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University of Southampton (2020)

Rangeland vegetation dynamics in Africa during 1982-2015

D’Adamo, Francesco

Titre : Rangeland vegetation dynamics in Africa during 1982-2015

Auteur : D’Adamo, Francesco

Université de soutenance : University of Southampton

Grade : Doctor of Philosophy (PhD) 2020

Résumé
Rangelands are domestic or wildlife grazing lands including grasslands, woodlands, shrublands, and some extent of deserts. In Africa, rangelands cover approximately 28% (ca. 8,300,000 km2) of the continent, where they provide essential ecosystem services (e.g., meat and dairy products, water, shade, recreation, pollination) in support of the livestock rearing activities of some 270 million people. The rangelands of Africa are found where most of global rural poverty and hunger are concentrated. In other words, they occur in countries defined as some of the most vulnerable to climate change and anthropogenic transformations. Studying the way ecosystems respond to these disturbances should prioritise developing regions that directly support millions of people. However, the response of African rangelands to global environmental change, and therefore their capacity to sustain people’s livelihood, has not been studied in detail. Based on three decades of optical and microwave satellite data, and a dynamic global vegetation model, this study represents the first African-scale assessment of long-term rangeland vegetation dynamics. Overall, findings revealed that African rangelands greened- up between 1982 and 2015 (ca. 3,500,000 km2 greening vs. ca. 700,000 km2 browning), thus supporting the recent evidence of a greening Earth. In addition, while most (ca. 2,400,000 km2) changes in rangeland vegetation resulted to be controlled by climate (climatic-driven rangelands), there exist substantial areas (ca. 1,800,000 km2) where this is not the case (non-climatic-driven rangelands). This evidence may imply that many biogeochemical models, where climate is the main input information for vegetation growth simulations, might not capture the complete trajectory of current and future changes in biosphere-atmosphere interactions. Importantly, the investigation of long-term changes in the vegetation composition highlighted that a switch in the woody and herbaceous vegetation coverage occurred. While climatic greening (ca. 2,200,000 km2) resulted from positive trends in both woody and herbaceous cover, non-climatic greening (ca. 1,400,000 km2) was associated with an increase in woody cover and a concomitant decline in herbaceous vegetation. Opposite evidence, i.e., decreased woody cover and increased herbaceous vegetation, was observed in non-climatic browning rangelands (ca. 400,000 km2). These results suggest that while greening boosts climate change mitigation via high carbon uptake, the encroachment of woody species likely shortens the resources available to pastoral communities. On the other hand, woody-controlled browning attenuates carbon sequestration rates, but higher herbaceous cover may inform of potential more forage for pastoralists.

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