Ecological and physiological studies of Clematis vitaba L.
Clematis vitalba L. is a deciduous perennial climber native to central Europe and southern England which has become a major invasive weed species in New Zealand native forest remnants. In forest remnants, C. vitalba tends to establish in forest gaps, forest margins and recently disturbed areas. A field distribution survey and, field and laboratory based studies of germination, growth, nitrogen (N) nutrition and light-level acclimation were conducted with the overall objective of determining reasons for the success C. vitalba in New Zealand forest remnants. The field site was at Dennistoun Bush, a native forest remnant located at Peel Forest, central Canterbury, New Zealand. Based on a distribution survey, a field based growth experiment and the investigation of naturally established C. vitalba (Chapter 2), it is shown that low light level is the primary factor associated with the limited establishment of C. vitalba in undisturbed forest and, that increased N availability following disturbance may be important in the successful establishment at high light levels. Chapter 3 sets out to determine if the pattern of C. vitalba distribution in the field can be related to the influence of environmental factors on germination. It is shown that moist chilling, applied N (as NO₃⁻ or NH₄⁺) and light can reduce seed dormancy when applied at levels that are likely to occur in the field. It is concluded that the potential for germination is unlikely to be an important factor limiting the establishment of C. vitalba in undisturbed forest. It is proposed that the interaction of chilling, N and light, combined with sporadic seed release are important in the rapid establishment of C. vitalba in disturbed sites and therefore the success of C. vitalba in forest gaps and margins. A model for the mechanism of dormancy reduction in C. vitalba seed is proposed. Chapter 4 investigates the influence of N on growth and N assimilation. The response of C. vitalba to increased applied N is compared to the response of barley (Hordeum vulgare). It is shown that the growth, N assimilation and N storage characteristics displayed by C. vitalba are consistent with plant species adapted to environments with high NO₃⁻ availability and, consistent with the establishment of C. vitalba in recently disturbed sites. This is further substantiated by the unusual characteristic where the potential for NO₃⁻ assimilation (nitrate reductase activity) is increased under NH₄⁺ nutrition. The stimulation of nitrate reductase activity (NRA) by NH₄⁺ is discussed in depth. It is suggested that the leaf dominated NO₃⁻ assimilation and the seasonal pattern of N remobilisation and storage displayed by C. vitalba may be important factors regulating canopy development and longevity. In Chapter 5, the low light acclimation potential of C. vitalba is determined. It is concluded that low light level is the most likely factor limiting C. vitalba establishment in undisturbed forest. It is shown that C. vitalba does not survive at 1% full sunlight, but can survive at 3% full sunlight and achieve substantial growth at light levels in the range 10% to 100% full sunlight. It is proposed that the light acclimation potential of C. vitalba is an important factor in the encroachment of C. vitalba into forest remnants. The relative importance of light acclimation characteristics are assessed. The factors considered to be important in the low-light acclimation of C. vitalba are; the ability to increase specific leaf area (SLA) while maintaining light absorption per unit leaf area; the ability to reduce photosynthetic capacity (measured as soluble protein and RUBISCO activity) relative to potential light capture and; the ability to reduce leaf shading. The factors considered to be important in the high-light acclimation are; an increase in leaf thickness and; the ability to increase and/or maintain photosynthetic capacity regardless of N availability. Evidence is presented that suggests that the changes in dry matter partitioning that occur as a consequence of changes in light level are closely related to tissue N concentration. Chapter 6 investigates the carotenoid composition of C. vitalba. It is shown that light level, but not applied N level, influence carotenoid composition. The carotenoid composition of C. vitalba is compared to a range of plant species. It is concluded that C. vitalba has a large xanthophyll-cycle pigment pool that is consistent with high-light adapted species and, that C. vitalba has a relatively low and inflexible chlorophylla to chlorophyllb ratio (chlorophyll a:b ratio) and low β- carotene concentration that are consistent with low-light adapted species. It is proposed that this combination of high- and low-light characteristics and in particular, the large xanthophyllcycle pigment pool, are important in the establishment of C. vitalba in forest gaps and margins where a rapid spatial and temporal variation in light level can occur. The carotenoid composition of C. vitalba is discussed in relation to the role of carotenoids in higher plants in general. It is shown that the influence of light on the xanthophyll-cycle pigments and β-carotene is consistent with a role for these pigments in high-light protection, that the influence of light on neoxanthin is consistent with a role in light harvesting and, that the influence of light on lutein is more consistent with a role in high-light protection than light harvesting. In Chapter 7 an invasive strategy of C. vitalba into forest remnants is proposed. It is suggested that the primary factors that account for the success of C. vitalba as an invasive weed in New Zealand native forest remnants are the ability to establish rapidly from seed following disturbance combined with; features of both low- and high-light adapted species; the ability to acclimate to a wide range of light levels regardless of N availability and; the potential for rapid growth in soils with moderate to high N availability.... [Show full abstract]