Fire Ecology

Fire ecology-Plant Mortality

Wildfire in North America is a natural phenomenon that burned millions of acres, years before European settlement. The relationship between ecosystems and wildfire have evolved in which that many species of plants and other processes depend on the natural disturbance to regenerate and keep a healthy ecosystem. Fires react differently depending on the environmental factors such as the climate, vegetation, and even by the health of the site like build up of fuel loads, or insect invasion. Depending on some of these factors fires can burn a very small area or can completely torch a large one. Many Plant species are impacted by fire for many reasons. There are some dominating plant species that take over the rest of the plants in the specific area of the fire. With frequent fires, these dominating plants are substantially reduced and provide room for the less dominating and sometimes more palatable species. Build up of woody debris or even an abundance of herbaceous ruminants can reduce germination due to poor light. Burning these areas can provide light for new growth and reduce fuel loads. With smaller fuel loads, fires are less intense. These frequent low intensity fires are important for our rangelands. When talking about "Fuels" having to do with fire ecology it is referring to the live and dead vegetation that potentially contributes to the fire. The quantities of fuel can differ, from a small portion, or all of the aboveground biomass, but this depends on the number of fuel properties but most importantly particle size, moisture content, and arrangement. Even though the increasing of vegetation biomass can be predictable, changes in fuel biomass over time can differ due to the transaction between growth and decay affecting fuel availability. (Brown 2) Fuel and soil moisture conditions have a major influence on upward and downward heat flows that affect plant responses. The Seasonal fluctuations in temperature and precipitation cause a progression of moisture content in dead woody fuels, litter, duff, 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 considerable variation in fire behavior and fire effects. For example, a winter and spring of above average precipitation results in wet woody fuels and duff that are in higher elevation forests which 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 with concomitant mortality of roots, buried regenerative structures and seeds, and tree cambium increase the potential for significant amounts of surface and subsurface heating. 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 thin fine fuels. During a dry season, especially in a drought, a much higher percentage of forest covering is apt to be scorched. 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 covering will be killed and consumed and few buried plant parts will be killed.(Miller 27,28) The chance of plant tissue being killed by fire depends upon the amount of heat it receives. The heat the plant receives depends on how great the temperature and for how long the plant is exposed to it. Most plants die if heated to temperatures between about 122 to 131 oF. Plant tissue withstands heat in a time-temperature dependent manner. Some plant tissues mainly growing points are more sensitive to heat when they are growing and their tissue moisture is high. Some other compounds of plants that might relate to their heat tolerance are salts, sugars, and lignins. Plants survival depends on the amount of stem tissues killed and this 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. The Plants survival also depends on where the damage is, such as crown damage. The Death of the plants might not occur till several years after its damage, the death is eventually caused by disease, fungus, or insects. The defense for plants against these agents is lowered by injury, and the site of damage provides an entry point for pathogens in conifers and hardwoods. Also plants that are weakened by drought, before or after fire wounding, are more likely to die. (Miller9) The survival of a plants stem varies on the conditions of the plant, sized protection, and location. Plants with a hard structure help the chances of its aboveground portion being killed by fire. Some important characteristics of aerial crowns are the branch density, number of live to dead crown material, location of the base of crown, and its complete size. One important factor for increasing survival is the height, most aerial parts of small plants are usually killed. Some species of trees that self-prune their dead lower branches for example red pines are less probable in having fire reaching their crown. Large buds like ones on the pines are more heat resistant. Small buds are more affected by fire because of their small mass. The upper portions of the crown may survive on taller trees. Large buds shielded by long needles can survive fires that scorch adjacent plant life. "The large shielded buds of ponderosa pine, lodge pole pine, western white pine, and western larch can survive at a 20 percent lower height than that where plant life is killed." For a tree crown to be scorched it is mainly caused by extremely high temperatures and the heat fluxes with the passage of the fire front. Long-term heating caused by burnout of fuel concentrations after the fire front has passed can also scorch crowns. Also it depends if the heat generated by the fire is lethal to plant life because of the ambient air temperature. "For example, at a 90

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