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

Thesis (PhD)--Stellenbosch University, 2012.

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.

AFRIKAANSE OPSOMMING: Naas habitatverlies word indringer spesies as die grootste bedreiging vir biodiversiteit beskou. Die indringing van riviersisteme wêreldwyd deur uitheemse plante dra by tot groot veranderinge in die biodiversiteit en ekosisteem funksie. In Suid-Afrika, veral in die Fynbos Bioom, is rivieroewers en -beddings van die landskappe wat die meeste ingedring word, meestal deur skadelike indringers soos bome en struike van Australiese genera soos bv. Acacia en Eucalyptus. Alhoewel grootskaalse bestuursoperasies besig is om die indringers te verwyder en ekosisteme te herstel, is min bekend omtrent die geleenthede en beperkinge vir die herstel van inheemse spesies na die verwydering van indringers. Die hoofdoel van hierdie tesis is om die nut te bepaal van die sleutel faktore van twee wyd aangehaalde restorasie modelle (suksessie en alternatiewe-toestand modelle) om die effektiewe bestuur van hewig ingedringde oewers te lei. Die Berg Rivier in die Wes Kaap, Suid-Afrika, is gebruik as studie area. Die Berg Rivier is hewig geimpakteer deur indringers, veral deur Eucalyptus camaldulensis. Die doel was om nuwe geleenthede vir restorasie in rivier areas te voorsien, deur die studies oor beperkinge vir restorasie en geleenthede vir inheemse spesie herstel te verbind. Hierdie tesis begin deur die beperkinge van restorasie na indringing te ondersoek, veral allelopatie wat een van die faktore is wat die impakte van Eucalyptus indringing verhoog en die herstel van natuurlike plantegroei na verwydering van indringer inhibeer. Verder bepaal ek die geleenthede vir beide passiewe (gebaseer op die suksessie model) en aktiewe restorasie (gebaseer op die alternatiewe-toestand model) wat volg op verskillende strategieë van verwydering van indringer bome. Die doel is om die effektiwiteit van die verskillende modelle vir volhoubare, doel georiënteerde bestuur te bepaal. Laastens het ek die grond verwante eienskappe ondersoek naamlik, water terugdrywing, grondvog en infiltrasie wat voordeel trek uit indringer verwydering en die daaropvolgende herstel van inheemse plantegroei. Resultate van allelopatie as ʼn restorasie beperking het getoon dat die teenwoordigheid van E. camaldulensis langs die Berg Rivier die herstel van inheemse spesies negatief beïnvloed. Eucalyptus camaldulensis is allelopaties en gee aanleiding tot grondwater terugdrywing. Ek beveel aan die verwydering van E. camaldulensis vanuit rivier sisteme omdat dit die potensiaal het om grond na nie-allelopatiese en nie-terugdrywende toestand te herstel wat die weg kan baan vir die herstel van inheemse plante groei. Die herstel van inheemse plantegroei het gemengde resultate gewys. Restorasie gebaseer op die suksessie model was oor die algemeen meer doelmatig, teenoor restorasie gebaseer op die idee van ʼn alternatiewe-toestand model, hoofsaaklik as gevolg van verskeie beperkinge wat aktiewe restorasie in die gesig staar. Inheemse spesie herstel was suksesvol op beide die totaal indringer verwyderde en uitgedunde areas, wat vier jaar vantevore behandel is. Dekking van inheemse bome en struike was hoër in beide heeltemal skoongemaakte en uitgedunde areas wanneer die vergelyk word met ingedringde areas. Dit dui daarop dat beide metodes inheemse plantegroei herstel promoveer en die ekosisteem op ʼn baan na herstel plaas. Om herstel deur uitdunning te verbeter stel ek ʼn vier-stadium proses voor om bestuurders te lei vir goeie herstel van sleutel inheemse spesies. Verskeie uitdagings geassosieer met aktiewe restorasie wat volg op val-en-stapel brand en val-en-verwyder is geobserveer in areas wat ʼn jaar van te vore behandel is. Ontkieming van aangeplante inheemse spesies se sade was laag in beide die val-en-verwyder en die val-en-stapel brand areas. Sekondêre indringing van uitheemse kruie en graminoiede, droë somers toestande en lae saad ontkieming hinder die vroeë inheemse spesie vestiging en herstel. Dus, vir aktiewe restorasie om sy doel te bereik moet effektiewe werwing en verspreidings strategieë in plek wees. Daar was geen werwing van inheemse spesies in die areas wat nie gesaai was nie. Dit kan toegeskryf word in die dominansie van uitheemse kruie spesies and graminoiede en die uitputting van inheemse spesies in die grond saadbank. Vermindering van water terugdrywing van grond ná verwydering van indringer bome is belangrik aangesien dit die potensiaal het om die sukses van inheemse plantegroei herstel te affekteer. Die areas waar inheemse plantegroei goed herstel het, het grondwater terugdrywing gevarieer van gemiddeld afstootlik in die uitgedunde areas na nie-afstootlik in die heeltemal skoongemaakte areas. Dus, suksesvolle inheemse spesie herstel het die potensiaal om die grondverwante ekosisteem funksies te verbeter, wat moontlik sal bydra tot die herstel van inheemse plantegroei. Ek kom tot die gevolgtrekking dat die indringer boom E. camaldulensis die inheemse rivier ekosisteem negatief affekteer en dat strategieë om hierdie spesie te verwyder nodig is. Herstel van inheemse plantegroei samestelling, struktuur en ekosisteem funksie hang af van die graad van ekosisteem verval en die oorblywende ekosisteem weerstandigheid. Behalwe die verwyderings en effektiewe restorerings doelwitte, moet restorasie pogings ook realistiese oplossings vir die oorkombaarheid van verskeie uitdagings en beperkinge ontwikkel voor enige restorasie plan geïmplementeer kan word. Suksesvolle herstel van rivier ekosisteme het die potensiaal vir verhoogde rivier vloei en mag moontlik lei tot ʼn verhoogde beskikbaarheid van water vir landbou, ontspanning, natuurbewaring en vir huishoudelike gebruik, en kan dus ʼn beduidende bydrae kan lewer tot water sekuriteit in Suid Afrika. Beide die suksessie model en die alternatiewe-toestand model beklemtoon die noodsaaklikheid om restorasie beperkinge te identifiseer. Hierdie studie identifiseer allelopatie as ʼn belangrike beperking tot restorasie en maak aanbevelings om dit aan te spreek en om restorasie te fasiliteer. Herstel gebaseer op die suksessie model was meer effektief as herstel gebaseer op die alternatiewe-toestand model wat verskeie beperkings in die gesig staar. Die alternatiewe-toestand modelle inkorporeer sisteemdrumpels en terugvoer wat moontlik kan verduidelik waarom gedegradeerde sisteme herstel uitdagings getoon het en weerstandig teenoor restorasie gebly het.

Doctoral