Effects of Fire on Plants- Overview

Plant response to fire is a result of the interaction between fire severity and characteristics of the plants in the fire, both their inherent resistance to injury and ability to recover. Fuel quantity and arrangement, fuel moisture content, topography, windspeed, and structure of the plant community itself cause the lethal heat zone created by fire to vary significantly in time and space. Fire can cause dramatic and immediate changes in vegetation, eliminating some species or causing others to appear where they were not present before the fire. However, in burned areas with a high component of surviving trees and resprouting understory vegetation, within a few years it can be difficult to determine that a fire recently occurred.

For the vascular plant groups discussed in this section, the recovering plant community, in the first few years after a fire, comprises individuals from the following categories:

• Plants that survived the fire with their form intact (See: Plant Mortality During and After Fire)
• Sprouts or suckers that grew from the base or buried parts of top-killed plants (See: Vegetative Regeneration After Fire)
• Plants that established from seed (See: Seedling Establishment After Fire)

Seedlings can be further described as:

• Plants that re-established from seed dispersed from surviving plants, usually trees
• Plants that re-established from seed dispersed from off of the burned site
• Plants that re-established from fire stimulated seed within the seedbank
• Plants that re-established from seed that developed on plants that resprouted after the fire

Certain species can only recover after fire by a single means. Some will only be present after fire if regenerative structures survive and produce sprouts, because their seedlings are unlikely to survive in postfire environments. Species of plants that cannot resprout after top-killing must establish from seed. However, some species can successfully recover from fire both by resprouting and by seedling establishment. Severity of the fire largely determines whether new plants are sprouts or seedlings.

Where fire top-kills plants that can vegetatively regenerate, sprouting will be a significant source of postfire vegetation. Where lethal temperature penetrates deeply enough to kill many regenerative structures, sprouting may be limited, but some buried seed may receive the proper stimulus to germinate and produce significant numbers of seedlings. A microsite that sustained lethal heat deeply enough to kill all stored seed probably had enough fuel and duff consumed to prepare areas of bare mineral soil seedbed. Reproduction on these sites occurs from seeds dispersed onto these burned surfaces. The availability of canopy-stored seeds depends on the height to which lethal temperatures reached into the tree crowns, while dispersal from wind-carried offsite seeds depends on distance and direction of prevailing winds. Surviving trees and resprouting plants may produce seeds that can establish within the next few growing seasons while suitable seedbed exists.

The immediate response of plants can differ within the same fire because of variations in the pattern of burn severity. For example, chamise and redstem ceanothus can sprout after a low to moderate severity fire treatment. High severity fires kill existing plants but they are replaced by new plants that develop from fire-stimulated seeds. The postfire community may contain both sprouts and seedlings of these species with the proportion related to the severity of the fire.

Postfire species composition is usually an assemblage of many of the species that were growing on the site and represented in the seedbank at the time of the fire. Vegetative regeneration is common to many species and can make a major contribution to the postdisturbance community (Ingersoll and Wilson 1990), contrary to the commonly held notion that seed reproduction is dominant. Resprouts from rhizomes, root crowns, or protected meristems can account for a substantial proportion of postfire recruitment (Lyon and Stickney 1976). Many of the seedlings present in the first few postfire years may have grown from seeds formed on resprouting species such as fireweed and heartleaf arnica. The only locations in which new species are likely to be added to the plant community are microsites that are severely burned and receptive to germination and establishment of seeds from species dispersed from off of the site (Stickney 1999).

Management Implications

Knowledge of plant response to fire can be critical to successful application of prescribed fire. Designing fire prescriptions requires knowledge of fire behavior, fire severity, species survival mechanisms, and methods of postfire vegetation recovery. Fire prescriptions should describe a set of weather and fuel moisture conditions that will control the rate and amount of fuel consumption by different size classes. Properly conducted, the prescribed fire will result in the desired amount of mortality, injury, resprouting, and seedling establishment from target species. To ensure that a desirable range of plant species establishes after prescribed fire, it is helpful to acquire predictive means of assessing how different prescriptions can produce different fire and plant responses (Whittle and others 1997). A fire prescription that takes both the surface and subsurface heat regime into account, thereby regulating its severity, is most likely to achieve desired fire effects.