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Wildlife, Animals, and Plants |
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FIRE EFFECTS
IMMEDIATE FIRE EFFECT ON PLANT:Garlic mustard is often top-killed when exposed to fire. A prescribed burn in the understory of a northern Illinois hardwood forest removed all aboveground garlic mustard biomass [30]. Prescribed burns in a central Illinois black oak forest conducted both in the fall and in mid-spring removed nearly all garlic mustard rosettes [54]. Although there was no immediate postfire survey of plants mentioned in the article, Luken and Shea [41] suggest that garlic mustard "plants are readily killed by mid-intensity dormant season fires". Emergent seedlings may also be killed by fire [54]. DISCUSSION AND QUALIFICATION OF FIRE EFFECT:It has been suggested that dense stands of garlic mustard may be able to resist low-severity fire, such that "abundant green garlic mustard plants"...may "literally extinguish fires" [49], but detailed descriptions of the direct effects of fire on garlic mustard plants (or visa versa) are scarce. Such observations may be confounded by the inherently patchy nature of mixed-severity fire regimes in many eastern deciduous forests where garlic mustard may commonly be found. For more information see the "Fire Ecology" section of this summary. PLANT RESPONSE TO FIRE:Garlic mustard has at least some ability to sprout from the root crown following damage by fire. By excavating charred rosettes, Nuzzo and others [53] found that adult plants resprouted from adventitious buds on the root crown located just below the soil surface following a mid-spring burn. In a northern Illinois oak woodland, garlic mustard reportedly resprouted several weeks following complete top removal by a prescribed fire conducted in late March [30]. Repeated fall burning (2-3 annual burns) did not reduce abundance or relative importance of garlic mustard in an eastern mesophytic forest understory in Kentucky [41]. DISCUSSION AND QUALIFICATION OF PLANT RESPONSE:There is some indication that garlic mustard is capable of sprouting following fire, but several questions remain. To what extent is postfire sprouting in garlic mustard influenced by fire severity? What, if any, physiological conditions promote or constrain postfire root crown sprouting? To what extent are resprouting plants successful at producing seed? Nuzzo and others [53] reported that a fall burn in a central Illinois black oak forest removed 79% of the litter layer, and very few adult garlic mustard plants were encountered in these plots the following spring. Conversely, many garlic mustard plants resprouted following a mid-spring burn at the same site that resulted in removal of only 32% of the litter layer. Spring burn plots retained a damp 0.4- to 0.8-inch (1-2 cm) layer of litter which seems to have protected the root crowns of top-killed plants, fostering survival via sprouting of multiple secondary shoots from adventitious buds located just below the soil surface [54]. Hintz [30] conducted a late-March prescribed burn in a mesic upland oak-hickory forest in northern Illinois. Garlic mustard established following the fire, although it is unclear whether these were sprouting burned plants or new spring seedlings. The burn was conducted near the time when seedling emergence might be expected, leaving some question as to which life-cycle stage was observed to be "sprouting". There is reference to "very little" garlic mustard producing seed that summer, intimating that at least some adult plants were present both prior to and after the fire. Luken and Shea [41] conducted a prescribed fire experiment in a northern Kentucky mesic deciduous forest in which they showed that garlic mustard plants could be removed by a fall burn. Yet it was also apparent from this experiment that populations can persist following even repeated burns. Garlic mustard remained the dominant species in the herb layer of both burned and unburned plots through 3 seasons of fall burning, and beyond. The authors proposed 3 possible explanations. First, persistence of individual garlic mustard plants immediately following fire may result from the patchy nature of many understory or mixed-severity burns. Under such conditions some extant plants may escape damage, and because of its ability to self-pollinate [3,15,17], the survival of a single plant may be sufficient to perpetuate a population. Second, the data of Luken and Shea [41] showed that burning resulted in higher densities of flowering stems compared with control plots. They speculated this as being due to either resprouting or release from competition. No observations of sprouting were reported. Third, even if all plants are killed, the existing seed bank may remain viable for several years [7,14], requiring subsequent annual burns to completely eradicate the population. FIRE MANAGEMENT CONSIDERATIONS:Control of invasive garlic mustard populations using prescribed fire, especially as a single management tool, appears to be difficult. Some temporary control is likely, but difficulties sustaining long-term control are confounded by a) the patchiness of understory and mixed-severity fires, b) the biennial nature of the species, c) the moderately persistent seed bank, and d) garlic mustard's propensity for rapid population increase (see section on Biological and Ecological characteristics) [41,54,67]. It may be possible to substantially diminish the number of individuals in a garlic mustard population with repeated burn treatments. But prescribed burning, especially during the growing season, could actually increase the relative importance of garlic mustard [3,41,54]. A prescribed burn conducted in May in a northern Illinois dry-mesic upland deciduous forest effectively reduced cover of garlic mustard, from a pre-burn 29.4% cover to 2.3% cover, postfire year 1. But May burning also damaged the native forb community, where total stem density of major herbs and small shrubs was reduced by 32% and average number of species per plot was reduced by 35%, postfire year 1. Although native plants subsequently showed gradual recovery, these effects were detectable for 3 years, most notably for Jack-in-the-pulpit and stickywilly. Garlic mustard recovery was more rapid. Within three years following burning garlic mustard had rebounded to 17.3% cover compared with a pre-burn level of 29.4% [67]. Dormant-season burns, while less likely to have negative effects on indigenous flora, also appear to be less effective at killing garlic mustard rosettes. After 3 years following a March prescribed burn at the above location, both garlic mustard and native herb cover had returned to approximate pre-burn levels [67]. It has been suggested that a narrow window of time exists during early spring in some areas and in some years, during which garlic mustard may be more effectively controlled by fire without damaging native plants. This hypothesis remains untested as of this writing [67]. Also, spring burns may increase seedling survival. Fires of insufficient severity may spare a sizable fraction of seedlings protected by the unburned portion of the litter layer. Additionally, a spring burn timed too early may permit survival of garlic mustard seedlings that germinate after treatment. In addition to greater initial seedling survival, removal of a portion of the litter layer may also provide a more favorable environment for growth and development of garlic mustard rosettes [54]. Apparently not all fires are equally effective at top-killing garlic mustard. The effectiveness of prescribed spring and fall burn treatments in reducing garlic mustard populations in an oak (Quercus spp.)-dominated dry-mesic upland forest in northern Illinois was directly related to fire "intensity". "Low-intensity" burns, with flame lengths up to 1.2 inches (3 cm), were patchy and frequently extinguished within plots. These "low intensity" burns had little to no effect on garlic mustard plants, whether seedlings or adults, regardless of season of burning. It was suggested that abundant green garlic mustard plants frequently extinguished the "low intensity" fires. "Mid-intensity" burns, with flame lengths up to 3 inches (15 cm), burned through most of the plots and significantly reduced the presence of garlic mustard. Adult plant densities were reduced by both spring and fall burns, as well as repeated fires, although single spring burns were most effective [49]. In areas with long fire-return intervals where favorable conditions for conducting effective prescribed burns may be rare to nonexistent, especially repeated annual burns, or where fire-sensitive native species exist, prescribed fire may be unsuitable as a management tool. Nevertheless, in areas with a fire-tolerant native flora, frequent prescribed burning may deter garlic mustard invasion by both directly killing invading plants, and perhaps in some areas by enhancing growth of native herbaceous competitors and thereby reducing habitat for garlic mustard colonization [49,87]. It is highly likely that managers who use fire to control garlic mustard may need to augment burn treatments with 1 or more additional control methods, such as pulling or herbicide use to achieve acceptable levels of control (see Management Considerations section for more information on other control methods).
Related categories for SPECIES: Alliaria petiolata | Fringed Onion |
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