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In this session, we will focus on four major topics that relate to fire and invasive species. We’ll begin with clarifying what we mean by “invasive species.” Our second topic addresses the stages of invasion as defined in the Sakai et al. paper, and talk a bit about characteristics of both successful invaders and successful invasions. We’ll next discuss ways in which invasive species might alter fuels attributes and ultimately fire regimes, based mostly on the Brooks et al. paper. And finally, we’ll discuss how prescribed burning might be used to control invasive weeds based on the DiTomaso et al articleweeds, based on the DiTomaso et al. article.

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There are a number of definitions of “invasive.” The definition provided here has many of the elements that characterize invasiveness: being vigorous, persistent, prolific or widespread, as thus causing serious ecological and economic impacts.This presentation is focused mostly on non-native invaders, or species introduced to the United States or to a specific region where they were not known to be native. However, native species can be invasive as well.

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Another term you’ll hear is “noxious.” This is a legal term, and the definition varies by state. Note that a number of highly invasive non-native plant species are not designated as noxious in most states. An outstanding example is cheatgrass, which is designated as noxious only in one state. A number of intentionally introduced species that can be highly invasive are not designated as noxious in most states. Examples include smooth brome, orchard grass, sweetclovers, and alfalfa.

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Sakai et al. (2001) recognized three stages of invasion. Colonization of new habitats requires that the first arrivals initiate new populations. After successful colonization, the next stage of invasion is characterized by establishment of a viable, self-sustaining population. Once initial colonization and establishment have occurred, invasive species may spread.

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Successful colonizers are often species we might think of as being “weeds.” Self-fertility is especially common in plants, but some insects and vertebrates can store sperm and also colonize from a single introduction. Plants with multiple reproductive strategies, that is, both vegetative reproduction and seeds, or plants with multi-seeded fruits may also be good colonists. Phenotypic plasticity enables colonizers to cope with a range of environmental characteristics.

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Traits needed for successful establishment of an invasive species may differ from those required for colonization. And, traits needed for successful establishment in natural systems may be different from traits needed in human-disturbed environments. Some studies have shown that r-selected species, or those with high reproductive rates, high growth rates, and quick maturity time are often more successful in becoming established than k-selected species that are characterized by lower reproductive rates and slower growth rates. Being highly competitive in the invaded environment is also a key trait for successful establishmentinvaded environment is also a key trait for successful establishment.

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Once populations are established, whether they spread or not depends mostly on dispersal ability. Species with good natural dispersal ability are more likely to spread. But, more often, spread of established populations is attributed to multiple introductions.

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It is important to be able to recognize the stage of an invasion. Even the most invasive species can be controlled when populations are small, in the colonizing stage. But, once populations are well established and have spread, it is difficult if not impossible to control many invasive species.

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We will now turn to ways that invasive species alter fire regimes. The effects of invaders are particularly dramatic when they alter disturbance regimes beyond the range of natural variation to which native species are adapted. Such alterations can result in community changes and ecosystem-level transformations. Invaders that alter fire regimes are widely recognized as some of the most important system-altering species on the planet.

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Fuel attributes are inextricably linked with fire regimes. Shifts in fuel conditions can result in directional shifts in fire behavior and fire regime attributes that may result in localized extirpation of species that cannot exist under the new regime.

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One of the best known examples of a an invasive species altering a fire regime is cheatgrass invasion of sagebrush grassland. The invasion of this annual grass in western North America has increased fire frequency to the point that native shrub steppe species cannot recover. This, in turn, negatively affects vertebrate species such as sage-grouse that are dependent on sagebrush habitat.

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There are many other ways that plant invasions can affect fuel properties, and thus fire regimes. Let’s look at effects of invasions on intrinsic fuel properties first. Fuel moisture content might be increased for example, by the introduction of succulent cacti, as has occurred in some European Mediterranean communities. Invasion of finely textured grasses can increase fine fuel loadings; whereas invasions of coarsely textured woody plants into grassland might have the opposite effect. Plants high in volatile oils can increase flammability.

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Plant invasions can change fire regimes due to alterations in extrinsic fuel properties such as fuel load, continuity and packing ratio. For example, introduction of African grass invaders increased grass biomass by 50% in Venezuelan savannas, resulting in hotter fires in these already fire-prone ecosystems. Plant invasions can also decrease fire intensity when they result in plant communities with lower fuel loads. Increases in fuel continuity can increase the occurrence and extent of wildfires, and the addition of ladder fuels increases vertical fuel continuity, allowing fires to travel from the surface into crowns of shrubs and trees Such invasions canfires to travel from the surface into crowns of shrubs and trees. Such invasions can change a surface fire regime to a crown fire regime. Changes in fuel packing ratios can either decrease or increase fuel flammability.

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There is often a positive feedback between plant invasions and fire regimes (Brooks et al. 2004; Mack et al. 1998). This has sometimes been called the grass-fire cycle (Vitousek et al. 1996). In this figure, Brooks et al. (2004) highlight the increasing impact as plants initially colonize (green to yellow) and then become increasingly abundant and increasingly effective at altering fuels and other ecosystem properties.

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Brooks et al. (2004) describe four phases in what they call the invasive-fire regime cycle. For each phase, they identify an appropriate management response in terms of evaluation (how bad is it?) and response (what’s most important if we are to act to break the positive feedback cycle?). The management actions are often designed to reverse the cycle to a previous stage.

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Recognizing the potential role invasive species might have on altering fire regimes should be a consideration in whether to introduce species and for prioritizing species that need to be controlled when they are in the colonizing stage. Once populations of such species have become established and have spread, restoration is difficult and costly, and may not be possible.

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We now turn to our final topic, based on the DiTomaso et al. article. Key points are that there are relatively few examples of invasive species being eradicated with a single prescribed burn. In fact, a single prescribed burn might enhance seed germination or sprouting of the invasive species, as well as reduce potential competitors. Thus, multiple burns, integration of other control measures such as herbicides, and post-fire seeding to restore competitive communities should be considered.

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The late-season winter annual, yellow starthistle, is susceptible to burning in early summer, before seeds are viable. However, seeds survive in the soil and germination is stimulated by burning. Therefore 3 consecutive years of burning might be required to reduce the seedbank. Another option is a follow-up herbicide treatment the year after the prescribed burn.

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Smooth brome, an intentionally seeded species, can be highly invasive in the Great Plains. Early season burns, when plants are being to bolt, are effective in suppressing smooth brome and encouraging warm season native grasses in tallgrass prairie. However, such early season burns can harm other native cool season plants.

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