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

Abstract

Abstract. 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 groundbased 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 þ3.34% 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 (À0.21% total change) from 2000-2013 while shrublands were greening faster than any other land cover type over the same period (þ8.93%). 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.

Drought Dynamics and Vegetation Productivity in Different Land Management Systems of Eastern Cape, South Africa—A Remote Sensing Perspective

“…Increasing vegetation productivity can also relate to bush encroachment which characterizes the invasion of alien species and especially the expansion of woody vegetation in areas where it is not naturally occurring [75,76]. This invasive vegetation suppresses palatable plant species and grasslands [76][77][78]. Bush encroachment is an ongoing problem in South Arica and in Eastern Cape Province [79,80] and could be one possible explanation for the depicted increasing trends mainly in communal land, where bush encroachment is an indicator of prevalent land degradation.…”

Mapping gains and losses in woody vegetation across global tropical drylands

“…However, the VOD wood shows much larger areas of positive trends as compared to VOD leaf (Fig. 4), indicating a potential underestimation of the spatial extent of shrub encroachment based on optical remote sensing data in this region (Saha et al, 2015). Manmade fires are used for controlling bush encroachment in Botswana and Zimbabwe (Gandiwa, 2011;Mudongo et al, 2016).…”

Vegetation Dynamics in the Upper Guinean Forest Region of West Africa from 2001 to 2015

“…However, trends in NDVI do not necessarily reflect changes in other vegetation characteristics, such as physiognomic structure and water content. Some research has suggested that NDVI can be used to estimate woody vegetation cover in savanna and woodland ecosystems [26,27], and that positive trends in NDVI can be interpreted as woody vegetation encroachment or densification [28,29]. In contrast, Mitchard and Flintrop [30] pointed out that the signals in the NDVI were more related to changes in the grass layer than woody vegetation cover in West Africa.…”