How Seasonal Influences Affect Plant Response to Fire

Carbohydrates

Carbohydrates, primarily starches and sugars, are manufactured by plants and provide energy for metabolism, and structural compounds for growth (Trlica 1977). Energy and material needed for initial plant growth following fire are provided by carbohydrates stored in undamaged plant parts, usually belowground structures. The timing of a fire, and its relationship to a plant’s carbohydrate balance, can be a factor in postfire recovery because the rate and amount of regrowth is related to carbohydrate reserves (Trlica 1977).

The importance of carbohydrate reserves to plant regrowth after fire depends on survival of photosynthetically capable material, such as leaf blades and sheath leaves on grass stubble (Richards and Caldwell 1985). If some photosynthetic tissue remains or new tillers rapidly regenerate, newly grown leaf material soon manufactures all the carbohydrates that the plant needs for growth and respiration (Caldwell and others 1981). Evidence from clipping and grazing studies has shown that the recovery of grass plants is more related to the removal of growing points than to the carbohydrate level at the time of defoliation (Caldwell and others 1981; Richards and Caldwell 1985). However, fire may have a greater impact on grass plants than severe defoliation because it kills all photosynthetic material and elevated meristems. New growth must be supported by stored reserves.

There is a seasonal cycle of depletion and restoration of total nonstructural carbohydrates related to the growth cycle of the plant. The most rapid depletion usually occurs during periods of rapid growth, but carbohydrates may also be used for flower and fruit development, cold-acclimation (“hardening off” for winter), respiration and cellular maintenance during winter dormancy, and warm weather quiescence (Trlica 1977). Restoration of carbohydrates occurs when production by photosynthesis exceeds demands for growth and respiration. The timing of fluctuations in the annual cycle of total nonstructural carbohydrates (TNC) differs among species because of variability in plant growth cycles and growing season weather (Zasada and others 1994).

The limited survival of chamise sprouts after spring prescribed fires has been attributed to low winter and spring carbohydrate reserves because of high spring demand for growth, flowering, and fruiting (Parker 1987b). Number and dry weight of shoots of salmon-berry were lowest on rhizome segments collected from May through July, which was also the seasonally lowest level of stored TNC (Zasada and others 1994). Salmonberry is most susceptible to physical disturbance during this time (Zasada and others 1994). For other species, the effects are most negative if the plant is burned late in the growing season because the plant uses a considerable amount of stored carbohydrates to sprout, but does not have enough time to restore reserves before winter dormancy (Mueggler 1983; Trlica 1977).

Severely burned chamise root crowns produced fewer sprouts than plants that experienced less heating, probably because more dormant buds were killed. Subsequent death of plants and limited sprouting may occur because insufficient carbohydrates are produced to sustain the root mass (Moreno and Oechel 1991). Root system dieback after excessive defoliation (Moser 1977) is considered to be a significant cause of plant mortality in grasses.

Repeated burning during the low point of a plant’s carbohydrate cycle can increase any negative effects of treatment. Reduced density, canopy cover, and frequency of Gambel oak in southwestern Colorado, after two summer burns 2 years apart, were attributed to an inability to restore spent carbohydrate reserves for the 9 months after top-killing and resprouting (Harrington 1989). In the Southeast, annual summer burning nearly eliminated understory hardwood vegetation in a loblolly pine stand (Waldrop and Lloyd 1991). Burning when carbohydrates were low eventually killed or weakened root systems. Annual winter burning resulted in significant increases in numbers of small diameter sprouts on these same plots, because burning occurred when reserves were fairly high and sprouts had a full growing season to restore reserves before the next treatment.

If burning occurs in close association with heavy use of the plant community by livestock or wildlife, either before or after the burn, plant recovery may be delayed or prevented because of the excessive demand on stored reserves. Heavy postfire grazing or browsing of perennial plants in the first growing season after a fire is likely to cause the most harm, particularly in arid and semiarid range communities (Trlica 1977).

Flowering

Burning has long been used as a tool to enhance flower and fruit production of blueberry. Flowering of grasses such as pinegrass and wiregrass has been noted to increase significantly after burning (Brown and DeByle 1989; Uchytil 1992). Burning during the growing season of April to mid-August causes profuse flowering of wiregrass in Florida, a marked contrast to a paucity of flowering that follows dormant season burning (Myers 1990b). Warmer soil temperature resulting from litter removal in these months may be the flowering stimulus (Robbins and Myers 1992). Increased light resulting from removal of the chaparral canopy stimulates flowering in golden brodiaea, a perennial forb that produces only vegetative growth in the shade (Stone 1951). This has also been observed in the Northern Rocky Mountains, in heartleaf and broadleaf arnicas, showy aster, and pinegrass. Increased availability of soil moisture and soluble nutrients also stimulates increased flowering.

In response to late spring fires, Henderson and others (1983) observed significantly greater flowering in big bluestem, little bluestem, sideoats grama, and Indian grass, all Wisconsin warm-season grasses. In-creased flowering was attributed to higher levels of carbohydrate production caused by improved growing conditions, such as mulch removal. Grass flowering and seed production draw heavily upon carbohydrate reserves. Higher net photosynthate production was observed in big bluestem after spring burning. In contrast, cool-season grasses that were actively growing during late spring experimental fires showed a marked reduction in flowering, possibly because growth initiated after the fires further depleted carbohydrate reserves already drawn down by early growth. There also may have been more damage to meristematic tissue because plants were actively growing at the time of burning.

Fires enhanced flowering of dominant forb and shrub species in longleaf pine forests on the Florida panhandle (Platt and others 1988), with the most significant effects resulting from growing season fires. These fires increased the number of flowering stems, decreased the average flowering duration per species, and synchronized the period of peak flowering of herbaceous plants, particularly fall flowering composites with a clonal growth form. Fire killed the elevated apical meristems, which no longer suppressed dormant buds on rhizomes, roots, and stolons. Multiple stems were initiated from these buds at times of the year when photoperiod strongly induced flowering. Dormant season fires had little effect on flowering periods because apical meristems were located at or below the ground surface, were little affected by fire, and continued to suppress secondary meristems the next growing season.

These mass flowering events are a means by which plants that regenerate after the fire redistribute themselves within the stand. For plants that germinate from soil stored seed, their profuse flowering in the first few years after fire resupplies the seedbank and ensures their presence after the next fire (Stickney 1990).

Phenology

Plant growth stage at the time of a fire can result in different plant responses. Fire effects can vary substantially during a specific season, such as spring, because several phenological stages can occur in that 3 months. Phenology and the accompanying variation in plant condition, not season, leads to observed differences in plant response to fire. Phenological differences that affect plant responses to fire include varying levels of stored plant carbohydrate, presence of elevated herbaceous meristems that are more susceptible to fire because of their location, and presence of actively growing tissues that are more sensitive to high temperatures than when they are dormant or quiescent. The seasonal growth pattern that is characteristic of each species can be significantly modified by temperature and moisture in a specific year.

As an example of seasonal influences, ponderosa pine trees scorched in late October survived higher percentages of crown damage than trees scorched in early June and mid-August. The increased survival of fall burned trees was attributed to reduced physiological activity, lower bud tissue moisture contents, bud protection by fully developed bud scales and needles, and replenished carbohydrate stores, allowing adequate reserves to support spring shoot and root growth (Harrington 1987a, 1993).

Phenology also affects flammability. Moisture content of 1 year and older foliage of Western conifers is lower in the early part of the growing season than later in the summer (Chrosciewicz 1986; Jameson 1966; Philpot and Mutch 1971), and may contribute to higher spring crowning potential (Norum 1975). Seasonal differences in the moisture content of surface vegetation can determine whether the vegetation is a heat sink or is dry enough to be a heat source and thus contribute to fire spread. Seasonal curing of herbaceous vegetation changes it from live to dead fuel.

Burning Conditions

Fuel and soil moisture conditions have a major influence on upward and downward heat flows that affect plant responses. Seasonal fluctuations in temperature and precipitation cause a progression of moisture content in dead woody fuels, litter, duff, soil organic layers, and soil. For a given vegetation and fuel type, burning conditions vary seasonally according to a general pattern; and the response of individual plant species to fires occurring under typical seasonal fuel and soil moisture conditions are fairly predictable based on their life-form.

Yearly variations in weather and associated departures from average moisture conditions can cause substantial variation in fire behavior and fire effects. For example, a winter and spring of above average precipitation results in wet woody fuels and duff in higher elevation forests that limit fire spread and fuel consumption. Below average precipitation in winter and spring can create dry enough conditions in forests to create potential for fires with high fuel consumption, and significant amounts of heat release both above and below the surface. Dry large fuels, duff, and mineral soil increase the potential for significant amounts of surface and subsurface heating, with concomitant mortality of roots, buried regenerative structures and seeds, and tree cambium.

Differences in seasonal weather in shrub/grass types can result in a large range in grass production, particularly annual species, which creates different fire behavior potentials. A wet year in the Great Basin leads to much more herbaceous biomass in sagebrush/ grass communities, a greater likelihood of ignition, and larger sizes of fires that do occur. However, whether these higher fuel loadings relate to greater consumption of basal fuels and higher mortality of bunch-grasses and sprouting shrubs has not been documented.

The pattern of fire effects across the landscape varies with burning conditions. Areas of tree crown consumption, crown scorch, and little crown damage can be intermixed. Heavily burned areas of the forest floor where significant amounts of fuel were consumed and most buried plant parts were killed can be adjacent to areas where prefire fuel loading was low, and little subsurface heating occurred. On rangelands, the pattern can vary between areas of significant heat release associated with consumption of shrubs and accumulated litter and other areas where little heat was generated due to sparse fine fuels.

During a dry season, especially in a drought, a much higher percentage of forest canopy is apt to be scorched or consumed. Lethal temperatures may be driven to greater depths because fuel and duff consumption is fairly complete. During a wet year or early in the year before significant drying has occurred, less canopy will be killed and consumed and few buried plant parts will be killed.