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Discussion papers | Copyright
https://doi.org/10.5194/hess-2018-227
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 28 May 2018

Research article | 28 May 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

Water-use dynamics of an alien invaded riparian forest within the summer rainfall zone of South Africa

Bruce C. Scott-Shaw1 and Colin S. Everson2 Bruce C. Scott-Shaw and Colin S. Everson
  • 1Center for Water Resources Research, School of Agricultural, Earth and Environmental Sciences, University of KwaZulu-Natal, Private Bag X01, Scottsville, Pietermaritzburg 3209, South Africa
  • 2Department of Plant and Soil Sciences, University of Pretoria, Private Bag X20, Hatfield, Pretoria 0028, 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 South African government initiatives, such as the Working for Water (WfW) alien clearing programme, to justify alien tree removal and implement rehabilitation unless hydrological benefits are known. Consequently water-use within a riparian forest in the upper Mgeni catchment of KwaZulu-Natal in South Africa was monitored over a two year period. The site consisted of an indigenous stand of eastern mistbelt forest that had been invaded by Acacia mearnsii, Eucalyptus nitens and Solanum mauritianum. The heat ratio method of the heat pulse velocity sap flow technique and the stem steady state techniques were used to measure the sap flow of a selection of indigenous and introduced species. The indigenous trees at New Forest showed clear seasonal trends in the daily sap flow rates varying from 8 to 25L•day−1 in summer (sap flow being directly proportional to tree size). In the winter periods this was reduced to between 3 and 6L•day−1 when limited energy flux was available to drive the transpiration process. The water-use in the A. mearnsii and E. grandis trees showed a slight seasonal trend, with a high flow during the winter months in contrast to the indigenous species. The water-use in the understorey indicated that multi-stemmed species used up to 12L•day−1. Small alien trees (<30mm) A. mearnsii, and S. mauritianum used up to 4L•day−1 each. The combined accumulated daily sap flow per year for the three A. mearnsii and E. grandis trees was 6548 and 7405L•year−1 respectively. In contrast, the indigenous species averaged 2934L•year−1, clearly demonstrating the higher water-use of the introduced species. After spatial up-scaling, it was concluded that, at the current state of invasion by 21%, the stand used 40% more water per unit area than if the stand were in a pristine state. If the stand were to be heavily invaded, at the same stem density of the indigenous forest, a 100% increase in water-use would occur over an average rainfall year.

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Bruce C. Scott-Shaw and Colin S. Everson
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Bruce C. Scott-Shaw and Colin S. Everson
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The research undertaken for this study have allowed for an accurate direct comparison of indigenous and introduced tree water-use. The measurements of trees growing in the understorey indicate significant water-use in the sub-canopy layer. The results showed that individual tree water-use is largely inter-species specific. The introduced species remain active during the dry winter periods resulting in their cumulative water-use being significantly greater than that of the indigenous species.
The research undertaken for this study have allowed for an accurate direct comparison of...
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