Water use and production potential of Karoo shrubs

[摘要] English: Variation in, and changing of climatological patterns, especially rainfall, as well as grazing intensity and frequency has the biggest influence on rangeland productivity and sustainable animal production in the Nama-karoo Biome. This study was conducted to quantify whole-plant productivity, nutritive value and morphological adaptations of Karoo shrubs to defoliation along a soil-water deficit gradient. Two Karoo shrubs, Nenax microphylla and Pentzia incana were investigated. The watering treatments included the following: 0 - 25% depletion (non-stressed), 25 �?50% depletion (mildly stressed), 50 �?75% depletion (moderately stressed) and 75 �?100% depletion (severely stressed) of field capacity. The defoliation treatments were defoliation intensity to a height of 50 mm, 125 mm and 200 mm; and defoliation frequencies of three-monthly, six-monthly and twelve-monthly which was also used as the control. Water availability proved to be the single most important factor influencing both above- and belowground rangeland productivity. Defoliation intensity had the lowest impact on productivity, while the impact of defoliation frequency was markedly higher on both above- and belowground phytomass production. The root:shoot ratio increased with increased water deficit as a means to improve the water absorption of the shrubs. Determination of water-use efficiency (WUE) included both above- and belowground phytomass, while it excluded evaporation which gave a more accurate estimation of WUE. This is of the few studies where root growth is also included in calculating WUE. The expression of WUE in terms of transpiration as was done in this study, is more sensitive for describing ecosystem functioning than evapotranspiration, as was done in most studies in the past. In general, the WUE of the shrubs increased when exposed to water stress and higher grazing pressure. The more frequent and intensely the plants were defoliated, the higher the nutritive value of the produced edible phytomass. The quantity of this produced phytomass was, however, very low. The increased CP (N-content) of the water stressed plants could have contributed to the increased WUE. Strong evidence of compensatory growth and WUE were recorded for both species. This compensatory ability especially enabled the shrubs to display increased recovery after defoliation when water is not limited. Water stress had no marked influence on the reproductive ability of the investigated shrubs. It was, however, proved that both defoliation frequency and intensity had a bigger influence on seed production and germination percentage, than water stress. Pentzia incana has a high density of reflective trichomes that provides protection against heat and solar radiation. It also has a high stomatal density which allows increased photosynthetic rates when growth conditions are favourable. The stomata of N. microphylla occur only on the abaxial (lower) side of the leaf which protects it from direct sunlight and heat. It also has a very high stomatal density which could contribute to its ability of compensatory growth when adequate water is available. Furthermore, the leaves have a shiny appearance which enables them to reflect solar radiation to reduce leaf temperatures. This study highlighted the complexity of the effect of external influences, like rainfall and grazing (defoliation) on the functioning of the rangeland ecosystem in the arid and semi-arid Nama-karoo Biome. Although the land user does not have control over plant water availability, control over defoliation is possible. Defoliation therefore should be the most important key aspect in sustainable ecosystem utilization.

[发布机构] University of the Free State