COMMUNITY CARBON AND WATER VAPOUR EXCHANGE
6.1 Introduction
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CHAPTER 6
Chapter 6 Canopy Carbon and Water Vapour Exchange 143
somewhat balanced. However, if carbon sequestration occurs, the build up in soil carbon can turn the system into a carbon sink. On the other hand, occurrence of disturbance such as fIre lead to loss of accumulated carbon sinks, thus causing the carbon balance of the system to become negative, which makes the system a carbon source. Systems that undergo recurring disturbance would therefore have a carbon balance characterised by a series of peaks and troughs as the dynamics change from positive to negative or sink to source.
Early experimental investigations on response of C3 vs. C4 communities to C02 enrichment in natural grass vegetation have been undertaken in the C3 tussock tundra (Grulke et al. 1990), in C3 and C4 monospecifIc stands in the salt marsh (Drake and Leadley, 1991), and in the C4 tallgrass prairie (Ham et al. 1993). Expectations were that communities would respond along predictions based on differences in photosynthetic pathways, whereby C3 species would constitute a stronger sink in their respective communities compared to C4 species. Initial [mdings from C3 tussock tundra studies indicated that elevated CO2 induced a negative annual carbon balance.
However, recent [mdings from the tussock tundra indicated a previously undemonstrated capacity for that ecosystem to adjust to decade long changes in climate by acting as a net sink for atmospheric CO2 during the summer growing season, yet remaining a source on an annual basis (Oechel et al. 2000). The response mechanism was attributed to adjustment at different levels (plant, soil, microbial, and whole-ecosystem) including nutrient cycling, physiological acclimation, and population and community reorganisation. In the wild C3/C4 salt marsh ecosystem, elevated CO2 signifIcantly increased net carbon exchange of the C3 community components, but had much less effect in the C4 community components (Drake and Leadley, 1991). The positive response of the C3 community was further supported by a modelling simulation (Rasse et al. 2003). In the C4-dominated tallgrass prairie, elevated C02 positively enhanced net carbon exchange only when water was limiting (Ham et al. 1993). In a C3 annual grassland, Freeden and co-workers (1995) reported increased net ecosystem CO2 uptake under elevated CO2, but the capacity of the response was reduced by acclimation due to a decrease in rubisco activity.
Most of these early studies on community fluxes were carried out in open-top chambers. Subsequent research in other ecosystems has predominantly employed the
Chapter 6 Canopy Carbon and Water Vapour Exchange 144
eddy correlation technique as part of the long tenn ecological monitoring of climate change impacts.
Measurement of community water vapour fluxes is important for detennination of community/ecosystem water balance. In Chapter 5, community ET was measured by lysimetry, and the data were interrelated with measurements of soil water status (change in pot mass as a consequence of soil water accumulation) to estimate water balance of the microcosm communities. In the work reported in the current Chapter ET was measured by flux exchange of water vapour from the canopy, the advantage being that shorter time scale mechanism of water savings is revealed. The use of several methods for assessment of community water use in this study was found necessary to gain confidence in the results, taking into consideration the importance attached to understanding effects of elevated C02 on community water use and its implications for community production. Effect of elevated CO2 on ET of grasslands is attributed to reduction in stomatal conductance (gs), and effect of gs on ET under elevated C02 is recognised as the second most responsive parameter after photosynthesis (Field et al. 1995). Exhaustive studies of community water vapour flux that have influenced the current scientific dogma on effects of elevated C02 on ecosystem ET were undertaken in the C4-dominated tallgrass prairie. Results of these studies showed a 22% reduction in daily ET under elevated CO2 (Ham et al. 1995) and a 50% reduction in stomatal conductance (Owensby et al. 1997), and therefore tying in with other work (Wand et al. 200 1) that shows a reduction in gs at the leaf level.
Carbon and water vapour flux responses to elevated CO2 are more readily comprehended at the leaf level than they are at the canopy level, because of the complexities of the canopy boundary layer and light regime. Such complexities occur because each leaf in a canopy modifies the environment of adjacent leaves through reduced irradiances, wind speed, and vapour pressure deficit. Furthennore, canopy fluxes are generally greater than the sum of fluxes of individual leaves due to contributions of the rhizosphere. As a result, carbon and water vapour fluxes of vegetation canopies cannot be adequately predicted from the study of individual leaves. The open-top chamber technique is widely used for canopy flux studies (Drake and Leadley, 1991, Grulke et al. 1990, Ham et al. 1995). Nonetheless,
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influence of chamber microclimate conditions cannot be overlooked. Measurement of chamber microclimate conditions performed during experimental set-up indicated an increase of 3-5
°c
in air temperature within the canopy, more than 5% reduction in PAR compared with conditions outside the greenhouse (Section 2.5). Nonetheless, Jones et al. (1985) previously suggested that physiological responses to elevated CO2 are often not sensitive to temperature changes less than 5°c
(Jones et al. 1985).Besides, a similar effect of chamber microclimate was prevalent in all microcosms in this study.
The main objectives of undertaking measurements of canopy carbon and water vapour exchange are to investigate whether (i) South African C4-dominated grassland communities can increase C02 uptake under elevated CO2 (ii) whether their water use will be reduced, and WUE will change under elevated CO2 and (iii) whether the response patterns in (i) and (ii) above relate to water input and subsequent canopy development/LA!.
6.2 Materials and methods and data analysis