Invasion of woody species into weed infested areas : a thesis presented in partial fulfillment of the requirements for the degree of Masters of Science in Ecology at Massey University

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Massey University
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When studying plant dynamics and succession, it is important to determine potential limiting factors affecting recruitment (Crawley 1990). The purpose of this study was to investigate factors affecting the establishment and survival of woody species in weed infested areas around the central volcanic plateau. This was achieved by first describing these communities, and quantifying the number of native seedlings and saplings found in both forested and non-forested (weedy) areas. Seed input was measured with seed traps, and factors affecting recruitment of seedlings were investigated by manipulative field experiments. Although some native woody species were dispersed into weedy areas, both seed and seedling densities of most species declined rapidly with increasing distance from the forest margin. Sowing seeds at densities equivalent to 625 per m significantly increased seedling establishment of Griselinia littoralis and Coprosma 'taylorii' but not Pittosporum tenuifolium var. colensoi. Removal of exotic grasses (clearing treatment) that dominated in non-forest areas resulted in much greater establishment of all woody seedlings, including introduced broom (Cytisus scoparius) and several native species that had dispersed naturally. Most species also showed greater establishment in plots that were caged to prevent predation. However, the effects of clearing and caging treatments on survival of seedlings were not as apparent as they were for establishment. In addition, experimental clearing increased the growth of transplanted G. littoralis seedlings. Overall, most native species had much lower seedling establishment and survival in weedy areas compared with native forest. This is explained by a combination of both seed and microsite limitation in weedy areas. In another experiment designed to test the effects of bird consumption on seed germination, bird dispersers increased germination percentages of native species by removing fruits from seeds. All species examined (G. littoralis, Coprosma robusta, Pseudopanax crassifolius, and P. tenuifolium var. colensoi) showed very low germination of seeds within fruit, and much greater germination of seeds that were cleaned either by passage through birds or by hand. For C. robusta, G. littoralis, and P. tenuifolium var. colensoi, passage through birds also significantly increased germination of seeds compared with those cleaned by hand. The rate of germination was less affected by different treatments than the absolute percentage germinating, but was generally faster in bird-voided compared to hand-cleaned seed. Seeds in both of these latter treatments germinated considerably faster than seeds within fruit. Invasion of native woody plants in weedy areas appears to be constrained by a combination of low rates of seed dispersal for most species, and low probabilities of seedling establishment and survival in areas without disturbance. The most likely future scenario for the majority of weedy areas studied is continued dominance of exotic species in the short term, with slow succession to native shrubland as well-dispersed, frost-resistant native species such as manuka (Leptospermum scoparium) establish after disturbance. Management options are discussed with the aim of accelerating the rate of succession in weedy areas to native forest.
Tongariro National Park, Plant succession, Seed dispersal, Plant ecology, Weeds, Native plant species, National park vegetation