The Role of Fire

Although shrubs dominate chaparral, the community comprises a rich diversity of growth forms, many of which are conspicuous only after fire. In addition to evergreen shrubs and trees, there are semi-deciduous subshrubs, slightly ligneous (hardened) suflrutescents, woody and herbaceous vines, and a rich variety of herbaceous perennials and annuals. A large number of these species arise from dormant seeds deposited into the soil decades earlier, following an earlier fire. Dormancy is broken in some seeds by heat but in many other species smoke from the fire triggers germination. In the first spring following fire, there is an abundant growth of herbaceous plants, which are relatively short-lived and are replaced by shrubs within the first five years. The postfire herbaceous flora often is dominated by annual species that live for less than one year, and species diversity is typically greatest in this first year after fire. Recovery of shrub biomass is from basal resprouts and seedling recruitment from a dormant soil-stored seed bank.

community a group of organisms of different species living in a region suffrutescent a shrublike plant with a woody base biomass the total dry weight of an organism or group of organisms w community a group of organisms of different species living in a region suffrutescent a shrublike plant with a woody base

compound a substance formed from two or more elements toxin a poisonous substance predation the act of preying upon; consuming for food compound a substance formed from two or more elements toxin a poisonous substance predation the act of preying upon; consuming for food senescent aging or dying ecosystem an ecological community together with its environment

The striking contrast between the diminished herb growth under mature chaparral and the flush of herbs after fire is thought to be caused by allelopathic (chemical) suppression of germination by the overstory shrubs. Many of the smaller shrubs, such as sage (Salvia spp.) or sagebrush (Artemisia spp.), release volatile, aromatic compounds, and it has been suggested that these compounds inhibit the growth of competing grasses and wildflowers. This theory holds that lire destroys these toxins, and that this occurs throughout the shrub land and in a zone at the border between shrub lands and grasslands, forming a meter-wide strip known as the bare zone. However, experiments in which animals have been excluded from the bare zone demonstrate that the lack of herbaceous plants in and around mature shrub lands is as much due to animal predation as it is to chemical inhibition. In addition, it appears that the vast majority of species that germinate after fire do so more because their dormant seeds are stimulated to germinate by fire.

Resource agencies often respond to wildfires with emergency revegetation programs, which drop grass seed on newly burned sites with the expectation that this will reduce soil erosion and eliminate the threat of mudslides and flooding. The rationale for this management is that burned sites have greatly increased surface flow of rainwater and thus high soil erosion. Emergency seeding is considered essential on sites following exceptionally intense fires because of the anticipated negative effects. Throughout the state of California the seed of choice has been the nonnative annual ryegrass (Lolium multiflorum). However, there is abundant evidence that this practice fails to substantially reduce threats of mudslides and flooding and competitively displaces the native flora.

Some scientists have suggested that chaparral shrub lands become senescent if they are free of fire for more than a few decades. Detailed studies, however, find that these shrub land ecosystems can retain productive vegetation for a century or more, and in fact some shrubs require long periods without fire for successful seedling recruitment. see also Allelopathy; Biome; Ecology, Fire.

Jon E. Keeley

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