FOR 433. Carolyn Hull Sieg. US Forest Service Rocky Mountain Research Station Flagstaff, AZ

Transcription

1 Fire and Invasive Species Carolyn Hull Sieg US Forest Service Rocky Mountain Research Station Flagstaff, AZ 1

2 Topics What is an invasive species? What are the stages of invasion and what characterizes successful invaders and invasions? How can invasive species alter fire regimes? How can prescribed burning be used to curtail invaders? 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. article. 2

3 What is an invasive species? Invasive species are species that are vigorous, persistent, prolific, or widespread enough to cause serious ecological and economic impacts (Vitousek et al. 1996, Novak and Mack 2001) Non-native, exotic, and alien species are species introduced to the United States or to a specific region where they were not known to be native There are a number of definitions of invasive. The definition I provide here has many of the elements that characterize invasiveness: being vigorous, persistent, prolific or widespread, as thus causing serious ecological and economic impacts. In this presentation, I will focus 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. 3

4 What is a noxious species? Noxious species are legally designated by each state, and often include only those species that are agricultural pests. Note: a number of highly invasive non-native species are not designated as noxious in most states. Cheatgrass (Bromus tectorum) is a good example, as well as a number of purposely seeded species such as smooth brome (Bromus inermis) and yellow sweetclover (Melilotus officinalis). 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. 4

5 Stages of Invasion The Sakai et al. paper recognizes three stages of invasion: Colonization Establishment of a viable, self-sustaining population Continued spread of the population The Sakai et al. paper recognizes 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. 5

6 Traits of successful colonizers Traits of successful colonizers may include: Self-fertility Multiple reproductive strategies Phenotypic plasticity Dalmatian toadflax (Linaria dalmatica) reproduces vegetatively and produces literally thousands of seeds that can remain viable in the soil for several years. Successful colonizers are often species we might think of as being weeds. Selffertility 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. 6

7 Traits needed for successful establishment Traits needed for successful establishment may include: High growth rates, faster time to maturity Competitive ability Note that these may be different traits than those required for colonization, and that establishment in a natural community may require different traits than those required for establishing in a humandisturbed environment Diffuse knapweed (Centaurea diffusa) has strong negative effects on biomass production of native grasses. 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 establishment. 7

8 Traits leading to spread Good dispersal ability is most often associated with invasions that spread Multiple introductions are often correlated with the eventual success of invasions. Multiple introductions are responsible for the successful spread of grasses such as buffelgrass (Pennisetum ciliare) 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. 8

9 Why is it important to recognize the stage of an invasion? Invasive species are relatively easy to control when populations are small (in the colonizing stage). Once populations are well established and have spread, it is difficult, if not impossible to control many invasive species. 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. 9

10 How can invasive species alter fire regimes? The effects of invaders are particularly dramatic when they alter disturbance regimes beyond the range of variation to which native species are adapted, resulting in community changes and ecosystem-level transformations. 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 systemaltering species on the planet. 10

11 What is a fire regime? Fire is a type of disturbance, and ecosystems are partly defined on the basis of disturbance regimes of specific frequency, intensity, extent, type, and seasonality. Frequency: measure of fire cycle or fire return interval Intensity: amount of heat released per unit of time, which is related to severity, or the effect of this heat release on ecosystem properties. Extent: includes size and spatial arrangement Type: ground, surface, crown Seasonality: annual window of fire activity Let s first review what is a fire regime. Fire regimes and other disturbance regimes partly define ecosystems. When we talk about a fire regime, we are referring to a number of attributes, including frequency, which is a measure of the fire cycle or time for fire to burn a given area or mean fire interval, the average time before fire reburns a given area. Intensity, is the amount of heat released per unit of time, and is related to severity, which is the effect of this heat on biotic and abiotic properties of an ecosystem. Extent includes both the size of the fire as well as the spatial homogeneity of the burning. Fire regimes are also characterized by fire type, including ground fires that you might find in a peat-bog, surface fires that are common in grasslands, and crown fires that burn in forest canopies. Seasonality refers to the annual window of fire activity and is largely determined by the ability of fuels to ignite and carry fire. 11

12 Fuel attributes are inextricably linked with fire regimes Shifts outside the natural range of fuel conditions can result in directional shifts in fire behavior and fire regime attributes. Plant invasions have the potential to cause changes in fuel properties, including: Fuel types and fuel layers Intrinsic fuel properties Extrinsic fuel properties Such changes can alter fire regimes. 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. 12

13 How cheatgrass alters fire regimes The introduction of cheatgrass has altered fuel load and continuity of sagebrush grasslands. Increased fire frequency threatens sagebrush and species such as the sage-grouse. Gary Kramer, USFWS 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. 13

14 Effects of invasions on intrinsic fuel properties Fuel moisture Fuel size classes (timelag moisture classes) Chemical volatility The introduction of succulent cacti can increase fuel moisture content. How would a heavy invasion of leafy spurge (Euphorbia esula) change intrinsic fuel properties of a tallgrass prairie? There are many other ways that plant invasions can effect 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. 14

15 Effects of invasions on extrinsic fuel properties Fuel load Fuel continuity Fuel packing ratio Lehmann lovegrass, introduced to southwestern U.S. deserts from southern Africa, produces about 4 times as much green biomass as native species. 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 can change a surface fire regime to a crown fire regime. Changes in fuel packing ratios can either decrease or increase fuel flammability. 15

16 Managing invasive species that alter fire regimes Exclusion of species prone to alter fire regimes before they invade or in the colonizing stage is the most cost-effective and successful way of preventing severe ecological and economic impacts. Once invaders have become established and spread, restoration of communities is difficult and costly. 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. 16

17 How can prescribed burning be used to curtail invaders? Key points: There are relatively few examples of invasive species being eradicated with a single prescribed burn Prescribed burning may actually enhance populations of some species, especially biennial plants and perennial forbs. Integration of other control measures as well as restoration of competitive native species is important. 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. 17

18 Using prescribed burning to curtail invaders Yellow starthistle (Centaurea solstitialis) is a winter annual species that can be curtailed with early summer burning, but 3 consecutive burns may be required to eliminate the seedbank. Integrating burning followed by herbicide is another option. 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. 18

19 Using prescribed burning to curtail invaders Repeated early season burns can be used to suppress smooth brome and encourage warm season plants, but may also damage other cool-season species. 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. 19

20 Conclusion Control of invasive species is possible at early stages of colonization, but as populations establish and spread, control is difficult if not impossible. The introduction of non-native invasive species can greatly alter fire regimes through changes in fuel attributes and lead to dramatic effects on native species that evolved with a characteristic fire regime. A single prescribed burn is unlikely to curtail most invaders. An understanding of the life history of the invader and associated species will help in identifying ways to integrate other control measures. 20