In California, C. vitalba has smothered over 1.5 acres of riparian oak-bay woodland at Muir Woods, where it forms large monocultures. The dense canopy suppresses all vegetation below by significantly reducing light availability (Wrubel, pers. comm.).

Restricts light availability for species under the canopy. Hill et al. (2001) report that, "Vines can climb the tallest forest trees, forming a dense, light-absorbing canopy that suppresses all vegetation beneath it. C. vitalba can be so vigorous that the weight of foliage and stems breaks the supporting trees, reducing once-healthy forest to a low, long-lived thicket of vines scrambling over stumps and logs". However Ogle et al. (2000) observe that the vines ascend to the canopy of forest but are unable to climb large diameter emergent trees unless shrubs and smaller trees provide a series of stepping stones to the crown of tall trees. Their study findings (study area Taihape reserve, New Zealand) indicate that the numbers and variety of understorey trees and shrubs that have been severely reduced following the infestation of C. vitalba correlates with observations of the growth habit of C. vitalba. Ogle et al. showed e.g. that not a single canopy tree species had been lost from the Taihape Reserves though 25% or so of the understorey trees and shrubs species had been lost.

GISD 2005
Hill et al. 2001
Ogle et al. 2000
Wrubel, E., Personal communication.

In California, C. vitalba has smothered over 1.5 acres of riparian oak-bay woodland at Muir Woods, where it forms large monocultures. The dense canopy suppresses all vegetation below and may impede movement of humans and animals (Wrubel, pers. comm.). In Santa Cruz County the infestation is 60 acres and there C. vitalba plants also grow over trees and may block movement (Hamey, pers. comm.). The changes caused to habitat structure by the growth of vines over trees and in thick masses on the forest floor would impede the movement of wildlife and change the diversity of plants in the understory.

Hamey, N., Personal communication.
Weber 2003
Wrubel, E., Personal communication.

This species may occasionally establish in undisturbed areas, but can readily establish in areas with natural disturbance. In Muir Woods, C. vitalba occasionally establishes in undisturbed forest and woodland, and frequently establishes in naturally disturbed riparian vegetation (Wrubel, pers. comm.). In Santa Cruz County C. vitalba is thought to have been introduced in the town of San Vicente which was located in the San Vicente Creek watershed. This historic town was established for workers at a nearby limestone quarry and is now uninhabited (Hamey, pers. comm.).

C. vitalba is able to establish in areas with natural or human-caused disturbance, but is not known to establish in undisturbed natural areas. C. vitalba requires high light for growth and reproduction, and is tolerant of moderate shade. In seedling germination studies done in forests, C. vitalba seedlings did not survive in undisturbed forests with low light levels. It was also found that nitrogen may be the limiting nutrient resource (Bungard et al. 1998). C. vitalba is a successful weed because it can tolerate dense shade but also has the ability to grow rapidly in high-light environments, meaning it can establish in areas with very small forest canopy gaps (Baars and Kelly 1996).

Baars and Kelly 1996
Bungard et al. 1997
Hamey, N. Personal communication.
Wrubel, E. Personal communication.

Once introduced, C. vitalba has the potential to spread rapidly into forest edges and canopy gaps. In Marin County it is reported as spreading rapidly and the population size has doubled since management began (Wrubel, pers. comm.). In Santa Cruz County C. vitalba was first detected in 2009 or 2010, and control work did not begin until 2012. The population is now estimated to be 60 acres and is increasing in density where it occurs (Hamey, pers. comm.). Given the recent detection of this species in California, and the fact that both known populations are under management, the question is scored as stable and will be changed if more information becomes available.

Hamey, N. Personal communication.
Wrubel, E. Personal communication.

Dumping of garden waste containing C. vitalba vines onto roadsides has been a significant cause of spread in New Zealand. This weed appears to invade along roadsides by seed, possibly aided by the turbulence created by motor vehicles (CABI).
Fragments are spread by water, and from garden cuttings (Washington 1999).

C. vitalba often grows on roadsides, and the risk of seeds being transported on road vehicles from known infestations to new sites within continents is high. In the past, ornamental Clematis species were often grafted onto C. vitalba rootstocks. It is likely that some incursions of C. vitalba have resulted from reversion of these rootstocks. The highest risk of introduction remains via intentional introduction of the plant as an ornamental, and plants and seed continue to be sold by nurseries, mail order catalogues and websites (CABI). In Santa Cruz County C. vitalba is thought to have been introduced in the town of San Vicente which was located in the San Vicente Creek watershed. This historic town was established for workers at a nearby limestone quarry and is now uninhabited (Hamey, pers. comm.). There is also a risk of inadvertent introduction and increase via seed collection and revegetation projects, since C. vitalba can be mistaken for native clematis species (Wrubel, pers. comm.).

CABI 2015
Hamey, N. Personal communication.
Washington Noxious Weed Control Board 1999
Wrubel, E. Personal communication.

C. vitalba is also naturalized and considered invasive in Oregon, Washington, Australia, New Zealand. C. vitalba is native to Europe, from southern England and the Netherlands to North Africa, and from Spain to the Middle East and the Caucasus (CABI).

It occurs in agricultural areas, coastland, natural forests, planted forests, range/grasslands, riparian zones, ruderal/disturbed, scrub/shrublands, and urban areas (GISD 2005). More specifically, outside of its native range, this species is found in forest lands and in the margins and openings of forested lands. It is also found in riparian areas established with willows, in waste areas, and in coastal and lowland areas (GISD 2005). Infestations of C. vitalba occur in forest reserves, gardens, road margins and other places around Taihape in New Zealand (Ogle et al. 2000), infestations of C. vitalba occur in every region of New Zealand except Northland (north of Auckland).

Some of the habitat types it has not invaded in California include scrub and shrub-dominated habitats, grasslands, and broad-leaved forests such as oak woodland.

CABI 2015
GISD 2005
Ogle et al. 2000

C. vitalba was first collected in California in 1957 from the Strybing Arboretum in Golden Gate Park. Since that is a cultivated location C. vitalba was not included in the Jepson Manual and was not thought to be naturalized in California until it was verified in Muir Woods and in Santa Cruz County in 2014 (CCH). In Santa Cruz County C. vitalba was first noticed in 2009 or 2010, and control work did not begin until 2012 (Hamey, pers. comm.). Both California locations are under management and occur in riparian and coast redwood forest edges. It is possible that C. vitalba is more widespread as it closely resembles the native C. ligusticifolia.

CCH 2016
Hamey, N. Personal communication
Hyland, T. Personal communication
Wrubel, E. Personal communication