Abstract. Peat swamp forests are the second rarest forest type found in South Africa while dune forests have been under severe threat through mining and agriculture. Both forest types exist in the conservation area, and World Heritage site, known as the iSimangaliso Wetland Park on the East coast of South Africa. The area is prone to severe droughts (Taylor et al., 2006) and recent attempts to understand the local water balance revealed that there was insufficient information on the water use of the indigenous forests of the area. The peat swamp forest and dune forest sites studied in this research were located within close proximity to each other, yet, are characterised by different landscape positions in terms of water availability. The coastal dune forest soil profile was generally dry and sandy and the tree roots did not have access to the water table. In contrast the peat swamp forest is located in an interdunal wetland where the trees have permanent access to water. The climate at both sites is subtropical with a mean annual precipitation of 1200 mm yr −1 . However, over 20 months of measurement, the first summer (October 2009 to March 2010) was drier (424 versus 735 mm) than the second summer (October 2010 to March 2011) emphasising the variability of the rainfall in the area and providing a wide range of conditions measured.The sap flow of an evergreen, overstory Syzygium cordatum and a semi-deciduous, understory Shirakiopsis elliptica were measured in the peat swamp forest using the heat ratio method. The Syzygium cordatum water use was not highly seasonal and the daily maximum water use ranged from approximately 30 L d −1 in winter to 45 L d −1 in summer whereas the Shirakiopsis elliptica water use was more seasonal at 2 L d −1 in winter and 12 L d −1 in summer. The water use of the Syzygium cordatum was not influenced by seasonal rainfall variations and was actually higher in the drier summer (October 2009 to March 2010). Three trees of different heights were monitored in the same way in the dune forest and the water use found to be highly seasonal. Over the entire measurement period, the water use was highest for an emergent Mimusops caffra (5 to 45 L d −1 ), whereas the water use of the Eugenia natalitia (2 to 28 L d −1 ) and Drypetes natalensis (1 to 4 L d −1 ) was lower. At the dune forest, the water use was highest in the wetter summer due to the reliance of the trees on rainfall to recharge the soil water. A split-line regression showed that on average, soil water limited tree water use 64 % of the time over the measurement period at the dune forest. For modelling tree water use at the dune forest, it was concluded that a two-stage model, taking soil water content into account (from multiple sampling points), would be necessary.
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 stud-
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.
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 25 L•day−1 in summer (sap flow being directly proportional to tree size). In the winter periods this was reduced to between 3 and 6 L•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 12 L•day−1. Small alien trees (
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. The objective of this study was to investigate the water use (transpiration rates) of a selection of introduced and indigenous tree species and quantify the hydrological benefit that could be achieved through a suitable rehabilitation programme. Consequently water use within a riparian forest in the upper Mgeni catchment of KwaZulu-Natal in South Africa was monitored over a 2-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 (HPV) sap flow technique and the stem steady state (SSS) techniques were used to measure the sap flow of a selection of indigenous and introduced species. The indigenous trees at New Forest, South Africa, showed clear seasonal trends in the daily sap flow rates varying from 8 to 25 L day−1 in summer (sap flow being directly proportional to tree size). In the winter periods this was reduced to between 3 and 6 L 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 12 L day−1. Small alien trees (< 30 mm) A. mearnsii and S. mauritianum used up to 4 L day−1 each. The total accumulated sap flow per year for the three individual A. mearnsii and E. grandis trees was 6548 and 7405 L a−1 respectively. In contrast, the indigenous species averaged 2934 L a−1, clearly demonstrating the higher water use of the introduced species. After spatial upscaling, it was concluded that, at the current state of invasion (21 % of the stand being alien species), 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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