Lisa Scott. 1998. Weed Invasion in Smith, I.M., and G.G.E. Scudder, eds. Assessment of species diversity in the Montane Cordillera Ecozone. Burlington: Ecological Monitoring and Assessment Network, 1998.

WEED INVASION

Lisa Scott

SYSTEMATIC REVIEW OF PLANT FAMILIES

The current assessment focussed on seven plant families (Table 1). These families were chosen by the author based on their regional or provincial significance, and/or their aggressive and successful invasion of natural habitats in the Montane Cordillera Ecozone. The earliest records for the species discussed in this report were based primarily on information gathered from the Royal BC Museum herbarium and the University of British Columbia herbarium. The first known collections in BC for most of these invasive plants were between the end of the nineteenth century and soon after the turn of the century. Collections were mainly from coastal areas where non-native plants presumably arrived on ships. Vancouver Island is the terminus of a transcontinental railway, which is likely a vector for the early spread of many species (J. Pinder-Moss, Royal BC Museum, pers. comm., 1997).

The Sunflower Family, one of the largest families of plants in the world, is undoubtedly the largest family of weeds occurring within the Montane Cordillera Ecozone. Representatives are found throughout the Montane Cordillera, but are particularly prevalent in south-central BC. They occupy a wide range of habitats, from moist valley bottoms to dry grasslands and open forests of the Bunchgrass, Ponderosa Pine, and Interior Douglas-fir biogeoclimatic zones (Powell et al. 1994). Some local infestations occur in the Interior Cedar-Hemlock, Montane Spruce and Engelmann Spruce-Subalpine Fir zones (Powell et al. 1994). Members of this diverse family generally establish in

areas of disturbance such as cultivated fields and pastures, along roadsides and irrigation ditches, and frequently in the vicinity of gravel pits, although some species can displace native vegetation in undisturbed areas (Myers and Berube 1983; Powell et al. 1994; Taylor 1995). The knapweeds (Centaurea) are one of the most well-known and ecologically harmful groups in the Asteraceae family. The aggressiveness of diffuse and spotted knapweed (Centaurea diffusa Lam. and C. maculosa Lam.) in BC’s southern interior has caused severe loss of production, environmental quality and aesthetics on thousands of hectares of urban, agriculture, forest and recreation lands. However, during the last decade, attention has shifted from the knapweeds to species of even greater concern within the Figwort, Spurge and Rose families.

The Figwort family includes two extremely aggressive non-native species: Linaria genistifolia ssp. dalmatica (L.) Maire and Petitmengin and Linaria genistifolia ssp. dalmatica Mill. These species are found in south-central BC, the Kootenays, and in the southwestern corner of Alberta. They occur along roadsides and in waste areas, gardens, cultivated fields, open grassland and transitional forest-grassland, and are primarily associated with the Bunchgrass, Ponderosa Pine and Interior Douglas-fir zones (Powell et al. 1994).

Leafy spurge (Euphorbia esula L. ), a non-native member of the large and diverse Spurge family, is regarded as a serious threat to rangelands, croplands and pastures in the United States (Callihan et al. undated). In Canada, populations are concentrated in the southern Prairie provinces and southwestern Ontario (White et al. 1993). Within the Montane Cordillera, it occurs in isolated pockets in south-central BC. Spurge’s wide variety of ecological tolerances has facilitated the successful invasion of grassland and open forests of the Bunchgrass, Ponderosa Pine and Interior Douglas-fir zones (Powell et al. 1994).

The Rose family includes one of the most invasive species found within the Montane Cordillera: sulphur cinquefoil (Potentilla recta L.). Sulphur cinquefoil is currently considered to be the weed of greatest concern in the south Okanagan (Scott 1997a) because of its rapid spread over large geographic areas and its wide ecological amplitude (Rice et al. 1991; Rice 1993; Powell 1996). This long-lived perennial infests disturbed areas, meadows, pastures and rangelands and can dominate a site within two to three years of first appearance. In the Montane Cordillera, sulphur cinquefoil is most common is southern BC); it has been documented in the Bunchgrass, Ponderosa Pine, Interior Douglas-fir and Interior-Cedar Hemlock zones (Powell 1996). Reports indicate that sulphur cinquefoil can successfully displace diffuse and spotted knapweed, and other aggressive species (Rice 1993; Powell 1996; C. Hinkley, Tonasket Forest Service, pers. comm., 1997). Although further research is required to determine it’s potential distribution in the Montane Cordillera, it is assumed that sulphur cinquefoil is contiguous in suitable habitats within the Bunchgrass, Ponderosa Pine and Interior Douglas-fir biogeoclimatic zones (Powell 1996).

Non-native representatives of the Borage Family are found primarily in the Ponderosa Pine, Interior Douglas-fir, Montane Spruce and Engelmann Spruce-Subalpine Fir zones in the southern interior of BC (Upadhyaya and Cranston 1991; Powell et al. 1994). Borages are commonly associated with soil disturbance along roadsides and pastures, and logged forest habitats. They tend to prefer dry habitats, although they also occur in riparian areas disturbed by livestock.

Although all of BC’s noxious weeds are terrestrial species, non-native wetland and aquatic species pose a very serious threat to the natural waterways of the Montane Cordillera. Purple loosestrife (Lythrum salicaria L.) for example, a member of the Loosestrife family, is a herbaceous perennial that rapidly invades wetlands, meadows and pastures where it displaces existing vegetation. Loosestrife is well-established and widespread in southern BC and is capable of establishing in the riparian areas of most mid- to low-elevation biogeoclimatic zones in the southern interior. Eurasian watermilfoil (Myriophyllum spicatum L.), an aggressive aquatic species belonging to the watermilfoil family, has rapidly taken over many lake shorelines and slow-moving rivers in southern BC (BC Ministry of Environment, Water Investigations Branch 1977). This submersed perennial herb reproduces primarily by vegetative fragmentation, and rapidly displaces virtually all other aquatic macrophytes (BC Department of Environment, Water Resources Service 1976).