CONSERVATION

Earth Overshoot Day: Climate Crisis Impacts on South Africa’s Plant Diversity

AUG 17, 2020 | Written by Zoë Chapman Poulsen. Photos by Nigel Forshaw, Zoë Chapman Poulsen & Isobel Johnson.

Earth Overshoot Day: A Critical Threshold for the Planet

Above: The agricultural landscape of the Overberg looking towards Caledon at the height of the 2017 drought. Photo: Zoë Chapman Poulsen.

This year’s Earth Overshoot Day falls on 22 August of this year, acting as a reminder to all of our impact on the planet. Earth Overshoot Day is the day of the year that we cross an important threshold, the date that humanity’s demand for ecological resources exceeds that which the Earth can regenerate during that year.

With 60% of humanity’s ecological footprint being carbon, there is no hiding from the climate crisis, which is already impacting on the ecosystems and biodiversity of southern Africa. This week on the BotSoc Blog we will be taking a closer look at how the changing climate is impacting and may continue to impact on South Africa’s ecosystems and plant diversity.

The Climate Crisis: Increasing Temperatures & Aridity for southern Africa

Above: In 2017 the Calvinia Dam sits empty after two years of no rainfall during the worst drought in living memory. Photo: Zoë Chapman Poulsen.

South Africa is one of Africa’s leaders in climate change research, and the findings are deeply worrying. Climate change plays a significant role in impacting on water resources, food and water security, health and ecosystem services. Recognition of the problem and finding solutions for climate change mitigation and adaptation are key.

South Africa’s average annual temperatures have increased by at least 1.5 times more than the global average over the last 50 years. These trends are forecast to continue with temperatures projected to rise by 3-6°C by 2081-2100. Models also suggest that most parts of South Africa are likely to become drier, with increasing frequency of extreme rainfall events already a reality.

On the Red List of South African Plants, a relatively small number of species have been identified as being at risk due to climate change, but this number is likely to increase as our understanding of how plants respond to the climate crisis improves.

On the Edge: Impacts of Drought

Above: Drought has lead to extensive mortality of shrubs in the Succulent Karoo near Louriesfontein, Namaqualand. Photo: Zoë Chapman Poulsen.

Increasing aridity and temperatures due to climate change have been forecast to lead to greater frequency and intensity of drought periods, particularly affecting semi-arid ecosystems in South Africa including the Nama Karoo and Succulent Karoo.

With these ecosystems normally reliant on low but stable winter rainfall to trigger growth of plants including both autumn and winter flowering geophytes (bulbs), extreme drought and increasingly unreliable rainfall are becoming commonplace. In Namaqualand in response to the current drought, researchers have noted extensive mortality of trees, shrubs and large succulents at a scale never seen before.

Nowhere to Go: Impacts on Montane Flora

Above: Protea cryophila. Photo: Nigel Forshaw. www.inaturalist.org

South Africa’s mountain ranges including the Cape Fold Mountains, Drakensberg and others are home to a range of specialist species adapted to very specific growing conditions in narrow microhabitats. These makes many of these species more vulnerable to a changing climate as migration to higher altitudes will lead to contraction of their distribution ranges.

It is possible that this may eventually lead to species extinctions when no more suitable habitat can be colonised by growing at increased altitudes. Furthermore, it has been shown that areas at higher elevation have a higher rate of warming. Several members of the genus Protea have been identified as being under threat due to the impacts of climate change on montane species, including Protea cryophila, Protea convexa and Protea montana.

Fiery Futures: Impacts of Changing Fire Regimes

Above: Protea seed cone after the Betty’s Bay fire in 2019. Photo: Zoë Chapman Poulsen.

Climate change projections also indicate that in fire prone and fire dependent ecosystems in South Africa such as fynbos, renosterveld, savanna and grassland vegetation types, fire regimes are likely to change to higher fire frequency and intensity. These ecosystems are dependent on fire for their survival, including for the reproduction of many different plant species.

However, if fire frequency increases too much, it is possible that it may move beyond a threshold at which the ecosystem has the capacity to recover post fire. Prolonged post fire drought in the first year after fire has been shown to significantly reduce survival of seedlings, particularly forbs and graminoids. Increasing fire frequencies have also been shown to likely impact on woody species such as members of the Proteaceae family, that require several years between fires to mature, flower and set seed before the next fire moves through the landscape.

Trees on the Move: Climate Change Driving Bush Encroachment

Above: Sub escarpment grassland with forest. It is possible that South Africa’s grasslands may be threatened by bush encroachment with increasing levels of CO2 due to the climate crisis. Photo: Isobel Johnson.

Increasing levels of atmospheric CO2 are thought to be associated with bush encroachment and increasing cover of woody vegetation, particularly in South Africa’s savanna ecosystems. These changes have been documented through the use of historical ground-based and aerial repeat photography, supported by findings from chamber based CO2 level simulation that showed increased vigour and growth of trees in response to increasing CO2 levels.

It is also possible that these changes may threaten other ecosystems in South Africa such as grasslands with being colonised by woody vegetation. Bush encroachment is likely to have a significant impact on ecosystem services such as grazing and game viewing as part of the tourism industry. Our savannas and other ecosystems may change beyond recognition in years to come.

Further Reading

Bentley, L.K. Robertson, M.P. Barker, N.P. (2019) ‘Range contraction to a higher elevation: The likely future of the montane vegetation in South Africa and Lesotho’, Biodiversity & Conservation (Volume 28): pp. 131-153.

Helme, N. Schmiedel, U. (2020) ‘Namaqualand Nightmare’, Veld & Flora, Issue 106: pp. 14-19.

Slingsby, J.A. Merow, C. Aiello-Lammens, M. Allsop, N. Hall, A.S. Mollmann, H.K. Turner, R. Wilson, A.M. Silander, J.A. (2017) ‘Intensifying postfire weather and biological invasion drive species loss in a Mediterranean type biodiversity hotspot’, PNAS, pp. 1-6 www.doi.org