• IMMEDIATE FIRE EFFECT ON PLANT
• DISCUSSION AND QUALIFICATION OF FIRE EFFECT
• PLANT RESPONSE TO FIRE
• DISCUSSION AND QUALIFICATION OF PLANT RESPONSE
• FIRE MANAGEMENT CONSIDERATIONS

IMMEDIATE FIRE EFFECT ON PLANT:

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:

PLANT RESPONSE TO FIRE:

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:

Response of established Canada thistle plants to fire is unclear, as there are mixed reports in the literature. A Canada thistle clone in a mid-boreal wetland site was not noticeably changed when burned in the spring with a propane torch to simulate both light and deep burns [98]. The authors concluded that there is a moderate to high probability that Canada thistle and other Eurasian xerophytic species will dominate these wet-meadows in the short term after fire, and that they will continue to dominate small areas for longer periods [100]. There were no significant differences (p<0.05) in Canada thistle cover after spring burning in the prairie pothole region of Iowa [146]. In Mesa Verde National Park, Colorado, populations of Canada thistle that were well established before an August wildfire resprouted immediately after the burn, and spread downstream in the canyons. Canada thistle and other non-native species (e.g., cheatgrass (Bromus tectorum) and musk thistle (Carduus nutans)) continued to dominate the severely burned areas and expanded their area by 260% 6 years after the wildfire [64,67]. In a native mixed-grass prairie in North Dakota, late-spring and late-summer burning increased seed production and seedling numbers in Canada thistle, but fewer thistles were observed during the years following the burn than before or during the year of the burn [202]. Dormant season (winter and early spring) burning in eastern Oregon resulted in fewer total and fewer functional flowerheads on reproductive shoots of Canada thistle when compared to unburned control. Also, Canada thistle plants on burned sites grew more slowly and associated vegetation was more productive than on control sites. It was concluded that burning reduced the relative abundance of Canada thistle and may be useful as a means of halting its invasion or spread by maintaining a productive stand of native vegetation [253]. The discrepancy in these reports is probably due to the large number of variables that can affect the response of Canada thistle to fire, including fire severity, for which we lack a standard nomenclature in the literature. Other important variables include vegetation and site characteristics, frequency, and season of burning.

Site differences such as soil moisture content, plant community, and slope aspect can influence fire severity and may influence the response of Canada thistle to fire. In a northwestern Minnesota prairie site, prescribed burning on a nearly level mesic site in badly disturbed prairie had no effect on Canada thistle flowering while flowering was inhibited on a level, wet-mesic site in badly disturbed prairie after burning [171]. On a forested site in western Montana that was harvested and burned, Canada thistle seems to have increased with both light and severe burning in the fall, with larger increases on south aspects compared with others [123]. Olson [163] provided evidence that prescribed burning in the spring either reduced or did not change canopy cover of Canada thistle in Minnesota. Results differed between sites, which differed primarily in plant community type and in time and frequency of burning. 

Frequency, severity and season of burning may have a considerable effect on Canada thistle response. In a study conducted on a mesic tallgrass prairie site in Colorado, plots that were burned frequently (5 times over 7 years) had lower density of Canada thistle than did and area that was burned only twice during the same period. Results were inconclusive, however, since the final season of the study saw increased spread of Canada thistle from the surrounding area, probably due to clonal growth from existing plants [152]. In a prairie site at Pipestone National Monument, Minnesota, 5 years of annual spring burning in mid- to late April, with fires of low to moderate severity, reduced the frequency of Canada thistle over time until it was absent after the 5th year [15]. Similarly, observations in tallgrass prairie sites in South Dakota indicate that late spring prescribed burning (when native species are still dormant) on a 4 to 5 year rotation (as per the historic fire regime) encourages the growth of native plants and discourages the growth of Canada, bull and musk thistles. Livestock use must be carefully timed following burning, since grazing early in the growing season can potentially negate beneficial effects of prescribed fire [44]. On a common reed marsh in Manitoba, Canada thistle response to burning varied with season of burn. Aboveground biomass, stem density, and seedling density were unchanged on spring burns, but increased on both summer and fall burns [214]. Results are presented below:

FIRE MANAGEMENT CONSIDERATIONS:

Research in this report suggests that response of Canada thistle to fire is variable and it depends on vegetation and site characteristics, as well as frequency, severity and season of burning. Prescribed burns may be effective at stimulating growth of native species and thereby discouraging the growth of invasives such as Canada thistle [183], and may be best if timed to emulate the natural fire regime of a site [44]. Hutchison [105] states that prescribed burning is a "preferred treatment" for the control of Canada thistle, and that late spring burns effectively discourage this species, whereas early spring burns can increase sprouting and reproduction. During the first 3 years of control efforts, he recommends that burns be conducted annually [105]. Season of burn is an important consideration for prescribed burning, as the timing of the burn will determine species composition and cover in the post-fire community [101,102]. Dormant season burning may be a preferred treatment method in some areas, because in many habitats it stimulates growth of native vegetation that subsequently competes with Canada thistle [162,253]. However, dormant season burning may not be as effective as late spring burning [105]. Controlled studies comparing the effects of these variables in different natural areas are currently lacking in the literature. 

Equations for estimating fuel loading of forb communities including Canada thistle are available [27].

The USDA Forest Service's "Guide to Noxious Weed Prevention Practices" [225] provides several fire management considerations for weed prevention in general that apply to Canada thistle. To prevent invasion after wildfires and prescribed burns, re-establish vegetation on bare ground as soon as possible using either natural recovery or artificial techniques as appropriate to site objectives. When reseeding burn areas, use only certified weed-free seed. Monitor burn sites and associated disturbed areas after the fire and the following spring for emergence of Canada thistle, and treat to eradicate any emergent Canada thistle plants. Regulate human, pack animal, and livestock entry into burned areas at risk for weed invasion until desirable site vegetation has recovered sufficiently to resist weed invasion.

Related categories for SPECIES: Cirsium arvense | Canada Thistle