Opportunities and contraints in the restoration of riparian ecosystems invaded by alien trees : insights from the Western Cape, South Africa

[摘要] ENGLISH ABSTRACT: Invasive alien species are widely considered to be the second most significant threat to biodiversity globally following direct habitat destruction. The invasion of riparian systems worldwide by alien plants has contributed to profound changes in biodiversity and ecosystem functioning. In South Africa, river banks and river beds are amongst the most severely invaded landscapes, with the most damaging invaders, especially in the Fynbos Biome, being trees and shrubs of the Australian genera Acacia and Eucalyptus. Although large-scale management operations are underway to clear invasive trees and restore ecosystems, little is known regarding opportunities and constraints of native species recovery after alien clearing. The core aim of this thesis is to consider whether key aspects of two widely cited restoration models (successional and alternative-state models) are useful for guiding effective management of severely-invaded riparian vegetation. As a study system, I used the Berg River in the Western Cape, South Africa which is severely impacted by invasive trees, especially Eucalyptus camaldulensis. By linking the studies of constraints for restoration and opportunities for native species recovery, the aim was to provide new possibilities for restoration in riparian zones.The thesis starts by examining constraints to restoration following alien invasion, in particular allelopathy which is one of the factors that exacerbate the impacts of Eucalyptus invasion and inhibit recovery of natural vegetation after clearing. I further assess opportunities for both passive (based on the successional model) and active restoration (based on the alternative-state model) following different strategies for removing invasive trees. The aim is to determine the effectiveness of the different models for sustainable, goal-directed management. Finally, I investigate soil-related properties namely water repellency, soil moisture and infiltration that benefit from alien clearing and subsequent recovery of native vegetation.Work on allelopathy as a restoration constrain showed that the presence of E. camaldulensis along the Berg River negatively affects the recovery of native species. Eucalyptus camaldulensis is allelopathic and induces soil water repellency. I recommend the removal of E. camaldulensis from riparian systems as this has the potential to restore soils to a non-allelopathic and non-repellent state that can pave way for native vegetation recovery.Native vegetation recovery showed mixed results. Restoration based on the successional model was generally efficient, whereas restoration based on tenets of the alternative-state model was inefficient mainly due to the several constraints active restoration faced. Native species recovery was successful on both completely cleared and thinned sites that were treated four years ago. Cover of native trees and shrubs was higher in both completely cleared and thinned sites compared to invaded sites, indicating that both methods promote indigenous vegetation recovery and set the ecosystem on a trajectory towards recovery. To improve recovery through thinning, I propose a new four-stage process to guide management in ensuring good recovery of key native species.Numerous challenges associated with active restoration following fell & stack burning and fell & removal were observed on sites that were treated one year ago. Germination of introduced native species was low in both fell & removal and fell & stack burning sites. Secondary invasion of alien herbs and graminoids, dry summer conditions and low seed germination hindered early native species establishment and recovery. Therefore, for active restoration to achieve its goals, effective recruitment and propagation strategies need to be established. Recruitment of native species was non-existent in the sites that were not seeded; this is attributed to the dominance of alien herbaceous species and graminoids and the depletion of native species in the soil seed bank.Reduction of water repellency of soils after removal of the invasive trees is important as it has the potential to affect the success of native vegetation recovery. On sites where native vegetation was recovering well, soil water repellency ranged from moderately repellent in thinned sites to non-repellent in completely cleared sites. Therefore, successful native species recovery has the potential to improve soil-related ecosystem functions, which will possibly help towards restoring indigenous vegetation.I conclude that the invasive alien tree E. camaldulensis negatively affects the native riparian ecosystem and that strategies to remove the species are needed. Recovery of native vegetation composition, structure and ecosystem function depends on the degree of ecosystem degradation and remaining ecosystem resilience. Besides having clear and effective restoration goals, restoration efforts should also develop realistic solutions to overcome numerous challenges and constraints, before any restoration plan is implemented. Successfully restored riparian ecosystems have potential to increase river flow and may lead to increased availability of water to agriculture, recreation, conservation and for domestic use, resulting in significant water security in South Africa.Both the successional model and the alternative-state model emphasize the need to identify restoration constraints. This study identified allelopathy as an important constrain for restoration and recommends measures to address it so as to facilitate restoration. Recovery based on the successional model was more effective than recovery based on the alternative-state model, which faced several constraints. Models of alternative-states incorporate system thresholds and feedbacks that might explain why the degraded system faced recovery challenges and remained resilient to restoration.

[发布机构] Stellenbosch University