Climate change may wipe out Eswatini’s 2 major savanna tree species: Study

Climate change may wipe out Marula and Knobthorn — two tree species in Eswatini — from their current ranges, according to a new study. These widely occurring and dominant southern African species are critical to the functioning of lowland savannas. 

Compared with other terrestrial biomes, tropical savannas face heightened risks from a rapidly changing climate, noted the study published in Scientific Reports on May 10, 2023. A biome is a large natural community of plants and animals that live in a specific habitat and climate.

A westward shift in the distributions of marula and knobthorn towards central Eswatini is anticipated based on projected climate scenarios from 2041 to 2070, the research added. Responses of these two keystone species to climate change may decouple them in future. Such changes in the habitat of keystone species are likely to have considerable cascading effects.

Also read: Poaching, habitat loss push Africa’s elephants to the brink

“Marula and knobthorn showed idiosyncratic responses across a range of climate conditions, with marula expanding beyond current ranges to make up for lost distributions, while knobthorn patches receded towards the core of their ranges,” the study noted.

This suggested that these trees may be losing suitable climates within current ranges. Marula showed evidence of tracking climate change by emerging in previously cooler and uncolonised areas. On the other hand, knobthorn is unlikely to do so. In addition, knobthorn’s patchy distributions were limited to a smaller geographic area — thus, any range loss can cause a local wipeout.

Knobthorn was unable to establish outside of its core range and faced limitations like land use, terrain and soil properties and herbivory to occupying broader climatic ranges, the study found. These barriers will make it unlikely for the species to establish in the predicted future distributions in Eswatini and across southern Africa.

The observed lack of movement in climate-suitable areas could also be due to shade — an important barrier to the regeneration of shade-intolerant savanna tree species like knobthorn, the study noted.

When it comes to marula, field data suggested that the tree populations are contracting at the warmer sites and expanding towards cooler temperatures outside their current adult range.

Also read: The long shadow of colonial forestry is a threat to savannas and grasslands

Marula would unlikely persist in some of the region’s hottest areas, with regional temperature increases for Eswatini projected to be 4°C between 2041-2070 and 2071-2100. However, the species’ ability to disperse and recruit in newly suitable climates beyond current range distributions could counter these effects, the study added.

Marula tree is considered to be Africa’s botanical treasure. Not only fruits but also its nuts are rich in minerals and vitamins. It was a dietary mainstay in South Africa, Botswana and Namibia throughout ancient times.

Knobthorn, a deciduous African tree, occurs in various savanna regions, often at low altitudes and in rocky areas The wood is hard as well as drought and termite-resistant — it has been used to make fence posts and mine props.

Africa showed an average increase in warming of approximately 0.3°C per decade between 1991 and 2021, according to State of the Climate in Africa 2021. This was faster than the warming of 0.2°C per decade, which occurred between 1961 and 1990.

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