Barriers to ecosystem restoration after clearing invasive Acacia species in the South African fynbos : soil legacy effects, secondary invaders and weedy native species

Nsikani, Mlungele Mlungisi (2018-12)

Thesis (PhD)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: A significant proportion of the world’s ecosystems are invaded by alien N2-fixing woody species such as Australian acacias. Invasive alien N2-fixing woody species often transform ecosystems through their negative impacts on soil chemistry, seed banks and microbial communities, and native plant diversity. Management interventions such as clearing are necessary to reduce these negative impacts. It is often assumed that clearing the invasive species will lead to a dissipation of their impacts and native plant diversity recovery. However, this is often not the case because the invasive species’ negative impacts can become persistent soil legacy effects and present barriers to restoration of viable native plant communities. Understanding barriers to restoration can lead to improved restoration outcomes. Using Acacia saligna invasions in the South African fynbos as case study, this thesis explored soil legacy effects, secondary invasion and weedy native species dominance after clearing invasive acacias. In chapter one, I reviewed global literature to understand how soil legacy effects of invasive alien N2-fixing woody species present barriers to restoration, and identify management actions that could potentially be used to address them. In chapter two, I investigated how long soil legacy effects of invasive A. saligna persist after clearing using soil sample analyses. In chapter three, I explored the effect of invasive A. saligna’s soil chemical and biotic legacies, and weedy native species on native species re-establishment using a greenhouse experiment. In chapter four, I identified species that are secondary invaders after clearing invasive A. saligna across several sites and investigated the effects of vegetation type and fire application on their establishment over three years after clearing using vegetation monitoring. In chapter five, I investigated interactions between secondary invaders and the extent to which soil nitrate levels, apparent after clearing invasive A. saligna, influence secondary invasion and weedy native species dominance using growth chamber and greenhouse experiments. I found that altered soil microbial communities, depleted native soil seed banks, elevated N status, secondary invasion and weedy native species dominance, and reinvasion can be barriers to restoration. Furthermore, management actions such as carbon addition, soil microbial treatments, herbicide or graminicide application and native species reintroduction can be used to address these barriers to restoration. Acacia saligna’s soil chemical legacies persisted up to ten years after clearing. However, they did not have direct negative consequences on the re-establishment of native proteoid shrubs but soil biotic legacies and presence of weedy native species reduced their growth. Secondary invasion was not habitat specific, was promoted by fire application and persisted up to three years after clearing at levels similar to or higher than the first year. Lastly, growth of secondary invaders and weedy native species increased with an increase in soil nitrate levels. I conclude that practicing restoration ecologists should manage soil legacy effects, secondary invaders and weedy native species after clearing invasive A. saligna to improve restoration outcomes.

AFRKAANSE OPSOMMING: 'n Merkbare deel van die wêreld se ekosisteme word deur uitheemse houtagtige stikstofbindende plant spesies soos die Australiese Akasias ingedring. Hierdie indringers transformeer dikwels ekosisteme deur hul negatiewe impak op grondchemie, saadbanke, mikrobiese gemeenskappe en inheemse biodiversiteit. Om hierdie negatiewe impakte te bestry, is bestuurspraktyke vir die afkap van die uitheemse indringers nodig. Daar is ‘n aanname dat die afkap van hierdie indringerspesies sal lei tot 'n einde aan hul impak en dat die inheemse biodiversiteit sal herstel. Dit is egter dikwels nie die geval nie, aangesien die negatiewe impak van die indringerspesies langdurige of permanente veranderinge in die grond kan nalaat en bied hindernisse vir die herstel en restorasie van inheemse plantgemeenskappe. Om hierdie veranderings en hindernisse te verstaan, kan lei tot verbeterde veldrestorasie resultate. Acacia saligna indringing in die Suid-Afrikaanse fynbos word as gevallestudie gebruik in hierdie tesis om die effek te ondersoek van die veranderde nalating van N2 in die grond, sekondêre indringers en die dominansie van onkruidagtige inheemse spesies. In hoofstuk een het ek die globale literatuur nagegaan om te verstaan hoe grond-nalatenskapseffekte van stikstofbindende uitheemse indringerplante hindernisse tot veldherstel kan bied. Daar is ook bestuursaksies ge-identifiseer wat moontlik gebruik kan word om dit aan te spreek. In hoofstuk twee het ek deur ontleding van grondmonsters ondersoek hoe lank die stikstof grondnalatingseffekte van A. saligna voortduur na die skoonmaak van die indringers. In hoofstuk drie het ek met behulp van ‘n kweekhuis-eksperiment, gekyk na die effek van A. saligna se nalatenskap op grondchemikalieë en grondbiotiese verwantskappe, en na die effek van onkruidagtige inheemse spesies op die hervestiging van inheemse spesies. In hoofstuk vier het ek sekondêre indringerspesies geïdentifiseer nadat A. saligna in verskeie areas skoongemaak is. Daar is na die effek van plantegroei tiepe, vuur-gebruik en die tydperk na die oorspronkike skoonmaak van indringers gekyk. Data is oor ‘n drie-jaar tydperk ingesamel deur moniteringsplotte. In hoofstuk vyf het ek gebruik gemaak van groeikamer en kweekhuis eksperimente om ondersoek in te stel na interaksies tussen sekondêre indringerspesies. Ek het ook die mate waarin grondnitraatvlakke, wat na die verwydering van A. saligna voorkom, sekondêre indringing en onkruidagtige inheemse dominansie beïnvloed, ondersoek. Veranderde grondmikrobiese gemeenskappe, uitgeputte inheemse saadbank, verhoogde stikstof, sekondêre indringing en dominante onkruidagtige inheemsespesies, en her-investasie deur A. saligna, is potensiële hindernisse vir veldherstel. Bestuurstegnieke wat potensieel hierdie hindernisse kan oorbrug, sluit in koolstof byvoeging, grondmikrobiese behandelings, onkruiddoder- of grasdodertoediening en die hervestiging van inheemse spesies. Acacia saligna se grondnalatingseffekte bly teenwoordig tot tien jaar na die oorspronklikke skoonmaak van die indringer. Acacia saligna se chemiese verandering van grond het nie noodwendig direkte negatiewe gevolge vir die hervestiging van natuurlike proteoïd struike nie, maar die nalatenskap van veranderde grondmikrobiese gemeenskappe en die teenwoordigheid van onkruidagtige inheemse spesies kan negatiewe impakte hê op hul groei. Sekondêre indringing blyk nie habitatspesifiek te wees nie, maar word bevorder deur brand na die oorspronklike skoonmaak. En dit kan tot drie jaar voort duur op vlakke soortgelyk of hoër as die eerste jaar. Die groei van sekondêre indringers en onkruidagtige inheemse spesies neem toe met 'n toename in grondnitraatvlakke. Ek kom tot die gevolgtrekking dat praktiserende restorasie-ekoloë grondnalatingseffekte, sekondêre indringing en onkruidagtige inheemsespesies moet bestuur nadat die indringer A. saligna skoongemaak is om veldherstel se uitkomste te verbeter.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/104929
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