Active restoration of woody canopy dominants in degraded South African semi-arid thicket is neither ecologically nor economically feasible
Will the planting of nursery-propagated woody canopy species within severely degraded Arid Subtropical Thicket once dominated by the succulent shrub Portulacaria afra (Spekboom) be an efficient restoration strategy to employ in addition to the current restoration protocol of planting a monoculture of P. afra truncheons?
Krompoort, a farm located on the northern footslopes of the Groot Winterhoek mountains, near Kirkwood, Eastern Cape, South Africa.
We planted, in degraded, intact and three differently aged post-restoration (P. afra truncheons) sites, nursery-propagated individuals of two woody canopy dominants (Pappea capensis and Searsia longispina), and two inter-canopy shrubs (Lycium ferocissimum and Rhigozum obovatum) in Sep 2008 (spring). The experiment was repeated again in May 2009 (autumn) and a succulent canopy species (P. afra) was added. We assessed restoration success in terms of the survival of planted individuals after 24 mo (spring planting) and 12 mo (autumn planting). The estimated cost of a restoration effort to establish two woody canopy dominant species with the large-scale planting of P. afra was calculated and compared with that of the already established protocol of planting only P. afra truncheons.
Survival after spring (24 mo) and autumn (12 mo) plantings of the two woody canopy species was less than 5%, whereas survival of L. ferocissimum was low (19%), R. obovatum good (70%) and P. afra excellent (100%). Contrary to expectations, survival was not related to a gradient of intactness encompassing degraded, restoration and intact treatments that are associated with increasing biomass and soil carbon. The costs of incorporating the four woody canopy species into the restoration programme's protocol were 2.4 times the costs of restoring with P. afra alone.
Planting propagules of canopy species other than P. afra is likely to render the programme economically unfeasible, depending on the price of carbon. We conclude that biodiversity goals for the restoration programme are better achieved via spontaneous recruitment of woody canopy and other species, even though this may take more than 40 yr post restoration.