Ecosystem structural changes controlled by altered rainfall climatology in tropical savannas

Abstract

Tropical savannas comprise mixed woodland grassland ecosystems in which trees and grasses compete for water resources thereby maintaining the spatial structuring of this ecosystem. A global change in rainfall climatology may impact the structure of tropical savanna ecosystems by favouring woody plants, relative to herbaceous vegetation. Here we analysed satellite data and observed a relatively higher increase in woody vegetation (5%) as compared to the increase in annual maximum leaf area index (LAImax, an indicator of the total green vegetation production) (3%) in arid and semi-arid savannas over recent decades. We further observed a declining sensitivity of LAImax to annual rainfall over 56% of the tropical savannas, spatially overlapping with areas of increased woody cover and altered rainfall climatology. This suggests a climate-induced shift in the coexistence of woody and herbaceous vegetation in savanna ecosystems, possibly caused by altered hydrological conditions with significance for land cover and associated biophysical effects such as surface albedo and evapotranspiration.

African dryland ecosystem changes controlled by soil water

“…The high percentage of captured covariance between VOD and soil water, and the clear consistent trends found in the time‐series, suggests that soil water was the dominant factor governing VOD changes in African drylands (Figure ). Overall, in accordance with previous studies documenting the effects of water availability on drylands ecosystem changes (Brandt et al, ; Brandt et al, ; Huber, Fensholt & Rasmussen, ; R. Fensholt et al, ; Ibrahim, Balzter, Kaduk, & Tucker, ; Wei et al, ; Zhang et al, ), our study showed that ecosystem changes are controlled by soil water in African drylands.…”

“…At the same time, we found that pixels with negative trends in NDVI and positive trends in soil water cover 33.4% of the African drylands, mainly in Somalia, southern Africa, and the north of about 14°N, and the same regions (with the exception of Somalia) showed an increase in VOD (Figure S7). This result is consistent with early observations by Andela, Liu, van Dijk, de Jeu, and McVicar (), which suggested that woody plants benefit more from increased soil water than herbaceous vegetation (Zhang et al, ). The contribution of soil water to vegetation changes decreased with increasing humidity (Figure S6).…”

“…The results add further weight to the idea, widely reported over the last decades (Mitchard & Flintrop, ; Skowno et al, ; Wigley, Bond, & Hoffman, ), that woody encroachment prevailed in African drylands during 1993–2012. Moreover, hot spot areas of increased VOD coincide with areas where woody vegetation increased has been reported (Tian, Brandt, Liu, Rasmussen, & Fensholt, ; Zhang et al, ). Contrary to the widespread greening in the African Sahel interpreted as a recovery from the great Sahelian droughts (Anyamba & Tucker, ; Herrmann, Anyamba & Tucker, ), our study observed a recently decreasing NDVI in the Sahel with the most pronounced decrease in eastern Sudan and western Niger.…”

“…Over the past few decades, an increase in woody plants, characterized by woody thickening and encroachment, has been widely reported in African drylands (Mitchard & Flintrop, ; Sankaran et al, ; Tian, Brandt, Liu, Rasmussen, & Fensholt, ; Wigley, Bond, & Hoffman, ; Zhang et al, ). The studies focusing on woody vegetation trends in African drylands tend to use dry‐season normalized difference vegetation index (NDVI; Brandt et al, ; Horion, Fensholt, Tagesson, & Ehammer, ; Mitchard & Flintrop, ), which is derived from the near‐infrared and red spectral reflections and provides a measure of chlorophyll abundance in the canopy layer.…”

Vegetation state changes in the course of shrub encroachment in an African savanna since about 1850 CE and their potential drivers

“…Furthermore, under restored watering conditions, photosynthetic rates recover faster when inhibited by stomatal than by metabolic factors, which in turn affects the competitiveness of C 4 grasses under optimal conditions (Bellasio et al, 2018). Such a feedback mechanism implies that at higher atmospheric pCO 2 concentration, an increase in the frequency of droughts and heavy rains may favor the expansion of C 3 vegetation (Zhang et al, 2019).…”