Abstract
The ecological system of the desert/grassland biome transition zone is fragile and extremely sensitive to climate change and human activities. Analyzing the relationships between vegetation, climate factors (precipitation and temperature), and human activities in this zone can inform us about vegetation succession rules and driving mechanisms. Here, we used Landsat series images to study changes in the normalized difference vegetation index (NDVI) over this zone in the Sahel region of Africa. We selected 6315 sampling points for machine-learning training, across four types: desert, desert/grassland biome transition zone, grassland, and water bodies. We then extracted the range of the desert/grassland biome transition zone using the random forest method. We used Global Inventory Monitoring and Modelling Studies (GIMMS) data and the fifth-generation atmospheric reanalysis of the European Centre for Medium-Range Weather Forecasts (ERA5) meteorological assimilation data to explore the spatiotemporal characteristics of NDVI and climatic factors (temperature and precipitation). We used the multiple regression residual method to analyze the contributions of human activities and climate change to NDVI. The cellular automation (CA)-Markov model was used to predict the spatial position of the desert/grassland biome transition zone. From 1982 to 2015, the NDVI and temperature increased; no distinct trend was found for precipitation. The climate change and NDVI change trends both showed spatial stratified heterogeneity. Temperature and precipitation had a significant impact on NDVI in the desert/grassland biome transition zone; precipitation and NDVI were positively correlated, and temperature and NDVI were negatively correlated. Both human activities and climate factors influenced vegetation changes. The contribution rates of human activities and climate factors to the increase in vegetation were 97.7% and 48.1%, respectively. Human activities and climate factors together contributed 47.5% to this increase. The CA-Markov model predicted that the area of the desert/grassland biome transition zone in the Sahel region will expand northward and southward in the next 30 years.
Highlights
Vegetation is an important part of the global ecosystem, which is very sensitive to both climate change and human activities
In 2001–2019, the desert area in the entire region decreased by 10.1 × 104 km2; the desert/grassland biome transition zone decreased by 33.5 × 104 km2; the grassland area increased by 45.9 × 104 km2; and the water body area decreased by 3.2 × 104 km2
The areas in which normalized difference vegetation index (NDVI) was positively correlated with precipitation accounted for 86.7% of the total area, and were distributed across most of the desert/grassland biome transition zone
Summary
Vegetation is an important part of the global ecosystem, which is very sensitive to both climate change and human activities. It acts as an indicator of global environmental change [1]. The desert/grassland biome transition zone is a cross-regional ecosystem, that has desert to grassland characteristics. It is very fragile and sensitive to climate change [2]. Studying the development and changes of the desert/grassland biome transition zone is of great significance for exploring the impacts of human activities and climate factors on changes in the surface environment in arid and semi-arid regions. Relatively few studies have focused on the desert/grassland biome transition zone, and their research topics mainly focus on the following aspects: first, long-term studies of the vegetation phenology of the desert/grassland biome transition zone from the perspective of climate change [2]; second, investigations of soil microbial community structures and metabolic characteristics in the desert/grassland biome transition zone [3]; and third, studies of the vegetation species in the desert/grassland biome transition zone [4]
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