Functional Ecology of Iceplant Taxa and Their Role in Eco-restoration of New Zealand Sand Dunes
Biological invasion by non-native plant species has often been cited as a cause of native biodiversity loss. While the outcome of species invasions depends on interactions between exotic and resident native species, most studies of biological invasions have focused solely on the direct negative impacts of non-indigenous species on native biota. Although investigations of the role of competition in shaping natural plant communities were dominant in the previous generations and are still popular, more recent experimental research has uncovered the striking influence of facilitation on community dynamics. This thesis aims to investigate competitive and facilitative influence of the invasive South African iceplant (Carpobrotus edulis) on Spinifex sericeus, a native foredune grass species, with particular reference to implications of these interactions for dune restoration in New Zealand. It further explores the growth rates, substrate preferences and mating systems of the exotic and native iceplant taxa found in New Zealand. I begin by briefly outlining the influence of competition and facilitation on natural plant communities with reference to the role of facilitation in eco-restoration. I also give a few examples where exotic species have been found to facilitate native ones. Secondly, a neighbour removal experiment was conducted on coastal sand dunes with the main aim of studying the effects of Carpobrotus edulis on establishment of Spinifex sericeus at the foredune region. Finally, I compared the growth rates of the most widely distributed iceplant taxa in New Zealand in different substrates and the breeding systems of the exotic Carpobrotus. Examples abound in literature of exotic plant species facilitating native ones especially in forestry. In the neighbour removal study, Carpobrotus edulis protected Spinifex seedlings against storm erosion, sandblasting and salt sprays while at the same time suppressing its leaf production. Suppression of Spinifex leaf production was more pronounced at top of the dune where stress elements is presumably more benign. There was no evidence of allelopathic suppression of Spinifex by C. edulis. Only Carpobrotus chilensis displayed some level of substrate preference by putting on relatively lower biomass in gravel. The exotic Carpobrotus spp. put on greater dry matter content than the native Disphyma australe and the Carpobrotus-x-Disphyma hybrid. The hybrid displayed a faster vegetative growth rate whereas D. australe allocated relatively more biomass to the roots than the shoot. Both Carpobrotus spp. are self compatible and highly capable of intrageneric and intergeneric hybridisation. Mass removal of the existing exotic iceplant stands from foredunes along high energy coasts is not advisable as they serve as useful stabilisers. The intergeneric hybrid is sexually sterile with sparsely spread stolons that could allow co-occurrence with other species and therefore is more suitable for foredune stabilisation. However, more research needs to be conducted on the ecology of the intergeneric hybrid.