Impacts of elevated CO2 on the growth, production and water use of a South African C4-dominated grassland community.

Nthabiseng Motete
University of Kwazulu-Natal, Durban
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A three year microcosm experiment consisting of four C4 grass species, one C3 grass, and a C3 geophyte was set up to investigate production and water use efficiency of a grassland community (coastal Ngongoni veld) in response to increasing concentrations of atmospheric C02 and different levels of simulated rainfall. The Ngongoni grassland community is dominated by species that possess a C4 photosynthetic pathway, predominantly of the NADP-me. Dominant C4 grass species irrespective of photosynthetic pathway include Andropogon appendiculatus, Eragrostis racemosa, Sprobolus pyramidalis, and Themeda triandra. Only one C3 grass species, Alloteropsis semialata sub-species eckloniana, is common in this grassland community. There are also a few forbs. The experimental system was assembled in a greenhouse, where microcosms were arranged in three rows representing four randomly arranged treatment groups with four replicates per treatment. Community canopy development and phenology were studied qualitatively from the beginning to the end of each growing season. Community above-ground production was determined at end-of-year harvests in a manner that differentiated contributions of different species. Above-ground biomass of grass species was further sorted by components in order to illustrate how these influenced canopy structure and possibly competitive interactions. Changes in aboveground biomass production of the grass species in the three years were used to infer species dominance changes in response to a factorial combination of CO2 and water treatments. Assessment of community water use was done by measurements of evapotranspiration using a weighing lysimeter, and by measurements of soil water content using a moisture probe. Fluxes of carbon and water vapour were also determined by canopy gas exchange in the second and third years of study. Leaf gas exchange measurements were performed at three intervals (beginning, middle and end) during the third year of study in order to investigate a correlation between photosynthesis and biomass production. Measurements done at the fmal harvest included total below ground biomass, distribution of roots with depth, and crown biomass (below-ground biomass could not be split into species-specific components).
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Rangelands Partnership

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