Factors affecting effective ecological restoration of Portulacaria afra (spekboom)-rich subtropical thicket and aboveground carbon endpoint projections
- Authors: Van der Vyver, Marius Lodewyk
- Date: 2018
- Subjects: Restoration ecology , Plant conservation -- South Africa Portulacaria afra -- South Africa Shrubs -- South Africa
- Language: English
- Type: Thesis , Doctoral , DPhil
- Identifier: http://hdl.handle.net/10948/23250 , vital:30468
- Description: Among the requirements to achieve effective ecological restoration on a landscape scale are a scientific underpinning, strong adaptive management and the setting of realistic endpoints. Efficacy and success depend on a complex interplay of factors of both local and regional influence. Biome-wide restoration experiments are rare, but valuable for evaluating efficacy of different protocols according to local context. The Thicket-wide Plot (TWP) experiment was initiated by the Subtropical Thicket Restoration Programme to inform landscape-scale ecological restoration of degraded Portulacaria afra (spekboom)-rich Subtropical Thicket. Thirteen different planting treatments of Portulacaria afra (spekboom) truncheons were planted as replicated rows on 0.25 ha fenced plots across the distribution range of spekboom-rich thicket habitat to better establish effective restoration protocols. I used a rule-based learning ensemble algorithm to evaluate 60 different predictors that potentially impact effective restoration, covering a range of climatic, topographic, edaphic, ecological and management related factors observed at 227 of these TWP plots. Percentage survivorship and aboveground biomass carbon sequestration rate (ABCsr) were taken as proxies for efficacy derived from subsequent monitoring data gathered 2 - 5 years after establishment, and a new allometric model I developed for spekboom. I found herbivory and identification of target habitat the two most important predictors of restoration efficacy, both well within the control of the practitioner. Highest survivorship and ABCsr were associated with larger truncheons planted deeper. The only exception to this pattern was found amongst the two smallest size truncheon categories. The application of external rooting hormone or watering treatment to truncheons were found of no significance to effective restoration, while all the pruned treatments were, in association with lesser planting depth, found to be significantly less effective than untreated truncheons planted deeper. When the restoration protocol is optimal (largest truncheons planted deepest in target habitats and protected from herbivory), my results show higher annual carbon sequestration rates (1.46 - 3.7 t C ha−1 yr−1 in aboveground biomass) than most older restoration sites that have been identified as benchmarks of success. In order to project ABC endpoints, I generated 40 new species-specific allometric models and used them to estimate biomass and carbon of intact and degraded stands on five sites spread out across the region. The highest AGC for stands in both intact and degraded states were estimated at 42.96 t Cha−1 and 12.98 t C ha−1 respectively, and the lowest at 26.32 t C ha−1 and 2.52 t C ha−1. Large canopy dominant (LCD) species contributed the largest AGC portion at three intact stands. The second largest portion was recorded for spekboom. The difference between paired stands on each sample site ranged between 23.8 and 32.8 t C ha−1, which provide a realistic target for biome-wide restoration that, when adopted together with current protocols, may be reached within three decades of inception.
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- Date Issued: 2018
Restoring the biodiversity of canopy species within degraded spekboom thicket
- Authors: Van der Vyver, Marius Lodewyk
- Date: 2011
- Subjects: Plant diversity , Biodiversity
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:10602 , http://hdl.handle.net/10948/1564 , Plant diversity , Biodiversity
- Description: I investigated the return of plant canopy diversity to degraded spekboom thicket landscapes under restoration treatment. I attempted the reintroduction of five nursery propagated and naturally-occurring plant species in severely degraded Portulacaria afra Jacq. (spekboom) dominated thickets that have been subjected to a restoration method involving the planting of dense rows of P. afra truncheons for various time periods and also in degraded and intact thickets. I also planted nursery propagated P. afra cuttings. An average of 30 propagules of each species, were planted in each of the chosen areas in two distinct seasons that exhibited distinct rainfall peaks. Sixteen propagules of P. afra were also planted in each treatment only once. Propagules of the two thicket woody canopy species (S. longispina and P. capensis) showed a total survival of 1% and 9%, respectively. Survival of L. ferocissimum and R. obovatum was 19% and 70% and all propagules of P. afra survived. Analyses showed that survival is primarily tied to a species effect, with R. obovatum and P. afra showing significantly better survival than the other species. Within the other surviving few species a significant preference for overhanging canopy cover was observed. The results show little significance of restoration treatment for propagule survival, suggesting that a range of conditions is needed for the successful establishment of canopy species that likely involves a microclimate and suitable substrate created by canopy cover and litter fall, combined with an exceptional series of rainfall events. I found that the high costs involved with a biodiversity planting endeavour, and the low survival of propagules of thicket canopy plant species (P. afra excepted), renders the proposed biodiversity planting restoration protocol both ecologically and economically inefficient. Restoration success involves the autogenic regeneration of key species or functional groups within the degraded ecosystem. Heavily degraded spekboom-dominated thicket does not spontaneously regenerate its former canopy species composition and this state of affairs was interpreted in terms of a state-and-transition conceptual model. Floristic analyses of degraded, intact and a range of stands under restoration treatment for varying time periods at two locations in Sundays Spekboomveld revealed that the stands under restoration are progressively regenerating canopy species biodiversity with increasing restoration age, and that intact sites are still the most diverse. The high total carbon content (TCC) measured within the older restored stands Rhinosterhoek (241 t C ha-1 after 50 years at a depth of 50 cm) rivals that recorded for intact spekboom thickets, and the number of recruits found within older restored sites rivals intact sites sampled. 2 The changes recorded in the above- and belowground environments potentially identify P. afra as an ecosystem engineer within spekboom dominated thickets that facillitates the build-up of carbon above- and belowground and the accompanying changes in soil quality and the unique microclimate aboveground, which enables the hypothetical threshold of the degraded state to be transcended. This restoration methodology is accordingly considered efficient and autogenic canopy species return was found to be prominent after a period of 35-50 years of restoration treatment.
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- Date Issued: 2011