Journal cover Journal topic
Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
doi:10.5194/hess-2016-650
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
16 Jan 2017
Review status
This discussion paper is under review for the journal Hydrology and Earth System Sciences (HESS).
Water-use dynamics of an alien invaded riparian forest within the mediterannean climate zone of the Western Cape, South Africa
Bruce C. Scott-Shaw1, Colin S. Everson1,2,3, and Alistair D. Clulow4 1Center for Water Resources Research, School of Agriculture, Earth & Environmental Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa
2South African Environmental Observation Network (SAEON), Grasslands-Wetlands-Forests Node, 1 Peter Brown Drive, Queen Elizabeth Park, Montrose, Pietermaritzburg 3201, South Africa
3Department of Plant and Soil Sciences, University of Pretoria, Private Bag X20, Hatfield, Pretoria 0028, South Africa
4Agrometeorology, School of Agriculture, Earth & Environmental Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa
Abstract. In South Africa the invasion of riparian forests by alien trees has the potential to affect the country's limited water resources. Tree water-use measurements have therefore become an important component of recent hydrological studies. It is difficult for government initiatives, such as the Working for Water (WfW) alien clearing programmes, to justify alien tree removal and implement rehabilitation unless a known hydrological benefit can be seen. Consequently water-use within a riparian forest along the Buffeljags river in the Western Cape of South Africa was monitored over a three year period. The site consisted of an indigenous stand of Western Cape afrotemperate forest adjacent to a large stand of introduced Acacia mearnsii. The heat ratio method was used to measure the water-use of a selection of representative indigenous species in the indigenous stand, a selection of A. mearnsii trees in the alien stand and two clusters of indigenous species within the alien stand. The indigenous trees in the alien stand at Buffeljags river showed significant intraspecific differences in the daily sap flow rates varying from 15 to 32 L day−1 in summer (sap flow being directly proportional to tree size). In winter (June) this was reduced to only 7 L day−1 when less energy was available to drive the transpiration process. The water-use in the A. mearnsii trees showed peaks in transpiration during the months of March 2012, September 2012 and February 2013. These periods corresponded to favourable climatic conditions of high average temperatures, rainfall and high daily vapour pressure deficits (VPD – average of 1.26 kPa). The average daily sap flow ranged from 25 L to 35 L in summer and approximately 10 L in the winter. The combined accumulated daily sap flow per year for the three Vepris lanceolata and three A. mearnsii trees was 5700 and 9200 L respectively, clearly demonstrating the higher water-use of the introduced Acacia trees during the winter months. After spatially upscaling the findings, it was concluded that, annually, the alien stand used nearly six times more water per unit area than the indigenous stand. This finding indicates that there would be a hydrological gain if the alien species are removed from riparian forests and rehabilitated back to their natural state.

Citation: Scott-Shaw, B. C., Everson, C. S., and Clulow, A. D.: Water-use dynamics of an alien invaded riparian forest within the mediterannean climate zone of the Western Cape, South Africa, Hydrol. Earth Syst. Sci. Discuss., doi:10.5194/hess-2016-650, in review, 2017.
Bruce C. Scott-Shaw et al.
Bruce C. Scott-Shaw et al.
Bruce C. Scott-Shaw et al.

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