FIRE EFFECTS GUIDE - National Wildfire Coordinating Group

is controlled by a phenomenon called apical dominance. Growth hormones, particularly
auxin, a plant hormone manufactured in actively growing stem tips and adjacent young
leaves, are translocated to dormant buds, which prevent them from developing into
new shoots (Schier et al. 1985). If these plant parts are removed, the source of growth
hormones is eliminated. The balance of plant hormones within the buds changes (ibid.).
Growth substances in roots, particularly cytokinins, that are translocated to the buds
can cause dormant buds to sprout, or stimulate bud primordia to differentiate into
shoots (ibid.). Cytokinins may already be present in buds, and a decrease in the ratio of
auxins to cytokinins provides the stimulus for bud outgrowth (ibid.). The buds that
become shoots are usually those nearest to the part of the plant killed by the fire. If
dormant buds are destroyed, new buds may form in wound tissue, called callus, and
subsequently produce shoots (Blaisdell and Mueggler 1956). Once new shoots are
actively growing, they produce growth hormones that are translocated to other dormant
buds that are farther away from the point of damage, suppressing their growth (Schier
1972; Bilderback 1974).
If the organic layer is thinned or disturbed, additional light may reach the tips of
rhizomes and stimulate them to grow towards the surface and produce shoots (Barker
and Collins 1963; Trevett 1956; Miller 1977). It has also been observed that
decapitating a rhizomatous plant causes laterally growing rhizomes to turn upwards
and become shoots (Schier 1983). Additional rhizomes often form in response to
vigorous aerial plant growth (Kender 1967), which may subsequently produce
aboveground shoots. Sprouts from new rhizomes or lateral roots may recolonize areas
where all reproductive plant parts were killed by a fire. Plants may sprout soon after a
fire, or not until the following spring if the fire occurs after the plants have become
dormant for the winter (Miller 1978). Warmer soil temperatures after burning may
enhance the amount of sprouting that occurs (Zasada and Schier 1973). The initial
energy required to support growth until the sprout is photosynthetically self-sufficient
comes from carbohydrates and nutrients stored in the reproductive structures or in
adjacent roots (James 1984).
Postfire sprouting ability can vary with plant age. Young plants that have developed
from seed may not be able to sprout until they reach a certain age, which varies by
species (Smith et al. 1975; Tappeiner et al. 1984). For a given species such as
bitterbrush, an older plant may be able to produce few, if any, sprouts that survive
(Ferguson 1988). Other factors also may lead to decreased amounts of postfire
sprouting in older bitterbrush. These include the higher amount of dead material within
old crowns, and the deep organic layer found beneath some old plants that cause
increased potential for lethal heating during a fire (Clark 1989; Miller 1988).
Aspen produces sprouts from healthy roots. Decreased amounts of postfire sprouting
observed from older aspen stands (Schier 1975) may be because the condition of
many of the roots has deteriorated to the point where they cannot sprout (Zasada, pers.
conv. 1989). Aspen stands in Alaska can resprout vigorously after fire when they are
150 to 200 years old, perhaps because the incidence of pathogens in Alaskan aspen
stands is relatively low (ibid.). In areas such as the Lake States, aspen stands are often
killed by cankers by the age of 50 to 70 years (ibid.) An aspen stand that is producing a
few understory suckers still has the capacity to sprout after a fire (DeByle 1988a).