USDA Forest Service

Pacific Southwest Research Station

Pacific Southwest
Research Station

800 Buchanan Street
Albany, CA 94710-0011
(510) 883-8830
United States Department of Agriculture Forest Service. USDA logo which links to the department's national site. Forest Service logo which links to the agency's national site.

Research Topics Fire Science

Post-fire shrub growth in conifer ecosystems

Small shrubs grow in the foreground with ponderosa pine trees in the background.
Three years after a fire, a resilient stand of ponderosa pine with a diverse understory including a shrub component develops on the Tahoe National Forest. Photo by Jens Stevens, University of California, Davis)

After a fire in Sierra Nevada conifer forests, a variety of shrubs and herbaceous species often regrow to take advantage of higher light levels and open soil conditions. Predominant  shrub species include Ceanothus (deerbrush, snowbrush, mahala mat) and Arctostaphylos (various manzanitas). Bear clover (Chamaebatia foliolosa, also known as mountain misery), a shrub, will reappear in abundance where it occurred before the fire. If a seed source is available, conifer seedlings will begin to establish and grow among the shrubs, though they may not be visible to the casual observer initially.

Shade produced by trees normally limits growth of understory shrubs, but when those trees are killed by a high severity fire, abundant shrub growth can result in a stand of montane chaparral that persists for many years. While these shrubs can initially limit the growth of tree seedlings by competing with them for light and soil moisture, many shrub species, especially Ceanothus, harbor beneficial bacteria in their roots which help to enrich the soil with nitrogen and can also enhance the growth of conifer seedlings. In 20 to 40 years, conifer seedlings can grow tall enough to start shading the shrubs, and the ecosystem will eventually progress to a conifer forest where shrubs exist only as scattered individuals and a seed bank in the soil, waiting for the next fire.

If a hot fire burns through the regrowing shrub and conifer sapling stand, young trees may be killed, resulting in further persistence of the chaparral. Due to fire suppression in Northern California, many historic chaparral patches have been over-topped and shaded out by firs and other conifers. In other areas, recent large fires with extensive high-severity patches could result in much larger montane chaparral areas where conifer seeds may not become established for many decades.

Management considerations

Long-term shrub dominance can be undesirable where land was previously used for recreation or timber production prior to fire, so land managers may choose to reduce shrub cover to speed conifer regrowth. When shrub competition is removed, planted conifers, such as ponderosa pines grow faster, especially in stands less than 20 years old, but this advantage is lost once the stand canopy closes. Managers of private timberlands may overcome the loss of nitrogen from shrub-root bacteria contributions with artificial fertilization.

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