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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).