The interactive effect of elevated CO2 and rising temperatures on the growth of Leucosidea sericea, Festuca costata and Themeda triandra
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Date
2021
Authors
Lebese, Evalt Malesela
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Abstract
The atmospheric concentration of greenhouse gases such as CO2 have been rising for the past two centuries, leading to a rise in global air temperature. Both atmospheric CO2 and temperature are projected to rise further in the twenty-first century and changes in either are known to affect plant performance (specifically net primary production) which differs between different plant functional types, for instance trees and grasses, or plants with a C3 or C4 photosynthetic systems. They are also predicted to drive the expansion of plants to higher altitude and extinction rates, as a result, they drive the depletion of the earth’s global biodiversity which is a major challenge of the twenty-first century. It has been noted that the tree Leucosidea sericea is encroaching into the formerly treeless high-altitude C4 grassland historically dominated by C4 grasses like Themeda triandra in the Drakensberg, South Africa. The expansion of patches of C3 grasses Festuca costata has also been observed and it is thought that the distribution of both these grasses and trees is changing due to elevated CO2, high temperature, or an interaction between the two as a result of climate change. Therefore, the aims of this study were to: (1) test the individual and interactive effect of elevated CO2 and temperature on the growth rate of a shrubby C3 tree L.sericea, a C3 grass F. costata and a C4 grass T. triandra; (2) to determine how variations in the competitive ability of L. sericea, F. costata and T. triandra are influenced by elevated CO2 and temperature, projected to be reached later in this century. A full-factorial experiment, conducted in growth chambers, with two levels of CO2 (400 ppm (ambient) and 650 ppm (elevated)) and two levels of temperature (ambient and +2oC above ambient) monitored the growth performance of the three species each representative of a plant functional typeover a six-month period. The leaf traits (total chlorophyll content and leaf area)for each species were also measured at the end of the experiment. The plants were grown in pairs in separate pots interacting with individuals of their own species and the other species. The competitive interaction of the three species examined in pairwise combination were measured by calculating the relative yield per-plant (RYP) for each target and neighbour species. The results showed that there was no significant effect due to the independent and interactive effect of CO2 and warm temperature on the growth rate and competitive interaction between any of the three species studied. This study suggests that there is no evidence that the overall effect of CO2 at 650 and warm temperature will result in growth enhancement in L. sericea, F. costata and T. triandra nor will they influence the competitive interaction between the three species. As a result, they will not help explain the observed upward movement of L. sericea into higher altitudes nor the spread of F. costata in the north-facing slopes previously dominated by C4 grasses like T. triandra in the Drakensberg. This study acknowledges that, apart from the limitations of pot size, success in plant establishment and sample size, there may have been other climatic and non-climatic factors such as precipitation, light availability, the functional traits of species, human disturbances, herbivory/grazing pressure and fire frequencies which were not considered in this study but might drive changes in community compositions in the Drakensberg. Therefore, it is suggested that future studies look into these factors which are likely to explain the upward movement of L. sericea and the spread of F. costata in high altitude of the Drakensberg Mountain
Description
A dissertation submitted to the Faculty of Science, University of the Witwatersrand, in partial fulfilment of the requirements for the degree of Master of Science, 2021