An accepted criterion for measuring the success of ecosys tem restoration is the return of biodiversity relative to intact reference ecosystems. The emerging global carbon economy has made landscape-scale restoration of severely degraded Portulacaria afra (spekboom)-dominated sub tropical thicket, by planting multiple rows of spekboom truncheons, a viable land-use option. Although large amounts of carbon are sequestered when planting a monoculture of spekboom, it is unknown whether this is associated with the return of other thicket biodiversity components. We used available carbon stock data from degraded, restored, and intact stands at one site, and sam pled carbon stocks at restored stands at another site in the same plant community. We also sampled plant community composition at both sites. The total carbon stock of the old est (50 years) post-restoration stand (250.8 ± 14 t C ha−1) approximated that of intact stands (245 t C ha−1) and we observed a general increase in carbon content with restora tion age (71.4 ± 24 t C ha−1 after 35 and 167.9 ± 20 t C ha−1 after 50 years). A multiple correspondence analysis sepa rated degraded stands from stands under restoration based on ground cover, floristic composition, and total carbon stock. Older post-restoration and intact stands were clus tered according to woody canopy recruit abundance. Our results suggest that spekboom is an ecosystem engineer that promotes spontaneous return of canopy species and other components of thicket biodiversity. The spekboom canopy creates a cooler micro-climate and a dense litter layer, both likely to favor the recruitment of other canopy species.
The attributes defining a restored ecosystem should include some measure of the return of species composition, ecosystem services, and ecological functioning (SERI 2004; Hobbs 2007; Benayas et al. 2009). Carbon sequestration projects, where trees are planted to earn carbon credits, should aim to provide investors with the benefit of both carbon and biodiversity credits (Bekessy & Wintle 2008). This requires confirmation of the restoration of a suite of species, representing different functional groups present in a reference intact system, via active or spontaneous return (White & Walker 1997; Herath et al. 2009).
Here we focus on the restoration of endemic-rich South African subtropical succulent thicket (Vlok et al. 2003; Cowl ing et al. 2005) for carbon credits (Mills et al. 2007, 2009). Component woody canopy (2–5 m) plants, e.g. Euclea undulata, Pappea capensis and Schotia afra are long-lived and reproduce mainly via ramets, or occasionally via seedlings that originate mostly from vertebrate-dispersed propagules (Midg ley & Cowling 1993; Sigwela et al. 2009). While relatively resilient to browsing by indigenous herbivores (Stuart-Hill 1992), subtropical thicket is highly vulnerable to browsing by domestic goats. Sustained, heavy browsing can transform the dense closed-canopy thicket into an open community compris ing scattered and degraded thicket clumps and isolated trees in a matrix of ephemeral herbs (Lechmere-Oertel et al. 2005a, 2005b). Particularly, vulnerable are drier (<450 mm/year) forms of thicket (Arid and Valley forms) (Vlok et al. 2003) dominated by the tree-like leaf succulent, Portulacaria afra (hereafter spekboom) (Stuart-Hill 1992; Lechmere-Oertel et al. 2005a, 2005b). Of the 16,942 km2 of solid (unbroken canopy) spekboom-dominated thicket, 46% has been heavily, and 36% moderately degraded (Lloyd et al. 2002). Spekboom is the first canopy dominant to succumb to browsing, being entirely eliminated in severe cases of degradation.
Spontaneous recovery of canopy species populations does not occur in browsing-degraded spekboom thicket, even decades after the cessation of goat-browsing (Lechmere Oertel et al. 2005a; Sigwela et al. 2009). Recruits of canopy species—both ramets and genets—are invariably associated
with the rich layer of organic mulch that accumulates beneath the intact thicket canopy (Sigwela et al. 2009). Heavy goat browsing alters the beneath-canopy microclimate and destroys the rich layer of organic mulch (Lechmere-Oertel et al. 2008). Deprived of an organic carbon-rich soil, and subject to ongoing browsing, adult canopy plants eventually die, thicket clumps steadily dwindle (Lechmere-Oertel et al. 2005a), and recruitment fails (Sigwela et al. 2009). The ecosystem is locked into a degradation trajectory that can only be reversed by active restoration such that the abiotic threshold imposed by changes in soil properties and microclimate is transcended (Briske et al. 2006).
The recent emergence of the global carbon economy has provided an unprecedented opportunity to finance the restora tion of degraded thicket via carbon credits (Galatowitsch 2009). Spekboom-dominated ecosystems store carbon in excess of 200 t/ha, a remarkable feature for a xeric ecosys tem and comparable to that of mesic forest ecosystems (Mills et al. 2005a). Spekboom contributes most of the above-ground carbon (Mills & Cowling 2006; Lechmere-Oertel et al. 2008) and, with its dense canopy, provides the relatively cool and dry conditions necessary for the accumulation of large levels of soil carbon (Lechmere-Oertel et al. 2005a, 2005b; Cowling & Mills 2010). Comparisons of degraded and intact stands reveal carbon losses of more than 80 t C ha−1 (Mills et al. 2005b). These losses are evident from the decrease in above ground biomass, but are also a result of the reduction in soil organic carbon stock (Mills & Fey 2004).
Using the opportunity provided by a long-standing (1976–1998) spekboom restoration trial implemented by a landowner, Mills & Cowling (2006) found that 112 t C ha−1, at a rate of 4.2 t/year, was sequestered in this 27-year period. These data provided the impetus for the South African government to fund spekboom restoration research and implementation at the landscape level. This initiative, the Subtropical Thicket Restoration Project (STRP), has the broad aim of creating an employment-intensive restoration economy funded by the carbon market (Mills et al. 2009). The current cost-effective methodology for restoration comprises the planting of spekboom truncheons (height: 250–650 mm; stem diameter: 15–35 mm) at 1–2 m spacing in parallel rows; the truncheons are sourced from surrounding relatively intact areas (Mills et al. 2007). Spekboom establishes readily from truncheons, which can be inexpensively harvested.
Van der Vyver et al. (2012) demonstrated that active restoration of woody canopy species other than spekboom was neither economically nor ecologically feasible. The question then arises: does the planting of spekboom monocultures facilitate the spontaneous restoration of other thicket species, especially woody canopy ones? The degradation dynamics of spekboom thicket are consistent with state-and-transition models (Milton & Hoffman 1994). We hypothesize that using spekboom as the focal species for restoration will achieve carbon sequestration goals and ensure that thresholds constraining the return of biodiversity are overcome (Briske et al. 2006). Here we test this hypothesis by examining total carbon stock and plant species composition within different post-restoration age stands, and compare them with intact and degraded sites.