Fire suppression has been one of the great success stories of wildland management organizations. Over the last 100 years or so, public fire-fighting agencies such as the U.S. Forest Service, the Bureau of Land Management, the Bureau of Indian Affairs, and the National Park Service have developed an impressive array of fire-fighting technologies that have remarkably reduced acreage burned by wildfires (Pyne 1982).

In California, fires ignited naturally and by Native Americans before European settlement burned as much as 13% of the state in any one year (Martin and Sapsis 1992). With effective fire suppression beginning in the early twentieth century, burned acreage plummeted to 15% of presuppression values. Since 1970, however, suppression efforts have become less effective. California, for example, has experienced a doubling in acreage burned by wildfires, while the number of wildfires in the state has increased only slightly (Martin and Sapsis 1992). Other western states have also seen sharp increases in burned acreage. In recent years, fires that burned tens and hundreds of thousands of acres have occurred in California, Idaho, Montana, Oregon, Washington, and Wyoming (Martin and Sapsis 1992; Agee 1993; Covington et al. 1994; Johnson et al. 1994). While most ecosystems occasionally experience very large fires (Romme and Despain 1989), the present-day frequency of such large fires appears unprecedented.

Ecosystems respond differently to fire suppression. Ecosystems that tend to be particularly cool and moist, such as certain boreal and subalpine ecosystems, burn so infrequently that the interval between fires is longer than the 75-100 years of effective fire suppression. Ecosystems that are extremely dry, such as deserts or cold, dry alpine ecosystems, are so unproductive that they accumulate fuel too slowly to have been affected by fire suppression (Martin 1982). Temperate ecosystems, where frequent, low-intensity wildfires had occurred in the past, are more likely to have been adversely affected by fire suppression (Agee 1993).



In these temperate, productive ecosystems, average fire size and severity have increased dramatically. Decades of fire suppression have left a legacy of increased fuel loads and ecosystems choked with an understory of shade-tolerant, late-successional plant species (Figs. 1 and 2). These structurally homogeneous ecosystems are conducive to the large, severe fires, especially during hot, dry, windy periods in late summer or early fall. Such ecosystems have fewer structural breaks to retard fire spread and intensity, and their increased accumulations of live and dead fuels may burn longer and more completely. Plant and animal mortality in these ecosystems is relatively high following the large stand-replacing fires that are now much more likely to occur in these ecosystems.



Ecosystem recovery following stand-replacing fires potentially follows four stand-development stages: stand initiation, stem exclusion, understory reinitiation, and old-growth (Oliver 1981; Larson 1990; Oliver and Larson 1990; Fig. 3). Stand initiation is a period in which a site is reoccupied by an influx of colonizing species combined with a diverse mix of late successional species. During stem exclusion, competition induces self-thinning of weakened plants, leading to a marked decline in species richness and structural diversity. Forest canopies close, leaving understory species with inadequate light. Understory reinitiation begins once larger trees die, leaving holes in the canopy large enough for light to reach the forest floor, where late-successional, shade-tolerant species can grow and survive. Over time, an old-growthlike forest develops with characteristic multiple-age classes of trees and multiple canopy layers. Plant species richness peaks during the stand initiation stage, declines during stem exclusion, then slowly increases as growing space is provided by individual tree mortality and the reestablishment of an understory (Schoonmaker and McKee 1988; Stuart et al. 1993). In contrast, structural diversity peaks during old-growth, allowing for a greater richness of epiphytes and invertebrates (Schowalter 1989).



Before fire suppression, ecosystems accustomed to frequent, low-severity wildfires supported diverse landscapes composed of a variety of plant communities and successional stages. The resulting landscape mosaic typically burned irregularly. Some landscape patches had light, discontinuous fuel and burned cool and quickly; others had heavy, continuous fuel and burned hot and slowly or did not have enough available fuel to burn at all. Recovery from fire was equally varied. Patches that burned hot resembled small stand-replacing fires with stand development patches dominated by shade-intolerant species. In contrast, few to no overstory trees were killed and only some of the understory plants were killed in cool-burning patches. Shade-tolerant species reestablished themselves in the understory. Stands harboring complements of both early and late successional species represented islands of high biological diversity.



The greatest effect of fire suppression on biological diversity is not on the diversity within a particular habitat (Whittaker 1977), but on the diversity of habitats across a landscape. Landscapes with high diversity resulting from fire perpetuate high species diversity by providing opportunities for the establishment and maintenance of early successional species and communities (Connell 1978; Reice 1994). Fire suppression, on the other hand, increases uniformity in habitats as competition eliminates early successional species, leaving only shade-tolerant understory plants to reproduce. For example, in the Klamath Mountains of northern California, recently burned landscapes had more (46-48) distinct habitat types that were more evenly distributed than equal-sized unburned areas (31) (Fox et al. 1992). Burned landscapes included habitat types dominated by early successional pines, shrubs, or herbaceous species, whereas unburned landscapes were more uniform in their cover of later successional fir-dominated communities.



Fire suppression has helped change the ecosystem dynamics of communities adapted to frequent, low-intensity wildfire. Complex landscapes are made simpler, some early and midsuccessional plants and animals are extirpated, shade-tolerant tree populations rapidly expand, and the relative importance of fire as a disturbance agent is reduced, while the importance of insects and pathogens is elevated (Covington et al. 1994). During droughts, for example, excessively dense forests become further stressed, enabling pathogens and insects to reach high population levels (Johnson et al. 1994). Trees killed by drought, insects, or pathogens create abundant fuel that exacerbates fire hazard. When fire occurs in such a system, it is often larger and more severe than one expected in areas with a natural fire regime. Such a scenario is being played out in the forests in the Blue Mountains of eastern Oregon and southwestern Washington (Langston 1995).

Re-creating the natural fire regimes of ecosystems adapted to frequent, low-severity fire seems an obvious management choice if we want to enhance biological diversity and reduce the risk of catastrophic wildfire. Paradoxically, while fires help maintain native biological diversity, they also create opportunities for invasive alien species to become established. In many cases, these species are superior competitors, predators, or parasites on our native flora and fauna (Hobbs and Huenneke 1992) and could actually reduce native biological diversity. Thus, restoring a more natural fire regime will have to be carefully considered to maximize ecosystem benefits while minimizing biological and social costs.