COMMUNITY WATER USE
5.4 Discussion
Three direct methods of measurement viz., weekly evapotranspiration by lysimetry, long- term change in pot mass, and soil water content, were used to assess treatment effect on community water use, in an attempt to answer the key question of whether community- level water use will be changed by long-term exposure to elevated CO2. The data obtained by all three methods support hypothesis 1 which postulated that long-term exposure to elevated CO2 will change community-level water use. There was also some evidence that evapotranspiration responses are dependent on water supply, thus supporting hypothesis 2. When community water use data were interpreted in the light of results of the previous two chapters (Chapters 3 and 4), it became apparent that evapotranspiration responses were related to stages of canopy development, as postulated by hypothesis 3. It is important to mention that an indirect assessment of treatment effect on community water use will also be considered from gas exchange measurements of canopy water vapour fluxes in Chapter 6.
To illustrate support for hypothesis 1, it appears that long-term exposure to elevated CO2
does change community-level water use, elevated CO2 reduced community evapotranspiration where the highest recorded cumulative reduction was 12% under elevated CO2
+
MAR relative to a 7% reduction under elevated CO2+
80%MAR in the first year. In the second year, an 8% reduction in cumulative evapotranspiration was recorded under elevated CO2+
MAR. Even though cumulative evapotranspiration was higher in the second year relative to the first year, there was more biomass produced per kg of water lost under elevated CO2+
MAR in the second year, hence WUE was higher in that treatment. This (WUE) remained relatively unchanged in other treatments. The lowest reduction in evapotranspiration under elevated CO2 was recorded in the third year.Two-way AN OVA interactions serve as a powerful tool for assessing if responses to one experimental factor are dependent on another experimental factor, and in this study the interactions are particularly important for assessing if responses to CO2 treatment are dependent on water supply because of the anticipated reduction in rainfall reliability predicted for South Africa (Ellery et al. 1991). The second hypothesis at the beginning of
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this chapter states that responses of community water use to C02 treatment will be dependent on water supply. Analysis of evapotranspiration data showed significant treatment interactions in the first and second years but not in the third, suggesting that responses to CO2 treatment were dependent on water supply only in the first and second year. It is speculated that the relatively lower annual cumulative water loss observed in the third year relative to first and second years, together with small differences in evapotranspiration that were observed between treatment in the third year could have resulted in lack of interaction between C02 and water treatments. Dependence of CO2 responses on water supply has been reported in other studies on grasslands, with a major trend being greater effect of CO2 response in years of low rainfall or soil water availability. In this study however, greater effect of CO2 response was observed at MAR.
Hypothesis 3 stated at the beginning of this chapter postulates that evapotranspiration responses will be related to canopy development, and the trends in monthly cumulative evapotranspiration fully support the hypothesis. Differences in water loss were not apparent at the beginning of the growing season because surface evaporation was the predominant process of community water loss due to lack of foliage. At the beginning of each growing season microcosm communities receiving higher water supply lost relatively more water irrespective of CO2 treatment, and in some instances effect of CO2 treatment was not statistically significant during the first two months of application of treatment. As the growing season progressed, the rate of water loss was higher in communities with greater leaf area development, and highest levels of monthly evapotranspiration were recorded at the time of full canopy in all three years. A striking difference in monthly water loss was observed towards end of the growing season as a result of interactions of CO2 and water treatment whereby a delay in senescence was induced.
Reduction of community evapotranspiration under elevated CO2 is a culmination of several phenomena operating at different scales of community organisation (stomatal conductance, leaf transpiration, sap flow, energy balance etc) and sometimes logistics do not permit assessment of all of these parameters in a single study. But, analysis of data in
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the literature shows trends of positive effects of elevated C02 on these various parameters that serve as indicators of community water use. The tallgrass prairie has been extensively studied in this regard, and reductions in stomatal conductance, canopy conductance, sap flow and evapotranspiration have been measured (Ham et al. 1995) as well as reductions in transpiration (Bremer et al. 1996). A 22% reduction in ET was measured in the tallgrass prairie relative to the 10% measured in the current study. In a model grassland community derived from the Negev in Israel, Griinzweig and Komer (2001) measured 2% reduction in ET under an elevated C02 treatment of 400 ppm and 11 % reduction under 600 ppm.
Consequences of reduction in ET for biomass production under different treatments could be summarised as follows: firstly WUE was higher under elevated CO2
+
MAR relative to other treatments in all three years. Secondly, increases in the amount and frequency of water supply from one year to another did not instantaneously enhance WUE in all treatments; WUE was enhanced only at elevated CO2 provided the increase in water supply did not exceed MAR. Thirdly, continued ample supply of watering as characterised by treatments in years two and three, enhanced soil water accumulation (section 5.3.3.2) without a corresponding enhancement in production.Reduction in ET resulted in higher volumetric soil water content measured under elevated CO2 in the current study, and the trend was further confirmed by a measurable increase in mass of plant pots due to water accumulation in the soil. Soil water content was found to increase with soil depth, hence the soil in the rooting layer was found to be on average 20% wetter than soil on the surface under elevated C02. Improved soil water status of 10- 28% was measured in a study using grassland assemblages (Volk et al. 2000). Deep drainage has also been observed to increase under elevated CO2 in some grassland studies as a consequence of soil water accumulation under elevated CO2 (Jackson et al. 1998;
Griinzweig and Komer 2001), especially during the wetter part of the growing season and not during the drier part of the growing season (Griinzweig and Komer 2001). In the current study, drainage loss was measured only when single water applications were in excess of the equivalent of 25mm rainfall event during the first year. Treatment effects on
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drainage loss were not significant, and that parameter was subsequently not measured in the second and third years.
Effect of swapping water treatments between MAR and 120%MAR in the third year were not profound, and even a reduction in biomass production that occurred in the last year could not be attributed to such an experimental manipulation.
In conclusion, the data presented in this chapter have unequivocally shown through application of three methods of assessment, namely evapotranspiration, change in pot mass as a consequence of soil water accumulation, and measurements of soil water content, that elevated CO2 reduces community water use. Consistent observations in that regard were made throughout three years, and evapotranspiration was reduced by approximately 12% under elevated CO2. The balance between evaporation and transpiration seems to regulated by leaf area index to a certain extent, because under 80%MAR, there was low leaf area index hence most water was lost by evaporation through the soil surface. The data also showed that community responses of water use were dependent on water supply. Microcosms that received high water supply under both CO2 treatments invariably underwent higher rates of evapotranspiration than microcosms receiving lower water supply. The highest response of community water use were recorded at elevated CO2
+
MAR. These results are in agreement with findings of similar studies on grassland communities in other parts of the world.Chapter 6 Canopy Carbon and Water Vapour Exchange 142