Abstract
Many studies have investigated the water relations of indigenous plants in the fynbos shrublands of the Cape, South Africa. These have mainly focused on understanding the mechanisms by which individual plant species respond to droughts, the frequency and severity of which is expected to increase due to climate change. However, comparatively little information exists on the dynamics of water use by indigenous plants in the region, and, in particular, how water use varies seasonally and between sites. In this study we determined water use by 3 fynbos vegetation types growing at 4 different sites, namely: (i) lowland Atlantis Sand Plain fynbos growing on deep sandy soils, (ii) Kogelberg Sandstone fynbos growing in a riparian zone on deep alluvial soils, (iii) dryland Kogelberg Sandstone fynbos growing on shallow sandy soils at a montane site, and (iv) alluvial Swartland fynbos growing in clayey soils. Evapotranspiration (ET) was quantified at each site during specific periods using a boundary layer scintillometer and energy balance system. A simple dual source model in which the stand ET was calculated as the algebraic sum of outputs from soil evaporation and transpiration sub-models was used to scale up the ET measurements to annual values. The data showed large differences in ET depending on site characteristics and on plant attributes. Dense stands of riparian Sandstone Fynbos had an annual ET of 1 460 mm which exceeded the reference ET of 1 346 mm. Dryland Sandstone Fynbos used only 551 mm of water per year while the Sand Plain Fynbos’ annual ET was 1 031 mm, which was similar to the reference ET of 1 059 mm. We conclude that some indigenous plant species use large volumes of water which should be accounted for in, e.g., groundwater recharge estimates, and calculations of incremental water gains after clearing alien invasive plants, among other applications. Keywords : Evapotranspiration, fynbos, scintillometer, Western Cape
Highlights
Indigenous vegetation in the Mediterranean-type climate region of South Africa is threatened by ongoing land transformation for agriculture, forestry, industrial development and urban developments (Jacobsen et al, 2007; West et al, 2012)
In studies to quantify the hydrological impacts of alien invasive plants, information on the water use by the indigenous vegetation is critical for determining the incremental water gains that can be achieved by clearing the alien plants (Doody et al, 2011; Dzikiti et al, 2013a)
This study provides the first assessment of the dynamics of water use by 3 types of fynbos vegetation in the Western Cape Province of South Africa at daily and annual time scales
Summary
Indigenous vegetation in the Mediterranean-type climate region of South Africa is threatened by ongoing land transformation for agriculture, forestry, industrial development and urban developments (Jacobsen et al, 2007; West et al, 2012). It is not surprising that numerous studies in the fynbos biomes have mainly focused on the mechanisms by which plant species respond to seasonal patterns in moisture availability and how these vary along rainfall gradients (Miller et al, 1983; Richards et al, 1995; Richardson and Kruger, 1990; Von Willert et al, 1989) and during droughts (West et al, 2012) These studies have largely targeted specific plant species and few were conducted at the community or vegetation-type levels (Jacobsen et al, 2007; Midgley et al, 2002). In the emerging water footprint concept, accurate baseline information on the water use by indigenous vegetation is critical for the determination of the water footprints of various land use practices such as agricultural and forestry activities, among others (Hastings and Pelgrum, 2012)
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