Rethinking catastrophe? Historical trajectories and modelled future vegetation change in southern Africa

Abstract

Most climate change projections for southern Africa describe a hotter and drier future for the subcontinent with catastrophic consequences for the environment and the socio-ecological sustainability of the region. We investigate whether evidence of the projections proposed for the climate and vegetation of the subcontinent is already evident. Results from the climate record indicate that the historical trend of increasing temperature is consistent with future projections for the region. Rainfall, however, appears not to have changed significantly.Results from an analysis of 1,321 repeat historical photographs indicate broad trends in vegetation trajectories in the major biomes of southern Africa. Contrary to early projections for the Succulent Karoo biome, biomass and cover have increased, largely in response to changes in land use practices in the region. Cover in the fire-adapted Fynbos biome has either remained stable or has increased over time with an unanticipated expansion of forest species, particularly in localities which have been protected from fire for long periods. The shrub-dominated Nama-karoo biome has seen an increase in grass cover, and rather than contracting, as suggested in the early modelled projections, the Grassland biome appears to have expanded westwards into former Nama-karoo biome sites. The Savanna biome has experienced a rapid increase in woody plant at rates that have not been anticipated by the models.An analysis of historical trajectories provides a useful context against which future trajectories can be evaluated. It also illustrates how land-use management has influenced vegetation change in the past and what might be done to mitigate some of the worst impacts of climate change in the future.

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

“…There is also some indication that African savannas are characterized by grassland/woodland phases occurring with a periodicity of 250-600 years (Gillson, 2004;Gil-Romera et al, 2010). Such woodland phases were not always led by the spread of encroaching species (Gil-Romera et al, 2010;Scott, Cooremans, de Wet, & Vogel, 1991), which suggests that a mere increase in woody cover, as reported by several studies since the 20th century (Hoffman, Rohde, & Gillson, 2019;van Rooyen, le Roux, van der Merwe, van Rooyen, & Geldenhuys, 2018;Wiegand, Ward, & Saltz, 2005), does not necessarily mean shrub encroachment. Accordingly, identification of encroached states should be related to the increasing cover of encroacher species and the suppression of perennial grasses (Gil-Romera et al, 2010).…”

Altered ignition catchments threaten a hyperdiverse fire‐dependent ecosystem

“…Most Fynbos species are adapted to cope with or depend on fire and their occurrence and distribution are limited by specific fire Forest expansion is becoming widespread in the CFR, representing a serious threat to biodiversity conservation (Hoffman, Rohde, & Gillson, 2019). While the forest species are indigenous, most are common and widely distributed, whereas the Fynbos ecosystems are predominantly unique and threatened, containing many endemic species (Rebelo et al, 2006) (Pooley, 2014).…”

Climate change impacts on water security in global drylands

“…43 In other regions, widespread dryland greening has been observed, 53,54 with trends driven by large-scale increases in woody cover and a limited increase in herbaceous production at desert-grassland interfaces. 45,55,56 Shrub encroach- ment and increases in woody vegetation have been recorded widely in North and Central American drylands, 57 the West African Sahel, 58,59 southern African shrublands, 60,61 central Asia, 62 and most tropical savannas, 63 with changes driven by seasonal combinations of changing temperature and rainfall, CO 2 fertilization, and changes in land management (e.g., decreases in large browsers, increased sustained heavy grazing, and fire suppression). 42,57,64 Increases in woody plants are sometimes accompanied by decreases in palatable grass species 65 and significantly reduced grass biomass.…”