Sap flow in Searsia pendulina and Searsia lancea trees established on gold mining sites in central South Africa

Abstract

The Witwatersrand Basin Goldfields (WBG) have seen over a century of continuous mining that has generated extensive tailings storage facilities (TSF), together with “footprints” remaining after the residue has been removed for reprocessing or consolidation into larger TSFs. These are now believed to number several hundred and cover a total area of 400–500 km(sup2). Acid mine drainage (AMD) from these structures is widespread and has resulted in contamination of soils, groundwater and surface water systems. Sustainable and long-term control measures are required to limit environmental contamination. The Mine Woodlands Project, initiated by the University of the Witwatersrand and AngloGold Ashanti Ltd, aims to investigate the use of trees for hydraulic control of mine seepage, as well as contaminant immobilization. A variety of exotic and indigenous tree species was planted in high density stands within site species trials located close to TSFs in the Orkney and Carltonville districts. The aim is to evaluate their survival and growth, as well as water use and contaminant uptake or immobilization. This paper describes a study of the annual pattern of sap flow rates in two species of indigenous tree (Searsia lancea (L. F.) F.A. Barkley and S. pendulina (Jacq.) Moffett, comb. nov.) established in plantation form. These species occur naturally in central and western South Africa. Sap flow was monitored continuously over a full year in eight stems representing each species, using the heat ratio version of the heat pulse velocity technique. Plot sap flow was estimated by scaling up according to the number and size of stems, and utilizing functions relating leaf dry mass and leaf area to stem diameter. The deciduous species S. pendulina was found to use 591 mm of water over a full growing season, while the evergreen species S. lancea was found to use 1044 mm over a full year. Differences in sap flow patterns between these species are attributed largely to different leaf dynamics. We conclude that S. lancea has potential for the hydraulic control of mine seepage water in phytoremediation systems in the WBG.