Seed and microsite limitation in Clematis vitalba on Banks Peninsula.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Clematis vitalba, introduced to New Zealand from Europe in the early 1900s, is an invasive weed which poses a high risk to native forest remnants, particularly in the central North Island and upper and central South Island. Its dispersal techniques and rate of spread make it a particular challenge for managers attempting to control its range expansion. Is the population abundance and distribution of C. vitalba on Banks Peninsula seed or microsite limited? I tested the presence of seed limitation in three populations of C. vitalba at two sites by measuring seed dispersal and checking for the presence of a soil seed bank. At a location where C. vitalba is absent due to control efforts, I investigated the presence of a long-term seed bank and long-distance dispersal. I also tested the degree of microsite limitation in C. vitalba at the two sites by surveying the natural distribution of seedlings, monitoring seedling emergence after seed addition and measuring the survival and growth of planted seedlings. Soil samples taken from the Port Hills and Akaroa sites contained low amounts of previous-season C. vitalba seed (an average of 0.0125 seeds per m² in the forest and 0.0025 per m² in the open) and cofirmed the presence of a small short-term soil seed bank. Seed rain was greater at Akaroa (3912 seeds caught) than on the Port Hills (1507 seeds caught), which is also reflected in a larger seed bank at Akaroa. The distribution of natural seedlings and growth of planted seedlings suggests that the optimum habitat for this species varies among different life stages. At the site cleared of C. vitalba more than eight years ago, there were no seeds in the seed bank but new seeds arrived by long-distance dispersal. Overall, these results suggest that microsite limitation has a greater role near the parent plant and seed limitation becomes more pronounced at increasing distance from the seed source. Clematis vitalba populations continue to spread from ongoing long-distance dispersal and short-distance dispersal with lateral expansion of established populations; established populations are more likely to be limited by the availability of microsites while expanding populations will be seed limited at the edge of the population where plant density is low.