Shadwell Eleanor, February Edmund
Department of Biological Sciences, University of Cape Town, Private Bag, Rondebosch, South Africa.
PeerJ. 2017 Jan 25;5:e2923. doi: 10.7717/peerj.2923. eCollection 2017.
In arid systems with no surface water, deep boreholes in ephemeral river beds provide for humans and animals. With continually increasing infrastructure development for tourism in arid wildlife parks such as the Kgalagadi Transfrontier Park in southern Africa, we ask what effects increased abstraction may have on large trees. Large trees in arid savannas perform essential ecosystem services by providing food, shade, nesting sites and increased nutrients for many other plant and animal species and for this are regarded as keystone species.
We determine seasonal fluctuations in the water table while also determining the water source for the dominant large tree species in the Auob and Nossob rivers in the Park. We also determine the extent to which these trees are physiologically stressed using leaf C, xylem pressure potentials, specific leaf area and an estimate of canopy death. We do this both upstream and downstream of a low water use borehole in the Auob River and a high water use borehole in the Nossob River.
Our results show that the trees are indeed using deep groundwater in the wet season and that this is the same water used by people. In the dry season, trees in the Auob downstream of the active borehole become detached from the aquifer and use more isotopically enriched soil water. In the Nossob in the dry season, all trees use isotopically enriched soil water, and downstream of the active borehole use stomatal regulation to maintain leaf water potentials. These results suggest that trees in the more heavily utilised Nossob are under more water stress than those trees in the Auob but that trees in both rivers demonstrate physiological adaptation to the changes in available water with smaller heavier leaves, no significant canopy dieback and in the dry season in the Nossob stomatal regulation of leaf water potentials.
An increase in abstraction of groundwater particularly at the Nossob borehole may cause an additional draw down of the water table adding to the physiological stress demonstrated in our study. The managers of the Kgalagadi Transfrontier Park have a mandate that includes biodiversity conservation. To fulfil this mandate, upper and lower thresholds for groundwater abstraction that allow for an adequate ecological reserve have to be determined.
在没有地表水的干旱地区，季节性河床中的深钻孔为人类和动物提供了水源。随着非洲南部喀拉哈里跨境公园等干旱野生动物公园旅游业基础设施的不断发展，我们不禁要问，用水量增加可能会对大树产生什么影响。干旱稀树草原上的大树通过为许多其他动植物物种提供食物、遮荫、筑巢地点和增加养分，发挥着重要的生态系统服务功能，因此被视为关键物种。
我们测定了地下水位的季节性波动，同时确定了公园内奥博河和诺索布河主要大树物种的水源。我们还通过叶片碳含量、木质部压力势、比叶面积和树冠死亡估计值，来确定这些树木生理胁迫的程度。我们在奥博河一个低用水量钻孔的上游和下游，以及诺索布河一个高用水量钻孔的上游和下游进行了此项研究。
我们的研究结果表明，这些树木在雨季确实利用深层地下水，而且这与人类使用的是同一水源。在旱季，活跃钻孔下游的奥博河中的树木与含水层脱离，转而使用同位素富集程度更高的土壤水。在旱季的诺索布河，所有树木都使用同位素富集程度更高的土壤水，活跃钻孔下游的树木则通过气孔调节来维持叶片水势。这些结果表明，用水量较大的诺索布河中的树木比奥博河中的树木面临更大的水分胁迫，但两条河流中的树木都表现出对可用水量变化的生理适应，即叶片更小更厚、树冠没有明显枯死，以及在旱季诺索布河中的树木通过气孔调节叶片水势。
地下水抽取量的增加，尤其是诺索布钻孔处的抽取量增加，可能会导致地下水位进一步下降，加剧我们研究中所显示的生理胁迫。喀拉哈里跨境公园的管理者有保护生物多样性的任务。为了履行这一任务，必须确定允许有足够生态储备的地下水抽取上限和下限。
