• 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:

Broom snakeweed is a fire-intolerant species that is severely harmed or, more often, killed by fire [45,78]. Broom snakeweed may be completely removed from an area immediately after fire [74]. In some instances, portions of plants survive despite serious damage [5].

Broom snakeweed is highly combustible when ignited [65]. In the spring, numerous dried flowers and supporting branches from the previous year are readily consumed by fire if there is sufficient fine fuel to move the fire from plant to plant [64].

DISCUSSION AND QUALIFICATION OF FIRE EFFECT:

Slow, hot fires create a longer duration of heat release which results in greater broom snakeweed mortality. Following fire in New Mexico, McDaniels and others [64] observed that fires with duration of heat exceeding 140 degrees Fahrenheit (60 ^oC) longer than 45 seconds reduced broom snakeweed by at least 70%; fires with durations less than 45 seconds gave highly variable results.

In west-central Kansas, broom snakeweed is severely harmed by spring fires [107].

PLANT RESPONSE TO FIRE:

Although usually killed by fire, plants occasionally sprout [45]. If the entire crown is not consumed, plants may produce shoots from undamaged primordial buds located on lower stems [64]. Reestablishment generally proceeds rapidly through large numbers of light, wind-dispersed seed [69,111].

Recovery time may vary with fire severity and with the season of burn but is generally rapid. In many parts of the Great Basin, broom snakeweed appears in abundance soon after severe fires in sagebrush communities [71]. After spring burns in northern mixed grass prairies, both increases and decreases in broom snakeweed have been reported [56].

DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:

Burning in southern Arizona reduced broom snakeweed cover from 0.66 plants/m² to 0.13 plants/m² 2 years after fire [34]. Following spring burns in New Mexico, most plants that survived burning (94%) produced shoots from the basal area at or near the surface [64]. After a southern Arizona burn at the beginning of the wet season, broom snakeweed was reduced by 95%. Following April and June burns on blue grama ranges of south-central New Mexico, the cover of broom snakeweed was reduced by 45% and 96% respectively [34].

In most of the Great Basin, broom snakeweed generally appears by the sixth year following fire if present in adjacent unburned areas [108]. Because of its ability to reestablish rapidly, broom snakeweed often assumes dominance on recently burned Utah sites with cheatgrass (Bromus tectorum) and rabbitbrush (Chrysothamnus spp.) [81]. Some areas develop nearly pure stands of broom snakeweed within 5 to 10 years, although in other locations plants may not dominate the site for up to 25 years [106,108]. Increases in broom snakeweed cover can be dramatic. In a Utah cheatgrass-sand dropseed-red threeawn (Aristida purpurea) community, only minor amounts of broom snakeweed were present prior to a summer wildfire. However, within 5 years after fire, broom snakeweed accounted for 31 percent of the perennial cover. Changes in cover and composition were documented as follows [16]:

                            (area burned in 1956)
                            1955     1957    1958    1960    1961
foliage cover       0.6      trace       0.9       6.1      5.9
% composition    0.8      trace       1.2       7.2      7.0

Broom snakeweed was present in 8 of 9 burn age classes in Utah, with peak frequencies occurring on 22-year-old burns in big sagebrush or Utah juniper (J. osteosperma) communities. Broom snakeweed populations declined gradually for 50 years, with frequencies of less than 10 percent reported on burns of 100 years or older [5]. I

FIRE MANAGEMENT CONSIDERATIONS:

In some areas fire can be used to control broom snakeweed during wet weather cycles [107]. Everitt and others [29] report that broom snakeweed can be controlled by fire during certain phenological stages. In southern Arizona, plants were most vulnerable when burned in October, April, and June [46]. Even cool fires can be effective in controlling broom snakeweed [3] so long as fuel loadings are sufficient to carry fire [10]. Gatewood [34] observed that the primary limitation to burning in broom snakeweed communities is inadequate fine fuel. Often dense stands of broom snakeweed (with essentially no grass) do not carry a fire well except under hazardous burning conditions (high air temperatures, high winds, and low relative humidities).

Most important variables in determining the success of a burn in broom snakeweed are litter fuel loading and fine fuel loading. Gatewood [34] reported that burn cover did not exceed 60% where fine fuel amounts totaled less than 600 kg/ha, where litter fuel was less than 1,200 kg/ha and where combined amounts total less than 1700 kg/ha. Litter moisture, fine fuel in canopy, temperature, and relative humidity are also important variables.

Gatewood [34] suggests that blacklines can be burned when the following conditions are met: 1) litter moisture is 3-4.5%, 2) fine fuel moisture is less than 20%, 3) relative humidity ranges from 20-45%, 4) temperature is 75-90 degrees Fahrenheit (24-32 ^oC), and windspeed is 5-9 miles per hour (8-15 km/hr). Prescribed fire should occur as close to the end of the dry season as possible or before vegetation greens up. Spring burns will kill any broom snakeweed seedlings that germinated during the winter. If precipitation has been below normal during previous growing seasons and if drought continues through fall and winter, Gatewood [34] recommends avoiding prescribed fire in broom snakeweed communities due to poor fire spread.

Once an area has been burned, Gatewood [34] notes that grazing should be discontinued until recovery has occurred (generally one growing season in normal to wet years, and two or more growing seasons in dry years). Grazing should be deferred until after fall freezes in areas burned in early spring. Because seeds remain viable in the soil, it may be necessary to burn at 5 to 10 year intervals to reduce broom snakeweed populations.

Related categories for SPECIES: Gutierrezia sarothrae | Broom Snakeweed