Investigating the life cycle of Tropaeolum speciosum to improve future management
As the adverse effects of exotic plant species increase, more scientists are interested in gaining a better understanding of the processes that facilitate plant invasions. Comparative studies have found some evidence that certain traits increase the likelihood of plants becoming invasive in their introduced areas. The object of this study, Tropaeolum speciosum, is invasive in New Zealand and damages forest ecosystems by heavily smothering young trees. It is listed as an Unwanted Organism under the Biosecurity Act 1993, but the major problem for its management is the lack of suitable eradication methods. In this thesis the breeding system and seed germination was observed as well as growth characteristics of its shoots and rhizomes. Special attention was given on investigating the vine's responses to different light intensity. Although the flowers of T. speciosum are self-compatible, protandry and herkogamy inhibit self pollination. Cross-pollination is most likely dependent on birds as specialised pollinators. Seeds appear to require a complex combination of environmental conditions for germination, but seeds seem to be viable for a longer time. This could indicate the development of a seed bank. Rhizomes, which were observed in a rhizotron, displayed rapid growth of up to 8 cm averaged per week. T. speciosum showed the ability to expand clonally by growing new ramets from rhizomes and these shoots can stay connected with the established mother plant. This allows resource translocations, which can be advantageous in resource limited habitats. Rhizomes are important for overwintering and nutrient storage and allow quick regrowth of sprouts early in the growing season. Furthermore they increase the weeds persistence and resistance to disturbance as they can grow at least 30cm deep and are able to regrow from pieces as short as 5 cm. An effect from stratification on sprout growth from rhizomes was not observed. Although temperature had a positive influence on growth, new sprouts were initiated at temperatures as low as 4°C. Warm temperatures reduced the survival of rhizome pieces as they were rotting. The observed speed of shoot growth was also fast. In the field, where a wider rhizomatous network can be expected, some sprouts grew 30 cm per week. An experiment in growth chambers revealed that heavily shaded plants had greater shoot length growth and reduced leaf development compared to unshaded plants. This allows the species to actively forage for light. These life traits of Tropaeolum speciosum help to explain its success in New Zealand's forest and should be considered for future management strategies.
Degree Name: Master of Science
Degree Discipline: Botany
Publisher: University of Otago
Research Type: Thesis
2 v. :ill. (some col.), maps ; 30 cm. Includes bibliographical references. Thesis typescript. University of Otago department: Botany. "May 2007."