Restoration of a degraded subtropical forest for climate change mitigation and adaptation in the city of Durban, South Africa.

Abstract

With unprecedented changes in climate and land-use patterns, a decrease in global biodiversity and ecosystem services has been occurring at an alarming rate. This has resulted in a widespread damage to the life-support systems upon which every living organism depends on. Reforestation of degraded forest ecosystems is now globally recognized as one of the best natural capital investment options, owing its contribution to biodiversity conservation, climate change mitigation and adaptation, and ecosystem services provision. The aim of this study was (1) to unravel confusions caused by the inconsistent use of terminologies describing different reforestation initiatives; (2) to investigate motivations behind recent reforestation initiatives; (3) to demonstrate the use of a restoration decision-making tool, Robust offsetting (RobOff); (4) to investigate the influence of climatic and edaphic factors on reforestation initiative, (5) to assess reforestation initiative success, and (6) to assess the impact of drought on reforestation initiative. A comprehensive review was conducted to unravel the confusion caused by the inconsistent use terminologies describing different reforestation initiatives, and to gain insight into motivations behind reforestation initiatives in recent literature (2000 to 2016). The results showed that there are 10 most common terminologies used to describe different reforestation initiatives. These terminologies were categorized into five groups based on their motivations, namely, (1) Creation or Fabrication, Reallocation and Replacement, (2) Ecological engineering, (3) Ecological restoration, (4) Reclamation, Reconstruction, Remediation, Renewal or Redemption, and (5) Rehabilitation. The recent reforestation initiatives were motivated by the need to reinstate resilient and more functional forest ecosystems (through planting of a higher diversity of native tree species). This is because species diverse forests are more resilient and functional with significant contributions to biodiversity conservation (fauna and flora), climate change mitigation (carbon storage) and adaptation (e.g., flood control) and ecosystem services that sustain society (e.g., food) and economy (e.g., employment opportunities). Using the Buffelsdraai Landfill Site Community Reforestation Project (BLSCRP) as a case study, RobOff, was employed to plan a complex large-scale reforestation. The complexity was caused by a mosaic of habitats (‗extant forest‘ and ‗former sugarcane fields‘) with varying levels of degradation, diverse reforestation actions (natural regeneration, current action, carbon action and biodiversity action), a limited budget and multiple goals (biodiversity, carbon stock and employment). RobOff results showed that investing in the restoration of ‗former sugarcane fields‘ through biodiversity action is preferable, because it achieved the highest biodiversity, carbon stock and employment opportunities. Field trials were conducted at the Buffelsdraai Landfill Site to assess the influence of microtopographic positions, and soil physical and chemical properties on the growth performance of the four most dominant planted native tree species (Bridelia micrantha, Erythrina lysistemon, Millettia grandis and Vachellia natalitia). Root-collar diameter, stem height and canopy width growth rates were assessed across the chronosequence of three habitats under restoration (0-, 3-, and 5-year-old), in the upland (dry) and lowland (wet) areas of each habitat. Erythrina lysistemon and V. natalitia were found to be good fast growing tree species suitable for restoration in both the upland and lowland areas, while B. micrantha was suitable for lowland area. Reforestation success of the BLSCRP was assessed using measures of plant richness, diversity, vegetation structure, invasive alien plants (IAPs) and ecological processes, contrasted across a chronosequence of habitats under restoration (0-year-old, 3-year-old and 5-year-old) and compared with a reference forest habitat (natural forest). The BLSCRP was largely successful, but low tree density and an increase in IAP cover with an increase in restoration age were identified as threats to the BLSCRP success. The 2015 El Niño event induced serendipitous drought occurrence in South Africa led to the assessment of its effect on planted tree sapling mortality and on the growth performance of the four most dominant planted tree species in the 0-year-old habitat. Drought effected mortality was highest in the lowland area (34.1%) and lower in the upland area (18.9%). Mortality rate of the nine most abundant species ranged from 10% to 52.5%. Erythrina lysistemon and V. natalitia had good growth rates in both the upland and lowland areas, and B. micrantha in the lowland area. The BLSCRP is highly likely to achieve its climate change mitigation and adaptation, biodiversity and ecosystem services restoration and employment creation in the city of Durban, provided the identified threats are addressed as soon as possible. The overall findings from this study showed that future large-scale reforestation initiatives around the globe should be designed to achieve biodiversity conservation, climate change mitigation and adaptation, and ecosystem services supply.