Plant Mortality During and After Fire

The likelihood of plant tissue being killed by fire depends upon the amount of heat it receives. The heat received by a plant is determined by the temperature reached and the duration of exposure. Most plant cells die if heated to temperatures between about 122 to 131° F (50 to 55° C) (Wright and Bailey 1982). Plant tissue withstands heat in a time-temperature dependent manner. Mortality can occur at high temperatures after a short period (Martin 1963), while death at lower temperatures requires a longer exposure (Ursic 1961). Additionally, some plant tissues, particularly growing points (meristems or buds) tend to be much more sensitive to heat when they are actively growing and their tissue moisture is high, than when their moisture content is low (Wright and Bailey 1982). The concentration of other compounds that vary seasonally such as salts, sugars, and lignins may also be related to heat tolerance of plants. Plant mortality depends on the amount of meristematic tissues killed. Susceptible tissue may not be exposed to heating by fire because it is protected by structures such as bark or bud scales, or is buried in duff or soil.

Plant mortality is often the result of injury to several different parts of the plant, such as crown damage coupled with high cambial mortality. Death may not occur for several years and is often associated with the secondary agents of disease, fungus, or insects. The resistance of plants to these agents is often lowered by injury, and wound sites provide an entry point for pathogens in conifers (Littke and Gara 1986) and hardwoods (Loomis 1973). A plant weakened by drought, either before a fire or after wounding, is also more likely to die.

Aerial Crown Mortality

A woody plant’s structure affects the probability that the aboveground portion will be killed by fire. Important aerial crown characteristics include branch density, ratio of live to dead crown material, location of the base of the crown with respect to surface fuels, and total crown size (Brown and Davis 1973). Height enhances survival, as the aerial portions of small stature plants are almost always killed. Species of trees that self-prune their dead lower branches, such as red pine, are less likely to have a fire carry into their crowns (Keely and Zedler 1998). Small buds are more susceptible to lethal heating than large buds because of their small mass (Byram 1948; Wagener 1961). Large buds, such as on some of the pines, are more heat resistant. The small diameter twigs and small buds of most shrub species make them fairly susceptible to fire. For conifers, long needles provide more initial protection to buds than short needles that leave the bud directly exposed to heat from the fire (Wagener 1961). Whether leaves are deciduous or evergreen affects crown survival in that deciduous trees are much less susceptible during the dormant than growing season.

In order for the aerial crown to survive fire, some buds and branch cambium must survive. For conifers with short needles and trees and shrubs with small buds, crown scorch is often equivalent to crown death because small buds and twigs do not survive (Wade 1986). The upper portions of the crown may survive on taller trees. Large buds shielded by long needles can survive fires that scorch adjacent foliage (Ryan 1990; Wade 1986). The large shielded buds of ponderosa pine, lodgepole pine, western white pine, and western larch can survive at a 20 percent lower height than that where foliage is killed (Ryan 1990). Crown consumption is a better indicator of crown mortality than scorch for fire-resistant conifers such as longleaf pine, which has long needles, large well protected buds, and thick twigs (Wade 1986). Crown characteristics that affect survival of trees after fire are listed in Table:Tree Characteristics Important to Surviving Fire and an Overall Species Resistance to Fire Rating.

The scorching of a tree crown is primarily caused by peak temperature heat fluxes associated with the passage of the flaming fire front (Van Wagner 1973). Long-term heating caused by burnout of fuel concentrations after the flaming front has passed can also scorch crowns. Whether the heat generated by fire is lethal to foliage also depends on the ambient air temperature (Byram 1958). For example, at a 90°F air temperature without wind, the height of foliage scorch can be approximately 25 percent higher than it would be at 77°F, because at higher air temperatures less additional heat is required to raise the foliage temperature to a lethal level (Albini 1976). Scorch is also affected by the degree to which heat is dissipated by wind (Van Wagner 1973). In western conifers, the percent of crown volume with scorched foliage is a better predictor of crown mortality than scorch height because it is a better measure of the amount of remaining live foliage (Peterson 1985). In southern pine species, nearly all trees can survive 100 percent crown scorch except during the fall when survival is about 95 percent (Wade 1985; Weise and others 1990). Heat-caused needle damage is detectable within a few days, sometimes within hours, and becomes more obvious over the next several weeks (Ryan and Wade 2000).

Stem Mortality

In fires where aerial crowns are not burned, trees and shrubs can be killed by girdling, caused by lethal heating of the cambial layer, the active growth layer just beneath the bark. Fire resistance of tree stems is most closely related to bark thickness, which varies with species, tree diameter and age, distance above the ground, site characteristics, and health and vigor of the tree (Gill 1995). Some species with thin bark have a fairly thick collar of bark at the base of the bole (Harmon 1984).

The insulating quality of bark is also affected by its structure, composition, density, and moisture content (Hare 1965; Reifsnyder and others 1967), factors that vary among species. For example, among central hardwoods, bark of silver maple has a high specific gravity and thermal conductivity, and can transmit heat to cambial layers in less time than bark with a low specific gravity and conductivity, such as bur oak and eastern cottonwood (Hengst and Dawson 1994).

Flame length (Brown and DeByle 1987), flaming residence time (Wade 1986), and stem char height (Regelbrugge and Conard 1993; Regelbrugge and Smith 1994) can be related to the amount of mortality of thin-barked trees. The cambium layer of thin-barked trees such as lodgepole pine and subalpine fir is usually dead beneath any charred bark (Ryan 1982). For Northwestern conifers in natural fuel situations, minimum bark thickness associated with consistent tree survival is about 0.39 inches (1 cm) (Ryan 1990). Wade and Johansen (1986) noted that bark as thin as 0.5 inch (1.25 cm) could protect young loblolly and slash pines during dormant season fires with low fireline intensity. A summary of tree bark characteristics related to fire survival is in Table:Tree Characteristics Important to Surviving Fire and an Overall Species Resistance to Fire Rating.

Cambium that grows beneath thick bark layers typically found on mature Douglas-fir, western larch, and ponderosa, Jeffrey, longleaf, slash, and loblolly pines is insulated from heat released by the flaming front. However, the cambium can be killed by long-duration heating, such as from burnout of logs and smoldering combustion in deep litter and duff layers. Complete basal girdling is generally only caused by smoldering ground fires because the amount and distribution of dead woody fuels is rarely adequate to lethally heat the entire circumference of a thick-barked tree (Ryan and Reinhardt 1988). The deeper the basal mound of dry duff that is consumed, the more likely that tree cambium is killed (Harrington and Sackett 1992; Ryan and Frandsen 1991). In thick-barked trees, crown injury is more often the cause of mortality than bole damage (Ryan and Reinhardt 1988).

Fire scars occur where the cambium is killed and often are not evident until the dead bark sloughs from the tree (Smith and Sutherland 1999). Because charring doesn’t happen unless the bark actually burns, charring often doesn’t occur until a subsequent fire burns the exposed surface. Once tree cambium is injured by fire or mechanical damage, it is often more susceptible to additional fire scarring, both because the bark is thinner near the scar, and because of pitch that is often associated with wounds. Fire scars can become infected by wood-inhabiting microorganisms including decay fungi. The survival of chestnut and black oaks after surface fires in Eastern hardwood forests has been attributed to their ability to rapidly and effectively compartmentalize the wound, forming a boundary around the injured and decayed tissue that reduces the spread of infection (Smith and Sutherland 1999).

Many large hardwoods survive fire but have charred bark on the lee side, which in thin-barked species is a telltale sign that the underlying cambium has been killed. Even though the bark often remains intact for 1 or 2 years, the damaged sapwood begins to decay, reaching the heartwood in several years and then progressing upward at a more rapid rate. Height of decay is directly correlated to age of wound (Kaufert 1933). On fast-growing bottomland hardwoods, wounds less than 2 inches (5 cm) wide usually heal over before rot enters, but larger wounds are nearly always infected, ruining the butt log (Toole 1959). Decayed sapwood disintegrates rather quickly, creating the hollow found on many old growth hardwoods in the South. Most hollow trees also develop an enlarged buttress. Toole (1959) found that bottomland hardwoods that initially survive fire suffer considerable mortality over the next several years from breakage of decay weakened stems. Loomis (1973) presented methodology for predicting basal wound size and mortality to surviving trees in oak-hickory stands.

Root Mortality

Structural support roots growing laterally near the surface are more susceptible to fire damage than those growing farther beneath the surface. Roots found in organic layers are more likely to be consumed or lethally heated than those located in mineral soil layers. The locations of structural roots are summarized for important tree species in Table:Tree Characteristics Important to Surviving Fire and an Overall Species Resistance to Fire Rating.

Feeder roots collect most of a tree’s water and nutrients, are small in diameter, and are usually distributed near the surface. Feeder roots located in organic soil layers are more subject to lethal heating and consumption than those located in mineral soil. Loss of feeder roots may be a more significant cause of tree mortality than structural root damage (Wade 1993). Feeder root death may not always kill the tree, but it can place the tree under significant stress. Increased amounts of root damage can result from fires that smolder in accumulations of litter beneath trees (Herman 1954; Swezy and Agee 1991; Wade and Johansen 1986). This can be a critically important factor if most of the feeder roots are located in thick duff layers, caused by the exclusion of fire or a regime of dormant season prescribed burning that consumed hardly any surface organic matter. There may be enough root injury or death to kill trees and shrubs, even though little or no damage is apparent to their aerial crowns (Geiszler and others 1984). While tree crown mortality can be related to fireline intensity, mortality of buried plant parts depends much more on the duration of all phases of combustion that regulates the downward heat pulse, than on the duration of the flaming front (Wade 1986).

Fire Resistance

Tree resistance to fire generally increases with age. Crowns become larger and for some species, the height to the base of the live crown increases, either from self pruning or removal of basal branches by surface fires. Bark thickness and stem diameter increase. A suppressed tree may develop fire resistance characteristics at a much slower rate than a vigorous tree of the same age and species resulting, for example, in a much thinner bark in suppressed loblolly pine (Wade 1993). The growth stage at which important species of trees become fire resistant and the degree of resistance of mature trees are summarized in Table:Tree Characteristics Important to Surviving Fire and an Overall Species Resistance to Fire Rating.