieved simultaneously with L-VOD from the multi-angular SMOS observations.

When averaged at a yearly scale, the L-VOD index has been found to be closely related to global patterns of plant aboveground biomass, a feature that was used recently to quantify annual changes in sub-Saharan aboveground biomass carbon stock.For more information.

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Climate change and risks

Aboveground vegetation biomass in the tropics no longer has a positive impact on carbon stocks

PRESS RELEASE - Scientists from INRA, CEA, CNRS, CNES* and several international universities have quantified - over the 2010-2017 period – the time changes in the carbon stocks of aboveground vegetation biomass in the tropics. One main result is that over the studied time period, these stocks are almost constant: gains in biomass carbon stocks in some regions of the tropics are compensated by losses due to deforestation or diebacks linked to climate impacts (in particular the El Niño events). The above-ground biomass in the tropics, which used to be a carbon sink, is becoming neutral. Tropical regions could even become a carbon source for the atmosphere in the near future, thus enhancing global warming. The results are published in Nature Plants on July 29, 2019.

12 December 2019


WeedElec, from "haute couture" to cutting-edge weeding

Launched at the beginning of 2018 under the aegis of 5 partners – including the EMMAH Joint Research Unit under the joint supervision of INRA and IRSTEA – the WeedElec project combines robots and drones for the detection and control of weeds using electric current. Claude Doussan is a researcher at INRAE Avignon, and presents its genesis and development.

17 December 2019


CA-SYS: a collaborative platform for experimentation in agroecology

Through the CA-SYS platform (Co-designed Agroecological System Experiment), INRA and its partners are designing experiments on innovative and sustainable agroecological systems. This arable infrastructure is using new experimental methods by combining varietal breeding, a minimisation of inputs, the exploitation of biological interactions and the organisation of agricultural space.

28 January 2020