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Chapter 3: Plant Invasions and Fire Regimes Matthew L. Brooks Chapter 3: Plant Invasions and Fire Regimes The alteration of fire regimes is one of the most Fire Behavior and Fire Regimes ____ significant ways that plant invasions can affect eco- systems (Brooks and others 2004; D’Antonio 2000; Fire behavior is described by the rate of spread, D’Antonio and Vitousek 1992; Vitousek 1990). The residence time, flame length, and flame depth of an suites of changes that can accompany an invasion individual fire (chapter 1). This behavior is affected by include both direct effects of invaders on native plants fuel, weather, and topographic conditions at the time through competitive interference, and indirect effects of burning. Individual fires can have significant short- on all taxa through changes in habitat characteristics, term effects, such as stand-level mortality of vegetation. biogeochemical cycles, and disturbance regimes. Ef- However, it is the cumulative effects of multiple fires fects can be far-reaching as they cascade up to higher over time that largely influence ecosystem structure trophic levels within an ecosystem (Brooks and others and processes. The characteristic pattern of repeated 2004; Mack and D’Antonio 1998). burning over large areas and long periods of time is Direct interference of invaders with native plants can referred to as the fire regime. be mitigated by removing or controlling the invading A fire regime is specifically defined by a character- species. In contrast, when invaders cause changes in istic type (ground, surface, or crown fire), frequency fundamental ecosystem processes, such as disturbance (for example, return interval), intensity (heat release), regimes, the effects can persist long after the invad- severity (effects on soils and/or vegetation), size, ing species are removed. Restoration of native plant spatial complexity, and seasonality of fire within a communities and their associated disturbance regimes given geographic area or vegetation type (Sugihara may be necessary to restore pre-invasion landscape and others 2006b; chapter 1). Fire regimes can be conditions. In this chapter, I describe ways in which described quantitatively by a range of values that invasions by nonnative plant species can change fuel are typically called the “natural range of variation” conditions and fire regimes, and discuss what can be or “historical range of variation.” The term “natural” done to prevent or mitigate these effects. requires a value judgment that can be interpreted in many different ways, and the term “historical” requires USDA Forest Service Gen. Tech. Rep. RMRS-GTR-42-vol. 6. 2008 33 a temporal context that is often poorly understood, as of thousands to millions of years. Climate factors are does the term “presettlement” (chapter 1). Any of these relatively stable at the scale of hundreds to thousand concepts may be inappropriate as a reference set for of years, although rapid changes have occurred dur- management decisions if conditions have changed so ing the 1900s due to anthropogenic influences on the much that the landscape cannot support a fire regime atmosphere (Houghton and others 1990). Ignition that is either natural or historical. The term “reference rates from natural sources such as lightning are re- fire regime” is used in this chapter to describe a range lated to climate and have a similar degree of stability. of fire regime conditions under which native species However, ignition rates from anthropogenic sources have probably evolved, or which are likely to achieve are related to human population levels, which can specific management objectives such as maximizing increase dramatically at the scale of tens to hundreds native species diversity and sustainability. Current of years. In contrast, fuels are created by vegetation, patterns of fire regime characteristics can be objectively which can change over a period of days to weeks due compared to reference conditions, and shifts outside of to land cover conversion to agriculture, some other the reference range of variation can be used to identify human use, or other disturbance. Vegetation can also “altered” fire regimes. be changed over a period of years to decades due to plant invasions that can displace native vegetation Biological and Physical Factors that Affect and change fuel properties. Thus, alterations in an- Fire Regimes thropogenic ignitions and vegetation characteristics are the two primary ways in which fire regimes can Fire regimes are influenced by topographic patterns, change rapidly. In this chapter, I focus on fire regime climatic conditions, ignition sources, and fuels (fig. 3-1). changes due to vegetation changes caused by plant Topographic factors are extremely stable at the scale invasions. Figure 3-1—Biological and physical factors influencing fire regimes. 34 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-42-vol. 6. 2008 Effects of Plant Invasions Canopy fuels include fine to very coarse fuels, have low to high horizontal continuity, and have relatively on Fuels _______________________ low ignitability. The vertical spread of fire from surface Fuels can be classified into layers based on their to canopy fuels often requires ladder fuels that provide vertical arrangement on the landscape. The fuel lay- vertical continuity between strata, thereby allowing ers typically used to describe fire spread are ground, fire to carry from surface fuels into the crowns of trees surface, and canopy fuels (chapter 1). Since plant or shrubs (fig. 3-3). Ladder fuels help initiate crown invasions are less likely to alter ground fuels (organic fires and contribute to their spread (National Wildfire duff and peat layers) in most ecosystems, surface and Coordinating Group 1996). They may consist of vines canopy fuels are emphasized in this chapter. or be comprised of the tallest surface fuels and the Surface fuels are typically dominated by litter plus shortest canopy fuels at a given site. herbaceous plants and shrubs, comprised mostly of fine In addition to fuel particle size and the horizontal (1-hour timelag: <0.25 inch (<0.6cm)) and medium-size and vertical distribution of fuel layers, other extrinsic (10-hour timelag: 0.25 to 1 inch (0.6 to 2.5cm)) fuels fuel properties related to the way fuels are arranged (Deeming and others 1977). In forested ecosystems, on the landscape can influence fire behavior (fig. 3-1; coarse (100-hour timelag: 1 to 3 inches (2.5 to 7.5 cm)) table 3-1). The amount of fuel, or fuel load, primarily and larger diameter surface fuels may also be plenti- affects the intensity of fires. The fuel bed bulk density, ful. Horizontal continuity of surface fuels is generally or amount of fuel per unit volume of space, affects the high in productive ecosystems such as riparian zones rate of combustion, which influences the frequency, but may be very low in low-productivity ecosystems intensity, and seasonal burning window of fires. such as desert uplands (fig. 3-2). These properties also affect residence time and the A B Figure 3-2—(A) High horizontal fuel continuity created by the nonnative grasses ripgut brome (Bromus diandrus) and cheatgrass (Bromus rubens) in a southwestern riparian ecosystem. (B) Low horizontal fuel continuity in a native Mojave Desert shrubland. (Photos by Matt Brooks, USGS, Western Ecological Research Center.) USDA Forest Service Gen. Tech. Rep. RMRS-GTR-42-vol. 6. 2008 35 Figure 3-3—High vertical continuity (ladder fuels) created by the nonnative giant reed (Arundo donax) in a southern California riparian woodland. (Photo by Tom Dudley, UC Santa Barbara.) Table 3-1—Primary effects of fuelbed changes on potential fire regimes.a Fuelbed change Fire regime change Increased amount (load) Increased fire intensity and seasonal burn window; in- creased likelihood of crown fire; increased fire severity Decreased amount (load) Decreased fire intensity and seasonal burn window; decreased likelihood of crown fire; decreased fire severity Increased horizontal continuity Increased fire frequency and size, increased spatial homogeneity Decreased horizontal continuity Decreased fire frequency and size, decreased spatial homogeneity Increased vertical continuity Increased fire intensity and likelihood of crown fire, which could increase size and spatial homogeneity Decreased vertical continuity Decreased fire intensity and likelihood of crown fire, which could reduce size and homogeneity Change in bulk density Change in fire frequency, intensity, and seasonality; change in fire severity Increased plant tissue flammability Increased fire frequency, intensity, and seasonal burn window; possible increase in fire frequency or severity Decreased plant tissue flammability Decreased fire frequency, intensity, and seasonal burn- window; possible decrease in fire frequency or severity a Modified from Brooks and others (2004) table 1. duration of smoldering combustion, which influence or fire regime property (Brooks and others 2004). For fire severity. Intrinsic fuel properties such as plant example, grass invasions into shrublands increase tissue flammability, influenced by moisture content horizontal fuel continuity and create a fuel bed bulk or chemical composition (for example, the presence of density more conducive to the ignition and spread salts or volatile oils), also affect fire behavior. of fire, thereby increasing fire frequency, size, and Nonnative plants can alter fuelbeds directly, based spatial homogeneity (in other words, completeness of on their own extrinsic and intrinsic properties as burning). At the same time, the replacement
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