Identifying priority areas for active restoration after alien plant clearing in the City of Cape Town

Mostert, Elana (2016-03)

Thesis (MSc)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Invasive alien plants (IAP) can have negative impacts on native ecosystems and in prolonged invasions, ecosystems can be transformed to a new alternative ecosystem state. Clearing IAP (“passive” restoration) does not always initiate native vegetation and ecosystem function recovery, therefore additional restoration measures (“active” restoration’) might be needed to set the ecosystem on the trajectory of recovery. Active restoration is more resource intensive compared to passive restoration. In some cases active restoration may be justified since, unsuccessful clearing may lead to the wasting of resources through re-invasion or secondary invasions. Restoring previously invaded or degraded vegetation can be motivated by using improved native biodiversity, ecosystem services or social benefits as incentives. To ensure the effective and efficient allocation of limited IAP control resources, some form of restoration prioritization is required. The aim of this study was thus to develop a framework to identify areas in need of active restoration and to prioritize areas for active restoration. The framework was illustrated in an urban setting by using Cape Town as a case study. In the first part of my thesis I developed two frameworks. Firstly, a framework was developed to identify areas that may need active restoration. Results of this framework are illustrated in a map indicating areas that would likely need active restoration. A second framework was developed to prioritize areas for active restoration, with a map as an outcome, indicating priority areas for active restoration. Both frameworks were built using an approach called Multi-Criteria Analysis, which is a method to construct a goal, combine stakeholder opinions and facilitate spatial restoration planning. Frameworks consisted of different criteria and sub-criteria to identify and prioritize areas for active restoration such as the extent and density of invasion, invasive species’ ecosystem impacts and conservation status of vegetation types. Criteria and sub-criteria were scored in terms of their relative importance relating to effects on vegetation recovery post-alien clearing and prioritizing areas. The framework is simple to implement and to illustrate findings and can be applied spatially and updated if new information becomes available. It can also be applied at different scales and to different ecosystems around the world; the importance of some criteria might be altered according to the ecosystem dynamics. In the second part of my thesis I conducted a field study investigating the impacts of invasions by two different types of invaders: pines and acacias; and compared their impacts on two different highly threatened lowland fynbos vegetation types. This study was also used to test the main assumptions made for the framework to identify areas for active restoration, developed in the first part of the thesis. Vegetation structure, composition and richness, and abiotic variables such as soil characteristics and litter biomass were used as criteria to determine whether ecosystems have been able to recover to a similar level than an uninvaded reference site post-clearing. Acacias changed abiotic and biotic variables after two cycles of invasion (and after one cycle of invasion in some cases) while lowland fynbos is resilient up to three rotations of pine planting. Pine-invaded areas generally had higher restoration potential than acacia-invaded areas. In terms of vegetation structure, perennial species and guild richness: acacias more negatively impacted invaded sites, whereas pine plantations recovered better in comparison to the reference site. Follow-up clearing generally promoted better ecosystem recovery in terms of overall species richness and structure but care should be taken not to damage indigenous shrubs. In conclusion, this study addressed two important aspects currently lacking in restoration, firstly by providing a framework for identifying and prioritizing areas for active restoration, to be used specifically in spatial IAP management. The two frameworks consider the multiple aspects involved in restoration, namely: biodiversity, ecosystem functioning and services, social and political aspects. Secondly, it is also a multi-species approach, considering the main woody transformers in the frameworks, testing the framework, and providing restoration recommendations for the two main lowland invaders: Pinus radiata and Acacia saligna. The overall outcomes of this study will serve as a tool for the City of Cape Town and land managers to improve active restoration efforts.

AFRIKAANSE OPSOMMING: Uitheemse indringerplante kan ’n negatiewe impak op inheemse ekosisteme hê, en langdurige indringing kan ekosisteme tot ’n nuwe, alternatiewe toestand transformeer. Die verwydering van uitheemse indringers (“passiewe” herstel) is nie altyd genoeg om die herstel van inheemse plantegroei en ekosisteemfunksionering teweeg te bring nie. Bykomende maatreëls (“aktiewe” herstel) kan nodig wees om die ekosisteem weer op die pad na herstel te plaas. Aktiewe herstel is meer hulpbronintensief as passiewe herstel. In sommige gevalle is aktiewe herstel egter geregverdig omdat onsuksesvolle verwydering van uitheemse indringers hulpbronne kan verkwis indien dit bloot tot hernude of sekondêre indringing lei. Verbeterde inheemse biodiversiteit, doeltreffende ekosisteemdienste of maatskaplike voordele kan as aansporing dien vir die herstel van plantegroei wat voorheen aan indringing of degradasie blootgestel was. Om te verseker dat die beperkte hulpbronne vir die beheer van uitheemse indringers doeltreffend en doelmatig toegewys word, word ’n vorm van prioritisering vereis. Die doel van hierdie studie was dus om ’n raamwerk te ontwikkel om gebiede waar aktiewe herstel nodig is uit te wys en te prioritiseer. Kaapstad dien as ’n gevallestudie om die toepassing van die raamwerk in ’n stedelike omgewing te demonstreer. In die eerste deel van my tesis ontwikkel ek twee raamwerke. Eerstens word ’n raamwerk ontwikkel om gebiede uit te wys wat dalk aktiewe herstel vereis. Die resultate van hierdie raamwerk word voorgestel op ’n kaart wat dié gebiede aandui. ’n Tweede raamwerk word ontwikkel om gebiede vir aktiewe herstel te prioritiseer. Weereens word die prioriteitsgebiede op ’n kaart aangedui. Albei raamwerke word met behulp van ’n benadering genaamd Multikriteriaontleding ontwikkel. Dié benadering word gebruik om ’n doel vas te stel, die menings van belanghebbendes te kombineer en ruimtelike herstelbeplanning te fasiliteer. Die raamwerke gebruik verskillende kriteria en subkriteria om gebiede vir aktiewe herstel uit te wys en te prioritiseer, soos die omvang en digtheid van indringing, indringerspesies se impak op die ekosisteem, en die bewaringstatus van plantsoorte. Tellings word aan die kriteria en subkriteria toegeken op grond van hulle relatiewe belang vir plantegroeiherstel na die verwydering van indringers, sowel as vir gebiedsprioritisering. Die raamwerke is eenvoudig om te implementeer, en bevindinge word maklik geïllustreer. Dit kan ruimtelik toegepas en bygewerk word namate nuwe data beskikbaar kom. Boonop kan dit op verskillende skale en verskillende ekosisteme oor die hele wêreld toegepas word; die belang van sekere kriteria kan bloot aangepas word na gelang van die ekosisteemdinamiek. In die tweede deel van my tesis onderneem ek ’n veldstudie om die indringingsimpak van twee soorte indringerplante, naamlik denne en akasias, te ondersoek. Die impak van dié twee spesies op twee hoogs bedreigde plantsoorte in die laaglandfynbosgroep word ook vergelyk. Die veldstudie word voorts gebruik vir die toetsing van die hoofaannames vir die raamwerke wat in die eerste deel van die tesis ontwikkel is. Plantegroeistruktuur, -samestelling en -rykheid sowel as abiotiese veranderlikes soos grondeienskappe en dooieplantbiomassa word gebruik as kriteria om vas te stel of ekosisteme ná die verwydering van indringers tot op dieselfde vlak kon herstel as ’n verwysingsterrein waar geen indringing plaasgevind het nie. Met akasias het die abiotiese en biotiese veranderlikes ná twee indringingsiklusse (selfs na een indringersiklus in sommige gevalle) verander, terwyl laaglandfynbos tot drie rotasies denne-aanplanting kon weerstaan. Gebiede met denne-indringing beskik oor die algemeen oor sterker herstelpotensiaal as dié met akasia-indringing. Wat plantegroeistruktuur, die voorkoms van meerjarige plante en rykheid aan funsionele groepe betref, het akasias ’n groter negatiewe impak op indringingsgebiede gehad, terwyl denneplantasies beter herstel het in vergelyking met die verwysingsterrein. Opvolgverwydering van indringerplante het oor die algemeen beter ekosisteemherstel bevorder wat spesierykheid en -struktuur betref, maar daar moet versigtig te werk gegaan word om nie inheemse struike te beskadig nie. Die navorsing vir hierdie tesis vul twee belangrike leemtes in huidige herstelaksies. Eerstens word raamwerke voorsien om gebiede vir aktiewe herstel uit te wys en te prioritiseer, wat bepaald vir die ruimtelike bestuur van uitheemse indringerplante gebruik kan word. Die twee raamwerke neem die veelvuldige aspekte van herstel in ag, naamlik biodiversiteit, ekosisteemfunksionering en -dienste, sowel as maatskaplike en politieke aspekte. Tweedens bied die navorsing ’n multispesiebenadering wat die vernaamste houtagtige transformatorspesies in die raamwerke bestudeer, die raamwerke toets, en dan aanbevelings doen oor herstel ná indringing deur die vernaamste twee laagland-indringers, Pinus radiata en Acacia saligna. Die algehele uitkomste van die studie dien as ’n instrument vir die Stad Kaapstad en grondbestuurders om aktiewe herstelpogings te verbeter.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98857
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