The invasion ecology of Acacia elata (A. Cunn. Ex Benth.) with implications for the management of ornamental wattles

Donaldson, Jason Ernest (2013-12)

Thesis (MSc)--Stellenbosch University, 2013.

ENGLISH ABSTRACT: This thesis explores how human dictated methods of introduction and species-specific traits interact to define spatial patterns in invasive plant populations using Acacia elata as a model species. I initially asked whether the relatively small invasive extent (when compared to congeners introduced for forestry or dune stabilization) of a species used widely for ornamental purposes (A. elata) is due to low rates of reproduction in South Africa. Results indicate that A. elata has similar traits to other invasive Australia Acacia species: annual seed input into the leaf litter was high (up to 5000 seeds m-2); large seedbanks develop (>20 000 seeds m-2) in established stands; seed germinability is very high (>90%); seeds accumulate mostly in the top soil layers but can infiltrate to depths of 40cm; and seed germination appears to be stimulated by fire. I argue that the current limited distribution of invasive A. elata populations is not attributable to species-specific reproductive traits. Following on from this I addressed the relative importance of species traits, the recipient environment, and introduction dynamics using species distribution models and spread models defined using a range of parameters representative of invasive Australian acacia species. Results indicate that in the early stages of simulated invasions, the number of trees introduced was the most important parameter influencing abundance and extent, while the placement of introduction foci in urban areas associated with ornamental introductions limited the extent and abundance of invasive populations. I suggest that conditions relating to human-mediated introduction events initially mask the influence of intrinsic species traits and help to explain the failure and success of species associated with specific human-shaped pathways. Third, I used scale-area curves to determine how these human-dictated introduction conditions influenced the spatial structure of populations. The fractal dimension (Dij) of A. elata populations indicates plot scale (2.5-25m) spread with high densities that are increasing. Artefacts of introduction history were observed at the regional scale (2.5-25km) - populations are contiguous around introduction foci such as towns and roads. Moreover, fragmentation of A. elata populations at local to landscape scales (0.25-2.5km) is due to a combination of the haphazard regional placement and limited secondary dispersal vectors, both of which can be linked to their introduction history. In summary, this study used a combination of field-based data, modelling techniques and broad-scale sampling methods to assess how intrinsic species traits and introduction conditions mediate invasive spread. I conclude that introduction history can have a long-lasting (100+ years) influence on the spatial structure and distribution of invasive plants, which can mask the influence of individual species traits.

AFRIKAANSE OPSOMMING: Alhoewel menigte plant spesies deur mense ingevoer word, word slegs 'n klein hoeveelheid van plante indringers. Die beperkte aantal spesies wat in nuwe habitatte vestig en versprei, kan grotendeels toegeskryf word aan die omgewing en mensgemaakte hindernisse ten opsigte van oorlewing, voortplanting en verspreiding. In hierdie tesis gebruik ek Acacia elata as ʼn model spesie om te ondersoek hoe die invoeringsproses sowel as spesie spesifieke eienskappe die ruimtelike struktuur van indringende populasies bepaal. Aanvanklik was my vraag of die relatiewe klein indringingsarea van ʼn spesie wat ingevoer is vir ornamentele doeleindes (Acacia elata), teenoor die groot indringingsareas van Australiese Acacia-spesies wat ingevoer is vir bosbou of duin stabilisering, die gevolg is van lae voortplantings tempo in Suid-Afrika. Resultate dui daarop dat A. elata soortgelyke eienskappe as ander indringende Australië akasia spesies het: jaarlikse saad produksie is hoog (tot 5000 sade m - 2); groot ontwikkelde saadbanke (> 20 000 sade m - 2); hoë saad ontkieming (> 90 %); saad versamel in die boonste grondlae, alhoewel dit kan infiltreer tot 'n diepte van 40 cm; laastens, ontkieming word deur vuur gestimuleer. Hierdie resultate stel voor dat die huidige beperkte verspreiding van indringende A. elata populasies nie die gevolg is van spesie spesifieke voortplantings eienskappe nie. Gevolglik het ek 11 indringende Australiese akasia-spesies ondersoek om die relatiewe belangrikheid van spesie eienskappe, die area waar hul geplant is, asook invoerings aspekte te ondersoek. Resultate wys dat die aantal bome wat ingevoer is, die belangrikste eienskap was ten opsigte van digtheid en verspreiding van ʼn indringende spesie, terwyl invoering naby fokuspunte in stedelike gebiede weer spesie digtheid en verspreiding beperk. Hierdie resultate toon dat spesies eienskappe aanvanklik nie ʼn groot invloed het op hul sukses nie, maar dat hul sukses eerder afhang van waar hul geplant word. Derdens, gebruik ek skaal - area kurwes om te bepaal hoe die invoering van A. elata die ruimtelike struktuur van populasies beïnvloed het. Die fraktale dimensie (Dij) van A. elata bevolkings dui op verspreiding by die plaaslike vlak (2,5 - 25m), wat voorstel dat verspreiding nie op hierdie skaal beperk is nie. Eienskappe van die rede/metode van invoering was wel waargeneem op die groter skaal (2.5 - 25km) waar populasies digter was naby dorpe en paaie. Die intermediêre skaal (0,25 - 2.5km) dui daarop dat A. elata populasies huidiglik beperk word deur 'n kombinasie van lukrake plasing en beperkte sekondêre verspreiding, albei wat gekoppel kan word aan hul invoer geskiedenis. Hierdie studie maak dus gebruik van 'n kombinasie van veld gebaseerde data, modellerings tegnieke en breë skaal steekproefmetodes om te evalueer hoe algemene spesies eienskappe en invoer toestande verspreiding bepaal. Sodoende verskaf ek ʼn raamwerk om die invloed van invoer toestande op die ruimtelike struktuur en verspreiding van ornamentele plant indringers te verstaan.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/85808
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