Satellite observed vegetation dynamics and drivers in the Namib sand sea over the recent 20 years

Abstract

Monitoring dryland vegetation trends and examining the drivers are of great importance to understand the dryland vegetation response to future climate changes. Recent findings through satellite data indicate that vegetation greenness has increased in several regions worldwide. These greening patterns are driven by human activities or combined human activities and environmental factors. However, the analyses of greenness trend for regions without direct human activities and shrub expansion in drylands are still lacking. To this end, this study investigates the vegetation trend across the Namib sand sea over March 2000 to December 2018 using monthly Normalized Difference Vegetation Index (NDVI) and examines several potential drivers including precipitation, temperature and atmospheric CO2 concentration. For the NDVI time series across the whole study region, a significant greening trend was found over March 2000 to September 2012 based on Mann–Kendall test but not over the whole study period. Structural equation modelling results indicated that precipitation and CO2 were the dominant drivers of greening. Temperature showed negative effects on vegetation greenness, indicating warming would reduce plant growth in the study region. Spatially, 75% of the region showed statistically significant greening over March 2000 to September 2012 and 39.30% for March 2000 to December 2018. The different vegetation trend results between the entire region and the pixel scale implied that location-specific greening could be masked by an overall trend. Our study suggested that precipitation (especially the large episodic precipitation events) and CO2 are dominant drivers of the observed greening in the Namib. Our findings fill an important knowledge gap of vegetation dynamics in regions without direct human activities.