Coltsfoot at roadside leading to the GMNP administration
building, August 1998. Photo: Michael Rose
The conspicuous proliferation of the exotic weed coltsfoot (Tussilago farfara) in Gros Morne National Park (GMNP), on the west coast of insular Newfoundland, Canada, has occurred only since 1973 when the park opened to the public. It occurs nowhere else in Newfoundland in such densities except from south of the park to Channel-Port aux Basques, the location of the ferry terminal from Nova Scotia. Coltsfoot has not been reported elsewhere as a prolific weed in a natural area.
The invasion of coltsfoot in the park was examined to determine which resource changes accompanying disturbance enabled population expansion. Resource levels were measured in 17 disturbance types of natural and anthropogenic origin notably hiking trails, roads, gravel quarries, shorelines, slopes, hydro corridors and insect kills and across a gradient from disturbed to undisturbed in 12 vegetation types. Balsam fir forest comprises 36 percent of the park and has the highest number of disturbance types.
Disturbances favouring coltsfoot were characterized by a pH of 6.8-8.3, high light intensity, increased bare ground, absence of duff cover and moist, gravelly substrates. These resource levels were typical of both natural and anthropogenic disturbances in which the canopy and duff cover were absent, and the pH of acidic native soils had been raised by the addition of quarried limestone or granitic gravel. These represent resource shifts or amplifications relative to undisturbed vegetation types in which coltsfoot was absent.
The difference in resource levels across the disturbance gradient indicates that coltsfoot is unable to colonize undisturbed native vegetation. Likewise, a change in resource levels over time, which favours other species and is unsuitable for coltsfoot, appears to be the mechanism of coltsfoots recession.
Coltsfoot is subject to grazing by other species, indicating its success in GMNP is not entirely a function of its escape from Old World predators, as is often thought to be the case for other invasives.
Not all disturbance types present resource levels favourable for coltsfoot establishment. However, resource levels associated with some disturbance types of anthropogenic origin indicate that park activities have played an important role in the invasion of coltsfoot in GMNP. Gravel of neutral to basic pH provided a medium for rhizome dispersal during the road and trail construction phase of the park from the late 1970s to mid 1980s. Redistribution of gravel from the stockpiles where it had colonized, allowed coltsfoot, a calciphile limited by low pH, to obtain exponential population growth over a relatively short period of time.
Invasion represents a change in resources from historic levels that accompanies certain disturbance types. In chronic (sustained) disturbances associated with roadsides and streams, coltsfoot may persist indefinitely. However, if native vegetation communities are not subjected to disturbances that produce favourable resource levels for coltsfoot, the biodiversity of native vegetation and the fauna that depend upon it, are not threatened.