• 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:

On prairie sites in Michigan with low to moderate spotted knapweed density and sufficient fine fuels to carry a fire, annual spring broadcast burning under severe conditions (when humidity and dead fine fuel moisture are as low as possible) serves to reduce spotted knapweed populations and increase the competitiveness of the native prairie vegetation. On some sites, 3 years of this regimen reduced spotted knapweed to the point where it could be controlled by hand-pulling individuals and increasing the fire return interval to 3-5 years. In dense infestations (>60 rosettes/m²), broadcast burning is ineffective due to lack of adequate fuel to carry the fire. In this case, spotted knapweed plants can be killed with repeated spot- burning (using a propane torch) of individuals and resprouts 3 to 4 times during the growing season until root reserves are depleted. Top-killing individuals allows seedlings to emerge in the time between burning treatments, so they are also killed with subsequent burning, thus depleting the seed bank. This treatment does not seem to harm existing prairie natives, but newly-germinating natives may be at risk, and seeding after the last burning may help them to recover [122].

In Douglas-fir-ponderosa pine communities in western Montana, spotted knapweed cover increased after spring prescribed fires with both high and low fuel consumption following shelterwood cuts. Fuel moisture and duff characteristics of the fires were as follows [58]:

Changes in spotted knapweed cover during the 4 years following treatments are as follows [8]:

Spotted knapweed was present on these sites before treatments were carried out, probably as a result of soil scarification during logging activities, as well as big game and livestock use of the site prior to these studies. Also, the harvesting done as a part of this study resulted in scarification of about 11% of the area, which evidently encouraged spotted knapweed establishment. Burning, especially high consumption, seems to further encourage the spread of spotted knapweed. Severe wildfire (as might occur if fuels are allowed to accumulate) would also probably favor expansion of knapweed [8].

Spotted knapweed was reported on a Douglas-fir forest site 5 to 10 years after wildfire in western Montana, though it was not recorded 3 years after the fire. No information on prefire vegetation was given [201].

After fall prescribed burning at a western Montana site in a Douglas-fir/ninebark habitat type dominated by snowbush ceanothus (Ceanothus velutinus), spotted knapweed was not present in the 1st postfire year, but its volume doubled 2 years following the burn compared with preburn volume [134]. Characteristics of that fire are as follows:

After a spring burn of lesser intensity on an adjacent site, no spotted knapweed was recorded before or after burning. Shrubs recovered faster and grasses increased on the spring burn, while herbaceous cover recovered faster and nongraminoid, herbaceous species increased on the fall burn [134].

FIRE MANAGEMENT CONSIDERATIONS:

It has also been suggested that burning prior to herbicide application can increase the efficacy of the herbicide and stimulate new growth from competitive species [176]. A study in western Montana, where dry conditions following burning limited spring germination of knapweed prior to the herbicide application, did not support this assertion [21], while results from Washington state supported the efficacy of burning prior to herbicide treatment [172]. A more recent project was begun in the fall of 1996 on the Lolo National Forest in western Montana, in which herbicide treatments were coupled with prescribed burning to improve habitat for elk. Prescribed burning in April 1997, followed by aerial application of picloram in June 1997 resulted in a 95% decrease in weed biomass (primarily spotted knapweed with small amounts of common mullein (Verbascum thapsis) and leafy spurge (Euphorbia esula)), an 86% decrease in native forb biomass, and a 714% increase in grass biomass as of July 1998 [66]. The area was retreated in 1999 with a lower concentration of picloram that was successful in killing spotted knapweed plants missed in 1997 and in killing the young knapweed that had sprouted after the original treatments. The end results to date suggest that the treatments were successful at reducing spotted knapweed cover and increasing native grass cover, although native forb density decreased and compositional changes were not recorded [94].

Rice [157] suggests controlling weeds (including spotted knapweed) with herbicides before reintroducing fire for the restoration of native plant communities, to avoid increasing the weed problem by using fire alone. A study is underway in western Montana comparing different combinations and timing of prescribed burning and herbicide applications [158].

Prescribed burning of spotted knapweed can be difficult, especially if no fine grass fuels are present, because fire does not usually carry through spotted knapweed stems easily. Grass fuel models work poorly for spotted knapweed unless associated grasses exceed 40-50% cover [235]. Spotted knapweed fuel loading varies between sites, thus affecting fire behavior. Intense fires in spotted knapweed have been observed under some conditions, and prescribed burning in spotted knapweed for fire hazard reduction may be a consideration [234].

A fuel model for spotted knapweed and guidelines for prescribed burning are available [234,235]. Calculation of fuel load is based on knapweed plant height and percentage of ground cover (old, standing plants and new plants), and litter depth and cover (including sparse grasses) [234]. The model was developed for early spring burns, and is valid only under specific fine fuel loading conditions. It is offered as a guideline to help the user select environmental conditions (wind, dead fuel moisture) that allow safe and effective burns. Fire managers familiar with fire modelling can create their own site-specific models for knapweed infestations using this general knapweed fuel model as it is summarized by Xanthopoulos [234,235].

When prescribed burning in spotted knapweed for fire hazard reduction in spring, sustaining a fire that carries without interruption and without risk of escape generally requires a predicted flame length of at least 20 cm (using a fuel model that does not include standing knapweed), and a controllable flame length less than 120 cm (as predicted using a complete fuel model). Fire behavior problems caused by discontinuous and nonuniform fuels are similar to the ones found in big sagebrush. Use these models with caution, since they have been verified with few actual test burns in spotted knapweed [234].

The following seasonal moisture data for spotted knapweed were collected on a south-facing slope in the Missoula Valley, Montana [234]:

The USDA Forest Service's "Guide to Noxious Weed Prevention Practices" [210] provides several fire management considerations for weed prevention in general that apply to spotted knapweed. To prevent infestation, re-establish vegetation on bare ground as soon as possible using either natural recovery or artificial techniques as appropriate to site objectives. When reseeding after wildfires and prescribed burns use only certified weed-free seed. Monitor the burn site and associated disturbed areas after the fire and the following spring for emergence of spotted knapweed, and treat to eradicate any emergent spotted knapweed 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.

Additionally, when planning a prescribed burn, preinventory the project area and evaluate cover and phenology of any spotted knapweed present on or adjacent to the site, and avoid ignition and burning in areas at high risk for spotted knapweed establishment or spread due to fire effects. Avoid creating soil conditions that promote weed germination and establishment. Discuss weed status and risks in burn rehabilitation plans. Also, wildfire managers might consider including weed prevention education and providing weed identification aids during fire training; avoiding known weed infestations when locating fire lines; monitoring camps, staging areas, helibases, etc., to be sure they are kept weed free; taking care that equipment is weed free; incorporating weed prevention into fire rehabilitation plans; and acquiring restoration funding. Additional guidelines and specific recommendations and requirements are available [210].

Related categories for SPECIES: Centaurea stoebe ssp. micranthos | Spotted Knapweed