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Assessment of African Vegetation Resources: New Perspectives from Satellite Passive Microwave Datasets

Periodic Reporting for period 1 - AfriVeg (Assessment of African Vegetation Resources: New Perspectives from Satellite Passive Microwave Datasets)

Reporting period: 2018-01-01 to 2019-12-31

• Problem/issue being addressed
Global climate change, caused by increasing greenhouse gas emissions (e.g. CO2), is occurring at rates much faster than anticipated and its effects are clearly felt worldwide, which poses major challenges for the sustainable development of human society as a whole and particularly those in the developing countries like Africa. Global terrestrial vegetation is a major CO2 pool with large spatial and temporal variations, but also large uncertainties due to our lack of knowledge on the underlying interaction between vegetation and climate. The poor understanding of vegetation-climate interaction limits the projection of further climate change, thereby our action to it. The impacts are especially severe in Africa as people’s livelihoods are strongly dependent on local ecosystem services, such as grazing, agriculture, firewood, and construction timber. In addition, the rapidly increasing African human population and fast-changing socio-economic conditions put more pressure on existing ecosystem services. From the remote sensing sector, the new advances in passive microwave technology, for example, the Soil Moisture Ocean Salinity (SMOS) mission, provides new perspectives for monitoring vegetation changes compared to the traditionally used greenness-driven approaches. This project is addressing the issue of how to make use of the novel satellite passive microwave technology to improve our understanding of vegetation changes at global and continental scales, with particular focus on Africa.

• The importance of society
The United Nations has identified 17 key Sustainable Development Goals (SDGs), which are an urgent call for action by all countries. This project is directly related to two of the 17 SDGs, which are climate action and life on land. The results and insights gained on global vegetation are beneficial for 1) improving the modeling of vegetation-climate interaction and thus more accurately projecting future scenarios of climate change and 2) for protecting and sustainably managing the vegetation resources, with particular importance for Africa.

• Overall objectives
The overall objective of this project is to accurately quantify vegetation spatial and temporal changes in terms of water and carbon. Specific scientific questions answered are:
1. How does the global plant water storage vary over the season and how they related to the seasonality in the canopy greenness?
2. How has the phenology of vegetation water content and greenness changed, respectively, during the last decades in the tropics and what are the main drivers behind the phonological changes?
3. What are the main environmental drivers of vegetation phenology (particularly the dry season greening) in Africa?

• Conclusions
This project has produced 2 peer-reviewed papers on top journals, including Nature Ecology & Evolution and Remote Sensing of Environment and 1 manuscript, answering the questions listed above. The results highlight that 1) the African Miombo woodlands exhibit a unique behavior regarding the seasonal coupling of water and greenness (i.e. large time lag between the two), which is different from the boreal and temperate forests, 2) satellite passive microwave can contribute to reveal a full picture of the land surface phenological changes in the tropics, with about 25% more areas as compared to only using optical remote sensing, and 3) groundwater interannual variability was the major driver of dry season greening in the African dry tropics.
• Work performed and main results achieved

- Global plant water storage
1. Data processing and analysis: I processed the global satellite passive microwave data of ESA SMOS and NASA AMSR-E/2 and the optical remote sensing data of MODIS, including noise reduction, seasonality extraction of the vegetation water content, vegetation greenness and soil water content variables.
2. Main results: We revealed the global patterns of seasonal variations in ecosystem-scale plant water storage and their relationship with leaf phenology. We find that seasonal variations in plant water storage are highly synchronous with leaf phenology for the boreal and temperate forests, but asynchronous for the tropical woodlands, where the seasonal development of plant water storage lags behind leaf area by up to 180 days. Contrasting patterns of the time lag between plant water storage and terrestrial groundwater storage are also evident in these ecosystems. A comparison of the water cycle components in seasonally dry tropical woodlands highlights the buffering effect of plant water storage on the seasonal dynamics of water supply and demand.

- Long-term phenological changes in the tropics
1. Data processing and analysis: I processed the AVHRR NDVI and VOD over the tropics, extracted the key phenological parameters, and calculate their trends over 1992-2012. I analyzed four potential environmental drivers of the phenological changes, i.e. fire change, land cover change, woody vegetation change, and rainfall change.
2. Main results: We find that about 50% of the pixels studied show significant phenological changes in either VOD or NDVI metrics. Drivers of phenological changes were assessed for pixels of high agreement between VOD and NDVI phenological metrics. We find rainfall variability and woody vegetation change to be the main forcing variables of phenological trends for most of the dry tropical biomes, while fire events and land cover change are recognized as second-order drivers.

- Environmental drivers of the African dry season greening
1. Data processing and analysis: I processed the MODIS EVI2, SMOS VOD, rainfall, and Sentinel-2 datasets over African dry tropics, extracted the leaf emerging dates (greening up) and rainfall onset dates over 2000-2019, and analyzed their causal relationship with groundwater, radiation, VPD, and temperature.
2. Main results so far: We find groundwater has a significantly stronger causal effect on the interannual variability of pre-rain leaf emergence than radiation, VPD, and temperature.

• Overview of the results and their exploitation and dissemination
The results have addressed issues around vegetation changes in water content from intra-annual to inter-annual, with a particular focus on Africa. Two peer-reviewed papers have been published in Nature Ecology & Evolution and Remote Sensing of Environment, along with the related social media articles. The results were also disseminated at two international conferences, i.e. the American Geophysical Union Fall meeting in 2018 and the ESA living planet symposium in 2019. The results were also disseminated in forms of university seminars and communications in two African countries, Senegal and Zambia combined with filed trips there in 2018 and 2019.
• Progress beyond the state of the art
The state of the art in remote sensing of vegetation studies was based on traditional optical sensors and greenness related indices. This project has done research around plant water content using the new satellite passive microwave technology at global and continental scales, which give us a new perspective on the way looking at vegetation resources, particularly in Africa.

• Socio-economic impact and the wider societal implications
Findings in this project are useful for Earth system models that will finally result in an improved prediction of further climate change influences.
Dry season greening of the dry tropical Miombo trees.