Summary: | 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.
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