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  • Spekboom (Portulacaria afra) Planting in Degraded Thickets Improves Soil Properties and Vegetation Diversity*
  • Bryce Panter (bio) and Sheunesu Ruwanza

The South African thicket biome is dominated by fragmented dense clumps of spiny shrublands (Cowling 1984, Vlok et al. 2003), which can support game farming, ecotourism, and firewood for local communities. However, substantial browsing by domestic animals, mostly goats and sheep, has transformed the thicket biome, leading to soil and vegetation degradation (Mills and Fey 2004). The problem is so severe that an estimated 46% of the naturally occurring Spekboom Thicket vegetation alone has been heavily degraded by domestic herbivores (Lloyd et al. 2002). In recognition of the problem, the South African government initiated the Subtropical Thicket Restoration Project (van der Vyver et al. 2013), whose dual mandate is to ecologically restore degraded thickets and use the initiative to create employment. The project identified Portulacaria afra (spekboom) as the plant that has the potential to restore these degraded thickets (Mills et al. 2007, Sigwela et al. 2009).

Spekboom is a succulent tree that reproduces vegetatively and establishes easily after transplant from cuttings. It has the potential to shade soils, return organic matter, and attract seed dispersers (Mills and Cowling 2010), facilitating soil and vegetation recovery after its introduction. Previous studies that have examined the efficacy of spekboom planting for restoration purposes have shown that spekboom introduction improves soil properties, especially carbon (Mills and Cowling 2010, van der Vyver et al. 2013), a result of spekboom's ability to alter litter composition and decomposition rate. Changes in soil properties can facilitate gemination and establishment of native plants underneath spekboom, suggesting that spekboom may function as a nurse plant in ecological restoration (Rens et al. 2008). To our knowledge no study has monitored both soil (physical and chemical properties) and vegetation (plant diversity) recovery following spekboom planting. We examined whether spekboom planting facilitates both soil and vegetation recovery by comparing soil physicochemical properties and vegetation diversity in natural, spekboom-planted, and degraded sites in Copper Moon Game Reserve, located in Eastern Cape, South Africa.

Three adjacent natural, spekboom planted, and degraded sites (approximately 50 m apart) were identified at Copper Moon Game Reserve (33°12' 59.65" S, 25°31' 06.31" E). Vegetation in the area is an ecotone between Albany Spekboom Thicket and Saltaire Karroid Thicket. Soils are sandy and derived from the underlying Cape and Karoo shale. Mean annual rainfall is approximately 409 mm and mainly falls in summer. Temperature ranges from an average 8°C in winter to an average 30°C in summer (Vlok et al. 2003).

The spekboom-planted site, which used to be a grazing area, was planted in 2005 with truncheons (woody cuttings) collected from the fenced natural site. Information regarding the impact of grazing in the spekboom-planted site is limited, however Google Earth images suggest that the site looked similar to the degraded site prior to planting. The spekboom-planted site was a trial site measuring approximately 50 × 50 m and is fenced to prevent animal browsing. The truncheons were planted at a depth of approximately 15 cm, at 1.5-m row spacing and 1-m plant spacing. The natural site is dominated by native thicket vegetation, Acacia karroo, Portulacaria afra, Boscia oleoides, and Crassula capitella (Mucina and Rutherford 2006). The degraded site, which is open to animal grazing, is dominated by grasses and a few scattered trees and shrubs. The sites are on a gentle north-east facing slope of [End Page 76]


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Table 1.

Comparison of measured soil properties and vegetation diversity between natural, spekboom-planted, and degraded sites at Copper Moon Game Reserve. Data are means and one-way ANOVA results are shown. Means with different letters are significantly different (p < 0.05), whilst those with similar letters are not significantly different (p > 0.05).

approximately 10–17°. The spekboom-planted site and the degraded site sit beside each other (approximately 2 km apart). The natural site sits above them on the slope.

Ten years after the initial spekboom planting (in 2015), eight plots, each measuring 10 × 10 m, were set up in each of the three sites. Within each plot, one...

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