Abstract
Brachystegia spiciformis Benth. is the dominant component of miombo, the sub-tropical woodlands which cover 2.7 million km2 of south-central Africa and which is coincident with the largest regional centre of endemism in Africa. However, pollen records from the genus Brachystegia suggest that miombo has experienced rapid range retraction (~450 km) from its southernmost distributional limit over the past 6000 years. This abrupt biological response created an isolated (by ~200 km) and incomparable relict at the trailing population edge in northeast South Africa. These changes in miombo population dynamics may have been triggered by minor natural shifts in temperature and moisture regimes. If so, B. spiciformis is likely to be especially responsive to present and future anthropogenic climate change. This rare situation offers a unique opportunity to investigate climatic determinants of range shift at the trailing edge of a savannah species. A niche modelling approach was used to produce present-day and select future B. spiciformis woodland ecological niche models. In keeping with recent historical range shifts, further ecological niche retraction of between 30.6% and 47.3% of the continuous miombo woodland in Zimbabwe and southern Mozambique is predicted by 2050. Persistence of the existing relict under future climate change is plausible, but range expansion to fragmented refugia in northeast South Africa is unlikely. As Brachystegia woodland and associated biota form crucial socio-economic and biodiversity components of savannas in southern Africa, their predicted further range retraction is of concern.
Highlights
At regional and global scales, climate broadly limits the distribution of plant taxa,[1,2] and the response of species to changing environments is likely to be largely determined by population responses at range margins.[3]
Besides the fact that White’s map of African vegetation[9] does not indicate miombo woodland presence in South Africa, the distribution is coincident with the present-day ecological niche models (ENMs) projection for B. spiciformis in southern Africa
With future climate-induced migration ruled out and opportunities for adaptation beyond phenotypic adjustment limited, what is the fate of South Africa’s only miombo woodland? We suggest that the medium-term persistence of the relict is plausible based on (1) future climate change projections (2100), which suggest that local climatic variables will remain within the ‘most suitable’ range of B. spiciformis, (2) the longevity of genets and the species’ ability to regenerate vegetatively and (3) its historical resilience
Summary
At regional and global scales, climate broadly limits the distribution of plant taxa,[1,2] and the response of species to changing environments is likely to be largely determined by population responses at range margins.[3] few studies have examined climatic determinants of range shift at the trailing margin of a savanna species in the southern hemisphere. Is the dominant component of miombo, the colloquial term used to describe sub-tropical woodlands dominated by Brachystegia, Julbernardia and Isoberlinia – three closely related genera in the family Fabaceae and subfamily Caesalpinioideae.[5] Miombo encompasses the woodland-dominated savanna ecosystems[6,7] which cover (Figure 1) an estimated 2.7 million km[2] of south-central Africa[8] and is coincident with White’s9 Zambezian Phytochorion, the largest regional centre of endemism in Africa[10]. Most of the dominant miombo canopy species, including B. spiciformis, are considered to be fire-tender species, which decline in abundance under regular burning and increase under complete fire protection.[5]
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