Clematis vitalba in a New Zealand native forest remnant: does seed germination explain distribution?

The pattern of distribution of the invasive vine Clematis vitalba in a typical New Zealand native forest remnant and the influence of chilling duration (chilling), nitrate (NO3-), and light on germination of C vitalba were investigated. In the forest remnant, C. vitalba was not established in undisturbed forest where light levels were less than or equal to 3% full sunlight and rarely established where light levels were less than or equal to 5%. Clematis vitalba was, however, established in forest gaps and margins where light levels exceeded 5%, particularly when these areas were associated with recent soil disturbance. Under controlled environment conditions, germination in the absence of chilling, NO3-, or light was low (3%). When applied alone, chilling and NO3- increased germination. Light, however, only increased germination when applied with one or both other factors. In the presence of light, germination increased up to c. 80% with increased chilling (0-12 weeks), and increased up to c. 50% with increased applied NO3- in the range 0.1-2.5 mol/m(3) but then changed little with additional NO3- up to 50.0 mol/m(3). In general, germination in the presence of two or three factors was greater than when either factor was applied alone. In the field, seeds remaining on vines over winter retained a high degree of dormancy and viability. We suggest that sporadic release of this seed effectively produces a seed bank. By comparing the levels of chilling, NO3-, and light that can be expected in the field with the levels that influence germination under controlled environment conditions, and considering a seed bank, we argue that C vitalba is likely to rapidly establish following soil disturbance. We suggest that rapid establishment may account for the distribution of C vitalba in recently disturbed, high light sites and, to some extent, the success of C. vitalba in New Zealand native forest remnants.