Exotic plant species in the mixedwood section of the southern boreal forest
Sumners, Wade H.
The objective of this study was to examine the distribution of exotic plants and determine the potential threats in the mixedwood section of the boreal forest. The invasion of exotic plants into natural areas is a growing concern among ecologists. Exotic species have no previous exposure to the invaded area and have been introduced, either intentionally or accidentally, by humans. These plants have the potential to suppress surrounding vegetation and acquire the majority of available resources. This dominance alters important ecosystem functions and negatively affects ecosystem structure and composition. This study examined three types of land use (roadside right-of-way maintenance, timber harvesting and wildfire) to identify the density, frequency and cover of exotic plants within the mixedwood forest. There were also separate categories of the time since disturbance (re-current, recent and mature) for each disturbance type. Data were collected in the summer field seasons of 2000 and 2001 in and nearby the Prince Albert Model Forest (approximately 70 km north of Prince Albert, Saskatchewan). Surveying was completed in mature forest, harvested and wildfire areas using 10 x 10 m quadrats. These quadrats were adjacent to or remote from roadside right-of-ways that were deliberately seeded with exotic species. Seeding the right-of-ways with exotic species occurred along principal and secondary highways. Surveying was also conducted within roadside right-of-ways using 1 x 1 m quadrats. These quadrats were adjacent to the recently disturbed and mature quadrats surveyed in the previous year. Each plant species observed in the quadrats had a cover value assigned, while stem counts were also conducted for exotic species. A total of 23 exotic species were observed within the quadrats. The exotic herb species belong to the Gramineae (9 species), Leguminosae (7), Compositae (5) families with one species each in Plantaginaceae and Boraginaceae. No exotic trees or shrubs were observed within the study sites. Areas that were recently disturbed either by timber harvesting or wildfire had 6 different exotic species with an average density of 0.2 ± 0.1 stems/m2 and a frequency of 72 %. Similar exotic frequencies and species in both recently harvested and burned survey sites suggests that these disturbances have a comparable affect on exotic distributions. Exotic species capable of wind dispersal had the highest frequencies in the recently disturbed survey sites. Common dandelion (Taraxacum officinale) had a frequency of 57 %, perennial sow thistle (Sonchus arvensis) was observed in 38 % of the sites and annual hawksbeard (Crepis tectorum) was at 25 %. Mature forest had a lower population of exotic plants, with only 2 exotic species observed. Taraxacum officinale and Canada bluegrass (Poa compressa) were observed in 13 % of the mature quadrats with an average density of 0.002 ± 0.002 stems/m2. The right-of-way quadrats contained the highest amount of exotics with 22 observed species. The average density of exotic species in roadside right-of-ways was 117 ± 22 stems/m2 with 94 % of the quadrats containing at least one exotic plant. The deliberate introduction, frequency of disturbance and the physical environment of roadside right-of-ways appear to influence the distribution of exotic plant species. The most frequently observed exotic species in the right-of-way areas were Taraxacum officinale (observed in 73 % of the quadrats at 8 stems/m2) followed by alsike clover (Trifolium hybridum at 45 % and 17 stems/m2), Sonchus arvensis (43 % and 4 stems/m2), creeping red fescue (Festuca rubra at 36 % and 31 stems/m2) and smooth brome grass (Bromus inermis at 31 % and 17 stems/m2). These species are either common in urban areas, agricultural weeds or have been deliberately seeded into right-of-way areas. The distribution of exotic species suggests that land use contributes to increased densities and frequencies of exotic plants. Each exotic species observed was ranked according to a system developed by Hiebert and Stubbendieck (1993). The ranking system was used to determine the current and potential threat of exotic plant species to become detrimental to ecosystem structure, composition and function. The ranking identified 14 species that were a lesser threat and easy to control, 8 species that were a lesser threat and hard to control, and one species, Bromus inermis, that was ranked as a serious threat and hard to control. Additional monitoring is required as the species observed in this study may be exhibiting a “lag phase” of population expansion, which typically precedes an exponential increase. Other species (scentless chamomile (Matricaria perforata) and caragana (Caragana arborescens)) that were not observed in the study area, but are known to occur within the region, are also a concern with respect to future exotic species invasions. Anticipated climatic changes are also expected to increase the distribution of exotic species as changes to environmental attributes will produce a longer growing season and increased plant growth and productivity.
DegreeMaster of Science (M.Sc.)
SupervisorArchibold, O. W. (Bill)
CommitteeWright, Robert A.; Thorpe, Jeff; Thomas, A. Gordon; de Boer, Dirk H.
Copyright DateMarch 2005