Thicket Restoration: Is Spekboom the only answer?

Restoring thicket has largely focused on using Portulacaria afra (spekboom), and to a much lesser extent on "late-successional" tree species, as the agents for returning ecosystem functioning. I write "late-successional" because we are a far way off from understanding the successional patterns in thicket; nonetheless, the case is quite well established that, in arid thicket, the presence of many tree species — such as Pappea capensis — is due to the presence of spekboom (e.g. van der Vyver et al., 2013). This is largely due to the increase in available soil moisture that is a by-product of a spekboom dominant layer (van Luijk et al., 2013; Wilman et al., 2014).

However, does this necessarily mean that spekboom is the best pioneer species to re-introduce or bolster into degraded lands? Recent field observations lead me to think that we might have overlooked an pioneer candidate: Crassula tetragona.

While looking investigating a donga (a dry gulley formed by erosion) in Addo that was surrounded by dense intact thicket, it struck me that although there were plenty of spekboom plants in the vicinity (e.g. the orange arrow in the picture below), I could only find one small individual in the donga — although, I argue elsewhere (The number one invasive species in arid lands globally: Spekboom?) that this is due to a lack of an adequate dispersal agent. What had colonised this inhospitable environment was Crassula tetragona (blue arrow in the picture below). Also, where it had colonised, colonies of clonelets were forming wherever leaf-clusters had broken off the parent plant. This clump is acting as a silt and litter trap, with the soil already at least 5 cm above the surrounding soil. This is an example of spontaneous rehabilitation and C. tetragona is changing the microclimate of this eroded donga testing.

However, this is in a herbivore-exclusion area and along a drainage line where there will be some moisture and soil (the donga had not hit bed rock yet). How will this species fair in a harsher environment?

Well, the images below were taken from Kaboega Private Game Reserve on a heavily eroded north-facing (and so hot!) slope. There is no topsoil - it is right down to the eroding shale bedrock.  The light green plants in the image below are C. tetragona. These plants are filling up the small rills on a slope where all the topsoil has been stripped off  — and they are doing it via vegetation growth: a crown piece gets knocked off (falls off?), and this readily takes root. This is an environment where a range of herbivores are present, including kudu and impala. Why is it not being eaten? This species is likely to be unpalatable (however, Curtis & Perrin, 1979, do list it as a preferred species for rodents).

In discussing this as an option with Jan Vlok, he highlighted that this is a species that does not do well in areas with high herbivore activity due to trampling. So some activity is okay, but not too much.

Given that this is a species that has is readily self-propagating across a range of arid and hot environments, it should certainly be considered as part of the rehabilitation suite. Coupling this with erosion control measures, such as mini-ponds, could provide the boost to create vegetated hotspots. 

References:

• Curtis BA & Perrin MR (1979) Food preferences of the vlei rat (Otomys Irroratus) and the four-striped mouse (Rhabdomys pumilio). South African Journal of Zoology, 14:224-229. DOI: 10.1080/02541858.1979.11447675
• van der Vyver ML, Cowling RM, Mills AJ, Difford M (2013) Spontaneous return of biodiversity in restored subtropical thicket: Portulacaria afra as an ecosystem engineer. Restoration Ecology 21:736-744.
• van Luijk G, Cowling RM, Riksen MJPM & Glenday J (2013) Hydrological implications of desertification: Degradation of South African semi-arid subtropical thicket. Journal of Arid Environments 91:14-21.
• Wilman, V, Campbell EE, Potts AJ & Cowling, RM (2014) A mismatch between germination requirements and environmental conditions: Niche conservatism in xeric subtropical thicket canopy species? South African Journal of Botany 92: 1-6.