Fire and nonnative invasive plants in the central bioregion

The Central bioregion is a vast area, stretching from Canada to Mexico and from the eastern forests to the Rocky Mountains, dominated by grasslands and shrublands, but inclusive of riparian and other forests. This bioregion has been impacted by many human induced changes, particularly relating to agricultural practices, over the past 150 years. Also changed are fire regimes, first by native peoples who used fire for a variety of purposes and then by European settlers, who directly and indirectly contributed to a great reduction in the frequency of fire on the landscape. Perhaps of even greater importance has been the introduction of nonnative plant species, which have come to impact every community type to some degree. Nonnative plants have a wide array of impacts on native ecosystems and populations in the Central bioregion, and these impacts continue to mount and evolve. Many long-time invaders, such as smooth brome (Bromus inermis), and leafy spurge (Euphorbia esula), have already spread to large areas, and their ranges may still be expanding. Others, such as tamarisk or saltcedar (Tamarix spp.) and buffelgrass (Pennisetum ciliare), are rapidly spreading at the present time, while still others have likely not yet shown their full potential for expansion. In this volume, as well as in this chapter, our emphasis is on the interaction of nonnatives with fire, how it affects them and how they affect it. The ecosystems of the Central bioregion have been shaped by fire, including fires associated with natural ignitions and those deliberately set by humans. Both grasslands and shrublands in this bioregion experienced frequent and widespread fires during their evolution (Stewart 2002). Prescribed fire is now widely used to manage some areas for their natural characteristics. Thus, while changed in character, both by conditions that now limit wildfire occurrence and spread and by prescribed burning, the Central bioregion remains one with a high fire frequency (Wade and others 2000). Fire interactions with nonnative plants can have important impacts. In some cases, fire can be a means of reducing impacts of nonnative species (chapter 4). In other cases, fire may facilitate the establishment and spread of nonnatives (chapter 2). Some nonnative species can radically change the fire regime itself (chapter3). Because of the widespread use of prescribed fire in this bioregion, it is important to know how nonnative species interact with fire and whether there are means whereby these interactions can be controlled.