Abstract
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 program, to justify alien tree removal and implement rehabilitation unless hydrological benefits are known. Consequently, water use within a riparian forest along the Buffeljags River in the Western Cape of South Africa was monitored over a 3-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 of the heat pulse velocity sap flow technique was used to measure the sap flow of a selection of 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 limited 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 had high average temperatures, rainfall and high daily vapor pressure deficits (VPDs – average of 1.26 kPa). The average daily sap flow ranged from 25 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 6 times more water per unit area than the indigenous stand (585 mm a−1 compared to 101 mm a−1). This finding indicates that there would be a gain in groundwater recharge and/or streamflow if the alien species are removed from riparian forests and rehabilitated back to their natural state.
Highlights
While extensive research has been undertaken on the water use of terrestrial ecosystems in South Africa, little is known about the water use and growth of trees growing in riparian areas
The mean annual precipitation (MAP) over the 3-year study was significantly higher than the long-term average (636 mm a−1) by 300–500 mm a−1 (2012 to 2014 being 1017, 902 and 1127 mm a−1, respectively)
There is a widespread belief in South Africa that indigenous tree species, in contrast to introduced tree species, use less water and should be planted more widely in land rehabilitation programs (Olbrich et al, 1996; Dye, 2001; Dye et al, 2008; Everson et al, 2007; Gush and Dye, 2008, 2009; Gush et al, 2015)
Summary
While extensive research has been undertaken on the water use of terrestrial ecosystems in South Africa, little is known about the water use and growth of trees growing in riparian areas. The deep fertile soils, with high soil moisture contents associated with riparian areas, make them ideal for plant establishment and growth (Everson et al, 2007). As such, these areas are extremely vulnerable to invasion by pioneer plant species, alien species that have historically been introduced for commercial forestry. Most riparian trees are phreatophytic, meaning they have access to a permanent source of water because their rooting system is within the shallow groundwater
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