Modern society and its activities generate waste products in large amounts. One invisible product is CO2, a gas that is formed when fossil fuels (oil, coal and natural gas) are burned. Levels of this gas in the earth's atmosphere are rising faster than they have ever done in geological history.
Plants use CO2 from the atmosphere during photosynthesis, the process which provides the building blocks for plant growth, and that ultimately gives rise to all organic matter on the face of the planet. Rising CO2 levels will affect the rate of photosynthesis, and hence plant productivity. Most work done internationally on this problem has focussed on crop plants and plants of northern hemisphere ecosystems. The wide range of responses of plants of different types makes it vital that we carry out research on our own unique flora in southern Africa to allow us to predict the impact of this atmospheric change over the next fifty years.
CO2 and productivity of key plant species
CO2 projects began at the NBI in 1992. So far, we have studied how doubling current CO2 levels affects the productivity of key plant species representing the Fynbos and Succulent Karoo (both temperate systems), and Savanna and Grassland Biomes (sub-tropical systems). For most plant types from temperate South African ecosystems, we have found positive photosynthetic responses to doubled CO2 levels, but growth has not increased as much as expected from theoretical predictions. We suspect that the species we have worked on discard or "dump" excess carbon fixed into the soil through material exuded from their roots (this may occur because many of these species have evolved relatively recently, and have never experienced such high CO2 levels in their evolutionary history). Contrary to predictions from prevailing theory, tropical grassland species have shown a strong positive growth response to doubled CO2, while using only 50% of the water used at current CO2 levels. This has major implications for the water runoff from the grassed catchment areas of the Drakensberg highlands, which supply the bulk of South Africa's water. Testing these ideas in innovative field and greenhouse experiments in Natal has confirmed our results from greenhouse work.
In a collaborative effort with Prof Ian Woodward of Sheffield University, UK, we have begun modelling South African vegetation responses to changing climate and CO2, including attempts to reconstruct responses under low CO2 conditions of the Last Glacial Maximum (LGM), 18000 years ago. This work is done with a Dynamic Global Vegetation Model (DGVM) that simulates changing vegetation structure and function (but not species composition). The work is revealing the critical role of CO2 in controlling the relative success of grasses and trees in the subcontinent, with major implications for understanding historical and future changes in savanna distributions carbon storage patterns, and even faunal success.
To shed further light on the role of evolutionary history on CO2 response, we have been growing cycad species in a range of CO2 environments of up to 3x the current level, to simulate conditions thought to exist 100 million years ago, when cycads evolved. This research confirms that cycads stand to benefit significantly from elevated CO2.
Dr GF Midgley, Specialist scientist
Ms N Motete, Scientist
Mr B Kgope, Scientist
Mr S Snyders, Auxilliary Services Officer: Greenhouse and Outdoor
Ms J Arnolds, Auxiliary Services Chemist: Laboratory
Prof FI Woodward, Sheffield University, UK
Dr PS Curtis & Dr MH Jones, Ohio State University, USA
Prof NW Pammenter, University of Natal, Durban, School of Life and
Prof WD Stock, University of Cape Town, Botany Dept.
Dr RJ Scholes, Council for Scientific and Industrial Research,
Prof WJ Bond, University of Cape Town, Botany Dept.
International programme links
Global Change and Terrestrial Ecosystems, CO2 network, Stockholm.
CO2 MAP (Meta-analysis project), Ohio State University, USA.