Examining the linkage between shrub encroachment and recent greening in water‐limited southern Africa

摘要

There have been reports of widespread increases in satellite‐derived vegetation indices in drylands across the globe. At the same time, there are numerous field‐based observations of increases in the density of small woody plant species, a process known as shrub encroachment. We compare ground‐based observations of shrub encroachment with information from 14 years of Tropical Rainfall Measuring Mission (TRMM) precipitation data, Moderate Resolution Imaging Spectroradiometer (MODIS) Normalized Difference Vegetation Index (NDVI) and MODIS land cover data for southern Africa, focusing on areas that receive less than 1000 mm of precipitation per year. Cumulative precipitation and maximum NDVI were computed for each year and linear regression analysis was used to correct for interannual variability in precipitation. Accounting for precipitation effects, we found an underlying, overall greening in southern Africa, with NDVI increasing +3.34% on average over the 2000–2013 study period. Over 15% of the region is undergoing statistically significant change, strongly biased towards greening. The strongest greening was in a coherent band stretching from northern Namibia to Lesotho. Ground‐based reports of shrub encroachment tend to overlap with strong greening; we suggest that these processes are linked. Reports that intersect our period of record showed overwhelmingly positive trends, indicating that our method is suitable for detecting shrub encroachment. Shrublands and savannas accounted for 78% of the study region. Savannas did not change appreciably (?0.21% total change) from 2000–2013 while shrublands were greening faster than any other land cover type over the same period (+8.93%). Fast rates of greening in this biome may be indicative of shrub expansion. Large‐scale shrub encroachment will have important consequences for dryland degradation and global carbon dynamics.