Semi-arid Grass Research Articles

Comparison of eddy covariance, chamber, and gradient methods of measuring soil CO 2 efflux in an annual semi-arid grass, Bromus tectorum

Eddy covariance measures net ecosystem exchange of CO 2 (NEE) at a scale between chamber-based measurements of CO 2 exchange processes and large-scale models of CO 2 flux dynamics. As the intermediate, it represents a link between small and large-scale estimates of NEE. Accuracy is therefore critical. However, estimates of nighttime ecosystem respiration based on scaled-up measurements of soil and leaf CO 2 exchange are most often larger than from eddy covariance. Identifying the source of the discrepancy is difficult due to large measurement uncertainties associated with high variability of fluxes in complex ecosystems. This study compared measurements in a simple system that allowed for minimal uncertainty. We compared measurements of soil efflux using (1) soil chambers, (2) the soil CO 2 gradient technique and ecosystem respiration using the (3) the eddy covariance method from a surface that was covered with living vegetation, straw, and snow in turn through a year. Results showed general agreement among measurements in a range of conditions during canopy absence indicating that each measurement technique is theoretically sound. However, we found disagreement among measurements in specific conditions that indicated certain limitations with each method. Nighttime eddy covariance measurements of ecosystem respiration were below the uncertainty limits of soil respiration measurements during the period of active canopy growth (leaf area index from approx. 0.3–1.0 m 2 m −2. This raises questions about the accuracy of nocturnal eddy covariance measurements over more complex surfaces. There was indication that the chamber method estimates were unrepresentative of the footprint in certain conditions due to within collar surface treatment and due to collar interaction with the environment. Lastly, the gradient method failed to represent surface fluxes during summer rain. To measure soil efflux in all conditions typical of this site, a combination of all three methods is recommended. A combined NEE estimate for 2005 for soil efflux was 406 ± 73 g C m −2.

Protein concentration in response to water stress in Themeda triandra Forsk

The influence of water stress on the concentration of water-soluble protein growth reserves was determined during three growth stages (vegetative, pipe and reproductive) as well as in three plant parts (roots, stubble and leaves) for the indigenous C4 semi-arid grass species, Themeda triandra. Four water stress treatments (T1 = 0–25%, T2 = 25–50%, T3 = 50–75% and T4 = 75–100% depletion of plant available water) were applied to plants grown in pots under greenhouse conditions. The water-soluble protein concentration (WSPC), determined in fractionated material obtained from stressed plants after drying in an oven, differed significantly (P<0.05) between plant parts as well as between growth stages. The WSPC in the leaves and stubble increased significantly with increased water stress for the vegetative, pipe and reproductive growth stages, while a similar increase was observed in the roots during the reproductive growth stage. The WSPC in the roots was variable among treatments during the vegetative and pipe growth stages. The results illustrate that under severe water stress, the decrease in aboveground vegetative growth causes an increase of water-soluble protein in aerial plant parts. Therefore, under severe drought conditions, the same number of animal units can still be carried owing to the higher nutritional value of the available fodder. This amplifies the importance of careful grazing and stock management in T. triandra dominant grassland, thus ensuring sustainable production and survival in an ecologically sensitive semi-arid climate.

Impact of water stress on growth reserves and re-growth of Themeda triandra (Forssk) following defoliation

The influence of water stress and defoliation on the concentration of total non-structural carbohydrates (TNCC) and water-soluble protein (WSPC) as growth reserves after defoliation were determined in the indigenous C4 semi-arid grass species Themeda triandra during three growth stages (vegetative, piping and reproductive) and in three plant parts (roots, stubble and leaves). Four water stress treatments (T1 = 0-25%, T2 = 25-50%, T3 = 50-75% and T4 = 75-100% depletion of plant available water) were applied to the plants in pots in a glasshouse. The TNCC declined drastically after severe defoliation over all the water treatments (P < 0.05), in all the plant parts (P < 0.05) and for all the growth stages (P < 0.05), while the TNCC did not recover (P < 0.05) (to the TNCC values as before defoliation) within 6 days after defoliation. The WSPC in the leaves increased from after defoliation, up to day 6 over all the water treatments (P < 0.05) and for all the growth stages (P < 0.05). Non-structural carbohydrates serve as the most important growth reserve and are utilised for re-growth after severe defoliation. Water-soluble protein can also be utilised as growth reserve, but only as an emergency measure for up to two days after defoliation. These results could make a meaningful contribution to ensure sustainable utilisation of the grassland ecosystem in semi-arid areas.

Relationships between water availability, carbon isotope discrimination and plant productivity in two semi-arid grass and shrub species

The positive relationship between water availability and carbon isotope discrimination (Δ) is well documented. There is also evidence that Δ is positively related to plant productivity. However, most of the data are concerned with cross species and genotypic variation in Δ relative to broad geographical differences in water availability or on general trends in local populations. We still lack quantified data on the response of localized populations to changing water availability, carbon isotope discrimination (Δ) and plant productivity, especially on a year to year basis. To provide data in this area, this study measured above-ground productivity in western wheatgrass (Agropyron smithii) and vegetative stem growth in big sagebrush (Artemisia tridentata), and carbon isotope discrimination in both species over several years of varying water availability. Estimates of Δ were regressed against spring soil moisture estimates, and plant productivity measure ments against Δ estimates. For sagebrush, carbon isotope discrimination estimates were positively related to spring soil moisture levels (p=0.027,r2=0.11) and productivity estimates (p=0.002,r2=0.23). Carbon isotope discrimination for wheatgrass also regressed significantly with soil moisture levels (p=0.039,r2=0.10) and productivity estimates (p< 0.001,r2=0.33).

Energy balance and photosynthetic and water use efficiency of cenchrus ciliaris grass

Energy balance studies ofCenchrus ciliaris grass cover at the semi-arid Palsana region India indicate that the mean contribution of net radiation to total radiation was 55 per cent in May when the grasses were dry and 44 per cent in October when grasses were actively growing. The albedo percentages varied from 18 to 20 per cent during May and 22 to 24 per cent during October. These are lower than those of arid zone grasses. The net long wave radiation is alos comparatively less in wet condition of this semi-arid grass land as compared to other arid grass land of western Rajasthan. The mean percentage of radiation penetrating into theCenchrus ciliaris grass cover was 84 per cent at mid canopy and 62 per cent in bottom of the canopy.

Physiological responses of the semi-arid grasses Thyridolepsis and Astrebla compared with Cenchrus.

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Physiological responses of semiarid grasses. IV.* Photosynthetic rates of Thyridolepis mitchelliana and Cenchrus ciliaris leaves

Photosynthesis response curves of mulga and buffel grasses to irradiance and to carbon dioxide concentration, together with studies of leaf anatomy and chloroplast structure, showed that the former was a C3 and the latter a C4 species. This is held to account for the large differences in growth rate between the two species. Although Mitchell grass has the anatomy of a C4 species, its photosynthetic responses were not examined. As its net assimilation rate is high at high temperatures, its generally low growth rates are associated with the poor development of its leaf surface. *Part III, Aust. J. Aguic. Res., 26: 447 (1975).

Physiological responses of semiarid grasses. II.* The pattern of root growth in relation to external phosphorus concentration

The effect of the external phosphorus concentration on the pattern of root development in mulga, Mitchell and buffel grasses was examined. During seedling and early vegetative growth, the pattern of root growth suggested that the emergence of axes, their extension and branching were coordinated in such a way that the mean rate of extension and distance between branches remained fairly constant for each component of the root system. Phosphorus deficiency did not greatly affect the length of the seminal axis or the number of its primary laterals, but resulted in a significant reduction in the length of these laterals; the length and number of nodal roots were also greatly reduced. Species differences in the pattern of root development are discussed in relation to seedling drought survival on infertile soils. *Part I, Aust. J. Agric. Res., 26: 423 (1975).

Physiological responses of semiarid grasses. III.* Growth in relation to temperature and soil water deficit

The optimum temperature for vegetative growth of mulga grass was about 25°C, and for Mitchell and buffel grasses 30°. Buffel grass had the highest yield at all temperatures, partly because of its higher growth rate which in turn can be ascribed to both a higher net assimilation rate and the diversion of a greater proportion of dry weight into leaf area. Seedlings with an ample supply of phosphate had higher relative growth rates than phosphorus-deficient seedlings at the commencement of the soil drying cycle, but their growth rates declined more rapidly as the soil water potential fell. This decline was associated with a reduction in the rate of phosphate absorption as well as a decrease in the tissue phosphorus concentration. *Part II, Aust. J. Agric. Res., 26: 437 (1975).

Physiological responses of semiarid grasses. I. The influence of phosphorus supply on growth and phosphorus absorption

Experiments were carried out under controlled conditions to investigate the physiological bases for species differences in yield and nutrient responses to variations in phosphorus supply. Buffel grass (Cenchrus ciliaris), and to a less extent Mitchell grass (Astrebla elymoides), showed a much larger yield response to increasing phosphorus supply than mulga grass (Thyridolepis mitchelliana). Mitchell and mulga grasses had much lower relative growth rates than buffel grass. Mulga grass required a lower external phosphorus concentration for optimal growth than Mitchell and buffel grasses; this was attributed to its superior system for absorbing and transporting phosphate from low concentrations, but was not associated with any yield advantage, yield being related more to the photosynthetic than to the nutritional characteristics of the plants. Differences between species in their external phosphorus requirements for growth and their distribution in semiarid Queensland are discussed.

Comparative Temperature and Moisture Responses in Gambel and Scaled Quail

ico, while Scaled Quail (Callipepla squamata) generally inhabit the more mesic semiarid grass plains of the Southwest and much of Mexico. The ranges of the two species overlap in southeastern Arizona and southwestern New Mexico (A.O.U. 1957; Leopold 1959). The habitats of these species are similar in the area of sympatry. The species are also similar in general body size, food habits, and in other parameters of their ecology. Comparative data relating to temperature and water relations would appear to be valuable in understanding these sympatric and closely related species. These data are also important in understanding adaptations of birds to hot, arid climates. Bartholomew and Cade (1963) reviewed the literature concerning water economy studies of birds. Since 1963 several studies dealing with water and temperature relations have provided additional information on several species of birds, but, to date, published data are lacking on minimum water requirements, metabolism, and evaporative water loss in Gambel and Scaled Quail. Gullion and Gullion (1964) studied the response of body weight to complete water deprivation in Gambel Quail. Bartholomew and Dawson (1958) studied the effect of various ambient temperatures on body temperature in Gambel Quail. No physiological studies of Scaled Quail appear in the literature. More physiological studies have been done on California Quail (Lophortyx californicus) than on the other two species. Bartholomew and MacMillen (1961) determined the minimum daily water requirement necessary for weight maintenance in California Quail, and Hudson and Brush (1964) and Brush (1965) studied the responses of body temperature, metabolism, and evaporative water loss to various ambient temperatures in the same species. Bartholomew and Dawson (1958) also studied body temperature in California Quail.