In the first year, increased community CO2 reduced water use more in MAR than under dry conditions. The geophyte (Eriospermum mackenii) did not respond to treatments in the above-ground organs in the first year.
List of abbreviations
CHAPTERl INTRODUCTION
Increasing atmospheric CO 2 concentrations and global climate change
Finally, we will consider the responses of C4-dominated grassland communities to elevated CO2 in a South African context. The C4 component of the mixed community also showed no measurable aboveground biomass response to elevated CO2 (Curtis et al. 1989a).
C 4 subtypes and elevated CO 2
1995) hypothesized that levels of C02 leakage may serve as predictors of the responses of C4 species to elevated CO2. Nijs et al. 1989); consequently, increased CO2 can ameliorate the negative effects of drought in many species (Morrison 1993).
MATERIALS AND METHODS
The air temperature inside the rooms was monitored at the top of the canopy (80 cm) and at the height of the middle of the canopy (40 cm). In general, there was no difference in the air temperature at the top of the canopy (80 cm) and at the height of the middle of the canopy (40 cm).
CANOPY STRUCTURE AND PHENOLOGY
Introduction
Thus, a treatment combination of CO2 + MAR affects canopy structure by increasing leaf biomass in the upper part of the canopy. The greatest decrease in canopy leaf biomass was observed in the part of the canopy above 40 cm (Figure 3.7).
Canopy structure and phenology
- Discussion
In contrast, at the species level, the effect of CO2 treatment on germination was not dependent on water supply (tables except in Eragrostis in the third year). Of the taller grasses, Sporobolus and Themeda responded most to increased CO2 + MAR, and their respective leaf biomass in the 40-60 cm layer was equal to 50% of each of their leaf biomass in the dense basal layers (5-20 cm or 20-40cm); while contributions from Alloteropsis and Andropogon in the 40–60 cm layer were each no more than 10–15% of their respective contributions in the dense basal layers (5–20 cm or 20–40 cm). Increased CO2 generally caused an increase in canopy height in Sporobolus and Themeda during the first two years. but then leaf biomass was reduced by the canopy in the third year, benefiting the small Eragrotis, whose leaf biomass increased.
Also, a change in the dominance of leaf biomass contribution was observed in the third year compared to previous years. The most striking change occurred between Sporobolus and Themeda, with the latter species making a dominant contribution to leaf biomass in the part of the canopy above 20 cm.
COMMUNITY PRODUCTION
Introduction
Consequently, it has been determined that outcomes of species competitive interactions cannot be generalized along biochemical and photosynthetic differences in C3 and C4 functional types (Wand et al. 1999). The meta-analysis by Wand et al. 1999) illustrated that both C3 and C4 functional types are susceptible to reduced production under conditions of limited resources. Water stress causes a reduction in the stimulation of leaf area by CO2 in C4 species, while overall stress reduces rate of carbon assimilation in C3 species and nutrient stress especially reduces biomass response in C3 species.
Water is a limiting resource in many grassland ecosystems, and predictions that climate change associated with increased atmospheric CO2 may be accompanied by variable rainfall in South Africa (Ellery et al. 1991) necessitate an understanding of how the production of the grassland ecosystem will respond to the predicted changes in atmospheric CO2 and water availability. To what extent will biomass production (community above and below ground and species above ground) depend on water treatment. iii) Will C02 reaction depend on water treatment.
Materials and Methods
Soil organic matter was quantified at the final harvest at the end of the third year. The largest increase in bulb mass was in the treatment of water with an average value of CO2 + MAR. Community aboveground biomass was greater under elevated CO2 + MAR than any other treatment combination in the first, second, and third years of the experiment.
Similarly, aboveground biomass production of the grasses was lower in the third year compared to the second year. The amount of accumulated surface waste at the end of the year was about 5-10% of the community's above-ground production.
CHAPTERS
COMMUNITY WATER USE
Introduction
The first study to report an increase in ecosystem water use efficiency under elevated CO2 was conducted in a mixed C3/C4 mesic tallgrass pond in Kansas (Knapp et al. The positive effects of increased C02 on water use are also reported in reconstructed grassland communities (Griinzweig and Komer, 2001, Yolk et al. 2000) The increased water use efficiency response in communities and microcosms exposed to elevated CO2 is unequivocally attributed to reduced stomatal conductance as a consequence of ci growth (Jackson et al.
Ecophysiological benefits of improved water use efficiency under elevated C02 include (i) longer periods of photosynthetic activity in otherwise water-limited ecosystems and (ii) increased carbon allocation to root biomass to increase soil water extraction capacity and better use of water-limited environments (Owensby et et al. 1997). The key question is whether water consumption at the community level will be altered by long-term exposure to elevated CO2.
Materials and Methods
- Community evapotranspiration by lysimetry
The maximum weekly water loss occurred in the middle of the growing season and typical values ranged between 1.7 and 3.0 kg. During the latter part of the growing season, little change was observed in the pot mass of microcosms exposed to increased CO2, while a decrease in pot mass was observed in the ambient CO2 treatments as well as increased CO2 + MAR. Statistically significant differences in the effect of treatments on pot mass were indicated by the non-overlapping of the regression coefficients.
Overall response pattern in the first four months of the growing season depicted a higher rate of water loss under elevated C02 treatments relative to ambient C02 at similar water treatments. Response patterns observed in the third year were very similar to those observed in the second year.
Discussion
A 22% reduction in ET was measured in the tallgrass prairie relative to the 10% measured in the current study. Second, increases in the amount and frequency of water supply from one year to another did not immediately improve WUE in all treatments; WUE is only improved against increased CO2 provided the increase in water supply does not exceed MAR. Reduction in ET resulted in higher volumetric soil water content measured under elevated CO2 in the present 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, so soil in the root layer was found to be on average 20% wetter than surface soil under elevated C02. In the current study, drainage loss was measured only when the single water application was greater than the equivalent of 25 mm of rainfall in the first year.
COMMUNITY CARBON AND WATER VAPOUR EXCHANGE
Introduction
In the wild C3/C4 salt marsh ecosystem, elevated CO2 significantly increased the 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 modeling simulation (Rasse et al. 2003). In the C4-dominated tallgrass prairie, increased C02 positively enhanced net carbon exchange only when water was limiting (Ham et al. 1993).
Exhaustive studies of community water vapor flux that influenced current scientific dogma on the effects of elevated C02 on ecosystem ET were undertaken in the C4-dominated tallgrass prairie. 1985) previously suggested that physiological responses to elevated CO2 are often insensitive to temperature changes of less than 5 °c (Jones et al. 1985).
Materials and methods and data analysis .1 Materials and methods
- Data analysis
Diurnal time course response curves of carbon and water vapor fluxes were determined for each of the four replicate treatments. The low rate of ET at the beginning of the growing season (October and November) was about 50%. However, the amount of irrigation applied in the first two months of the growing season (October and November) was about one and a half times higher than the amount of irrigation applied in the last two months of the annual measurement (April and May).
The response pattern for ET in the third year was generally similar to the response pattern for the second year. Incidentally, the lysimeter data from the third year also indicated lower ET rates (Chapter 5).
Discussion
The magnitude of respiratory fluxes in the third year was 5–10% higher than in the second year, and that observation was attributed to continued accumulation of groundwater. The share of dark breathing relative to CO2 assimilation was higher in the third year than in the second year, partly because lower CO2 assimilation was measured in the third year. The relatively lower CO2 assimilation figures measured in the third year resulted in a less efficient use of PAR per land area.
The annual timing of community carbon exchange suggests that stimulation by elevated C02 was higher earlier in the growing season compared to later in the growing season (Figure 6.3). The trend was observed in the second and third year, and the observation serves as a validation of the data presented in chapter 5.
LEAF LEVEL GAS EXCHANGE
Results for Eragrostis, Sporobolus and Themeda showed stimulation of CO2 assimilation to high cuvette CO2 concentrations in AlCi response measurements (Figure 7.1), as plants grown at elevated CO2 had higher Jrnax. Data on the light response of Eragrostis, Sporobolus and Themeda at growing CO2 concentration showed higher Arnax at elevated CO2 (Figure 7.2), and the differences between treatments were more pronounced for Eragrostis. Response of AlCi Alloteropsis parameters to treatment presented along with results of two-way ANOV A.
Response of AlCi parameters of Eragrostis to treatment, presented together with the results of a two-way ANOV A. Response of AlCi parameters of Themeda to treatment, presented together with the results of a two-way ANOV A.
