Tree health in near pristine, heavily invaded and restored riparian zones : the role of pests and fungal pathogens

Maoela, Malebajoa Anicia (2015-03)

Thesis (PhDConsEcol)--Stellenbosch University, 2015.

Thesis

ENGLISH ABSTRACT: Riparian zones represent an essential component of landscape biodiversity and are ecologically and socio-economically significant habitats. The riparian zones in South Africa are however threatened by numerous invasive alien plants (IAPs). These are dominated by several fast-growing Australian Acacias that are responsible for widespread replacement of native species. The impacts of IAPs on native ecosystems are widely acknowledged and have justified extensive eradication programmes such as the Working for Water (WfW) programme. To date, the WfW programme has only been partially successful in the eradication of IAPs in riparian zones in the short-term. However, studies that assess the success of this initiative seldom focus on the health of riparian tree communities. This under-representation of riparian tree health studies is largely because of a lack of knowledge of the diversity and abundance of pests and disease-causing agents associated with native plants in these systems. The aim of this study was to examine the role that folivorous insects and fungal pathogens play in tree health of riparian tree communities that differ in invasion treatments. The predominant two native riparian tree species Brabejum stellatifolium and Metrosideros angustifolia and one alien tree species Acacia mearnsii were selected. Three perennial river systems within the South-Western Cape Province were selected as study areas. Within study areas, nine sites of three invasion treatments were chosen: three near pristine; three heavily invaded (predominantly Acacia mearnsii) and three restored sites (seven years after clearing of A. mearnsii). The following were done to accomplish the main aim: (i) foliage-active arthropod communities were collected from selected tree species and compared in terms of alpha- and beta-diversity within and among sites with different invasion treatments, (ii) the levels of damage caused by leaf pathogens and folivorous insects on focal tree species were determined and compared among sites (iii) leaf damage patterns were studied in context of native plant diversity and host abundance, (iv) the influence of changes in predatory arthropod numbers and community composition, as well as changes in total leaf nitrogen and phosphorus levels on the numbers of free-feeding folivorous insects (sap-sucking and leaf-chewing), in relation to the level of invasion were investigated, and (v) physiological parameters and nutrients in Brabejum stellatifolium leaves with varying levels of fungal infection and folivore damage were investigated. Heavily invaded sites had low arthropod species richness and abundance as compared to restored and near pristine sites, which were more similar. Beta-diversity was also influenced by invasion treatment. However few significant differences in arthropod community composition could be detected between restored and near pristine sites, and this pattern was similar for all tree species and arthropod taxonomic groups. Arthropod richness and community composition are therefore potential indicators of ecosystem health after restoration. Comparison of sites within each invasion treatment revealed that rivers have distinct signatures in terms of their arthropod communities. A study of different damage levels caused by leaf pathogens and folivorous insects showed that on native trees, folivore and fungal pathogen damage was significantly higher at restored and heavily invaded sites than at near pristine sites. For A. mearnsii, damage caused by both folivores and pathogens increased at heavily invaded sites, but returned to near pristine site levels at restored sites. Differences in native plant diversity did not explain these patterns, as restored sites had similar diversity levels to the near pristine sites. Surprisingly, as native host abundance increased, respective damage caused by fungal pathogens and folivores decreased. For A. mearnsii, increased host abundance was significantly correlated to increased folivore and pathogen damage. Insects and diseases not only reduced leaf area but also affected balances of physiological processes, thereby possibly delaying their recovery in a restoration scenario. Photosynthetic rate and other physiological parameters generally decreased with increasing damage severity caused by weevils and fungal pathogens in the canopy leaves of mature trees. Folivorous insect densities associated with tree species showed different responses to changes in predator densities and plant nutritional status. Brabejum stellatifolium and Acacia mearnsii had highest leaf nitrogen levels at restored and heavily invaded sites respectively, as compared to near pristine sites. Increased folivore abundance was only significantly and positively correlated to levels of leaf nitrogen content for B. stellatifolium and A. mearnsii. The results suggest that leaf nitrogen content is important for folivore food plant selection on B. stellatifolium and A. mearnsii, but not on M. angustifolia. There was no relationship between leaf damage and nitrogen levels or altered arthropod communities on M. angustifolia Therefore, individual plant species should be treated as separate entities when assessing plant health in a restoration scenario. Combined, these results indicate that; (i) native plant species and arthropod communities recover after alien plant clearing. Recovered arthropod assemblages may indicate the recovery of crucial processes like pollination, decomposition, and seed dispersal thereby strengthening the significance of conservation of arthropods from maintenance of biodiversity and ecosystem function and structure, (ii) the responses of sites to plant invasion treatment are site-specific, and therefore important to consider rivers individually when conceptualising activities of restoration, (iii) the role of fungal pathogens and folivorous insects in riparian ecosystems is more important than previously understood and there is a need for more research to provide better insights on why certain fungal pathogens and folivorous insects become major pests and diseases once the IAPs are cleared, (iv) folivorous insect responses to plant nutritional quality and predator densities is plant species-specific. Hence, individual plant species should be treated as separate entities when assessing plant health in a restoration scenario, and (v) insect feeding on vegetative parts of the plant and leaf fungal pathogens infections are damaging to the host plants. This may pose a serious threat to riparian tree communities already stressed by invasive alien plants, thereby delaying their recovery in a restoration scenario. Results of this study have potential to serve as valuable tools in riparian ecosystem management in South Africa.

AFRIKAANSE OPSOMMING: Oewergebiede verteenwoordig ‟n noodsaaklike komponent van landskapbiodiversiteit en is vanuit ‟n ekologiese en sosio-ekonomiese oogpunt belangrike habitats. Die oewergebiede van Suid-Afrika word egter deur verskeie indringer uitheemse plante (IUP). Hierdie word gedomineer deur verskeie vinnig groeiende Australiese akasias wat verantwoordelik is vir die wydverspreide vervanging van inheemse spesies. Die impak van IUP‟e op inheemse ekosisteme word allerweë erken en het as regverdiging gedien vir uitgebreide uitroeiingsprogramme soos dié van Werk vir Water (Working for Water (WfW)). Tot op hede was die WfW-program slegs gedeeltelik suksesvol in die uitroei van IUP‟e in oewergebiede in die kort termyn. Studies wat die sukses van hierdie inisiatief ondersoek, fokus egter selde op die gesondheid van oewerboom-gemeenskappe. Hierdie onderverteenwoordiging van oewerboom-gesondheid is grootliks as gevolg van ‟n gebrek aan kennis oor die diversiteit en volopheid van plae en siekteveroorsakende agente wat met inheemse plante in hierdie stelsels geassosieer word. Die doel van hierdie studie was om die rol van blaarvretende insekte en swampatogene in die boomgesondheid van oewerboom-gemeenskappe wat op grond van die behandeling van indringers verskil, te ondersoek. Die vernaamste twee inheemse boomspesies op oewers is Brabejum stellatifolium en Metrosideros angustifolia, en een uitheemse boomspesie, Acacia mearnsii, is gekies. Drie standhoudende rivierstelsels in die Suidwestelike Kaap Provinsie is as studiegebiede gekies. Binne die studiegebiede is nege liggings van drie verskillende behandelings van indringers gekies: drie feitlik ongerep; drie wat swaar ingedring is (hoofsaaklik Acacia mearnsii) en drie gerestoureerde liggings (sewe jaar na die verwydering van A. mearnsii). Die volgende is onderneem om die doel te bereik: (i) blaar-aktiewe geleedpotige gemeenskappe is uit geselekteerde boomspesies versamel en vergelyk in terme van alfa- en beta-diversiteit binne en tussen liggings met verskillende behandelings van indringers, (ii) die vlakke van skade wat deur blaarpatogene en blaarvretende insekte op die boomspesies waarop gefokus is, veroorsaak is, is bepaal en tussen liggings vergelyk, (iii) die patrone van blaarskade is in die konteks van inheemse plantdiversiteit en gasheervolopheid ondersoek, (iv) die invloed van veranderinge in die getal roofgeleedpotiges en gemeenskapsamestelling, asook veranderinge in die totale vlakke van stikstof en fosfor in die blare, op die getal vryvoedende blaarvretende insekte (die wat sap suig en wat blare kou) is ondersoek in verhouding tot die vlak van indringing, en (v) fisiologiese parameters en voedingstowwe in Brabejum stellatifolium blare met verskillende vlakke van swambesmetting en blaarskade is ondersoek. Swaar ingedringde liggings het lae rykheid en volopheid van geleedpotige spesies gehad in vergelyking met gerestoureerde en feitlik ongerepte liggings, wat meer eenders was. Beta-diversiteit is ook deur die behandeling van indringers beïnvloed. Minder beduidende verskille in geleedpotige gemeenskapsamestelling kon egter tussen die gerestoureerde en feitlik ongerepte liggings waargeneem word, en hierdie patroon was eenders vir al die boomspesies en taksonomiese groepe van geleedpotiges. Die rykheid en gemeenskapsamestelling van geleedpotiges is dus potensiële aanwysers van ekosisteemgesondheid ná restourasie. Die vergelyking van liggings binne elke verskillende behandeling van indringers het getoon dat riviere duidelike „handtekeninge‟ het in terme van hulle geleedpotige gemeenskappe. ‟n Studie van verskillende vlakke van skade wat deur blaarpatogene en blaarvretende insekte veroorsaak is, het getoon dat blaar- en patogeenskade op inheemse bome beduidend hoër was op die gerestoureerde en swaar ingedringde liggings as op die feitlike ongerepte liggings. Vir A. mearnsii het die skade wat deur beide blaarvreters en patogene veroorsaak is, noemenswaardig op die hoogs ingedringde liggings vermeerder, maar dit het omgekeer na feitlik ongerepte vlakke op die gerestoureerde liggings. Verskille in inheemse plantdiversiteit het nie hierdie patrone verklaar nie, aangesien gerestoureerde liggings soortgelyke diversiteitsvlakke gehad het as die feitlik ongerepte liggings. Wat verbasend was, was dat soos inheemse gasheer volopheid toegeneem het, die skade wat deur swampatogene en blaarvreters veroorsaak het, onderskeidelik verminder het. Vir A. mearnsii was ‟n toename in gasheervolopheid beduidend gekorreleer met ‟n toename in blaar- en patogeenskade. Insekte en siektes het nie net blaaroppervlak verminder nie, maar het ook die balanse van fisiologiese prosesse beïnvloed en het daardeur moontlik hul herstel in ‟n restourasie-scenario vertraag. Die tempo van fotosintese en ander fisiologiese parameters het oor die algemeen verminder met ‟n verhoging in die erns van die skade wat deur kalanders en swampatogene in die blaardak van volwasse bome veroorsaak is. Die digtheid van blaarvretende insekte wat met boomspesies verband hou, het verskillende reaksies op veranderinge in roofdierdigthede en plant voedingstatus getoon. Brabejum stellatifolium en Acacia mearnsii het die hoogste blaarstikstofvlakke gehad in die gerestoureerde en swaar ingedringde liggings onderskeidelik, in vergelyking met die liggings wat feitlik ongerep was. Verhoogde blaarvretervolopheid was slegs vir B. stellatifolium en A. mearnsii beduidend en positief gekorreleer met vlakke van stikstofinhoud in die blare. Die resultate stel voor dat blaar stikstofinhoud belangrik is vir blaarvretende voedselplantseleksie op B. stellatifolium en A. mearnsii, maar nie op M. angustifolia nie. Daar was geen verhouding tussen blaarskade en stikstofvlakke of veranderde geleedpotige gemeenskappe op M. angustifolia nie. Individuele plantspesies moet dus as aparte entiteite beskou word wanneer plantgesondheid in ‟n restourasie-scenario geassesseer word. Saam toon hierdie resultate dat: (i) inheemse plantspesies en geleedpotige gemeenskappe herstel ná uitheemse plante opgeruim is. Groepe herstelde geleedpotiges dui moontlik op die herstel van belangrike prosesse soos bestuiwing, ontbinding, en saadverspreiding, wat dus dui op hoe belangrik die bewaring van geleedpotiges is vir die behoud van biodiversiteit en ekosisteemfunksie en -struktuur, (ii) die reaksies van liggings op die behandeling van plant indringers is liggingspesifiek en dit is dus belangrik om riviere individueel te oorweeg wanneer restourasie-aktiwiteite konseptualiseer word, (iii) die rol van swampatogene en blaarvretende insekte in oewer-ekosisteme is belangriker as voorheen begryp en dit is nodig dat meer navorsing onderneem word om beter insigte te verkry in hoekom sekere swampatogene en blaarvretende insekte vername plae en siektes word wanneer IUP‟e opgeruim word, (iv) blaarvretende insekresponse op die voedingskwaliteit van plante en roofdierdigtheid is plantspesie spesifiek. Dus moet individuele plantspesies as afsonderlike entiteite behandel word wanneer plantgesondheid in ‟n restourasie-scenario geassesseer word, en (v) insekvoeding op vegetatiewe plantdele en blaar swampatogeniese infeksies beskadig die gasheerplante. Dit kan ‟n ernstige bedreiging inhou vir oewerboomgemeenskappe wat reeds onder stres is as gevolg van indringer uitheemse plante en wat dus hulle herstel in ‟n restourasie-scenario kan vertraag. Die resultate van hierdie studie het die potensiaal om as belangrike gereedskap in oewer-ekosisteembestuur in Suid-Afrika te dien.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/96648
This item appears in the following collections: