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

[摘要] 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.

[发布机构] Stellenbosch University