14 Jul2003

Overstory #124 - Fire

Written by Kurt W. Cremer.

Introduction

Effects of fire on plants, animals and soils are numerous, complex and important. In Australia most types of natural vegetation have evolved to cope with fire and are largely dependent on and shaped by fire. Effects of fire include:

overheating and thus killing part or all of the above-ground portions of existing plants,
combustion of plants and litter with the consequent release of nutrients
loss of some of these nutrients through volatilisation and leaching
elimination of some growth-inhibiting chemicals and microorganisms
preparation of seedbeds suitable for germination and rapid early growth of new plants
release of seed from standing plants
stimulation of germination in some species such as Acacia
removal of competing plants and hence the opportunity for new plants to become established
better availability of nutrients and water to those plants that survive the fire with little or no damage
exposure of the soil to erosion
changes in water infiltration and retention

Fire is usually both beneficial and damaging, the net effect depending on circumstances and aims. This article deals only with direct health hazards - injuries caused by overheating, how to cope with them, and how to prevent them.

Background

Most plant tissues are killed at relatively mild temperatures - at 50 deg C after a few minutes and at 65 deg C after a few seconds. The temperatures produced by burning vegetation are far higher -- about 400 deg C for smouldering combustion and up to 1000 deg C or more for flames. They may last from a few seconds in sparse grass to several hours in logs or deep humus.

The two main effects of overheating, scorching of foliage and fire scarring or ringbarking of stems, are discussed below under separate headings. They can occur separately or together. Branches and roots can also be overheated: branches quite commonly, but roots only if they are covered by less than 30-60 mm of mineral soil.

Foliage

Uninsulated tissue such as foliage can avoid overheating only by being either remote or shielded from the flames. Wind can serve to cool foliage and disperse or divert convected heat. The chance of foliage being scorched diminishes rapidly with height. Conversely, the height to which foliage is scorched reflects the intensity of the fire and the height of the flames. Dense foliage from lower crowns or adjacent trees can provide valuable shielding from radiation. Similarly, bark on branches is often killed only on the lower side that faced the fire and was heated by both convection and radiation.

Bark

Insulated tissues, such as the cambium of tree trunks, can escape overheating even if exposed to fire at close range, depending on the amount of insulation and the duration of the fire.

The protective value of bark depends mainly on its thickness, irrespective of type or moisture content, provided that the bark itself does not burn away. Thin stems (less than 20 mm) of all species are highly vulnerable to any fire because their bark is very thin. But as the stem thickens, so does the bark, and many species eventually become highly resistant, except where fires are very prolonged.

Duration

of heating Some experimental results (McArthur 1968) indicate that if the surface of bark is kept at 1000 deg C, the cambium will be killed in about 2 minutes if the effective bark thickness is 10 mm, in about 6 minutes for 20 mm thick bark and about t3 minutes for 40 mm thick bark. With lower temperatures at the surface of the bark, the cambium takes longer to heat.

In dry grass, flames persist at any one point for about 4-20 seconds and in eucalypt litter for about 60-150 seconds, depending on the quantity and arrangement of the fuels. A mound of humus or logs heaped against a tree's trunk can burn for hours.

The risk of damaging a tree's trunk thus depends as much on the thickness of its bark as on the amount and type of fuel near the trunk. Generally, trunks have to be well over 10 cm in diameter before they will stand prolonged fires, and even the thickest bark can be killed if much fuel is heaped against it.

Effects of injury

Foliage and branches

Any direct damage to foliage will be noticeable as a grey or brown discoloration, sometimes within hours, always within a day or two after the fire. In the mildest form of damage, only parts of the leaf blade are damaged; the stalk and the supporting branch survive, but some of the associated exposed buds are killed. The leaf may be retained, or it may be shed in a few days as a result of an abscission layer formed at the base of its stalk (petiole). More severe scorching causes the whole of the leaf to turn brown. In this case, the petiole and thin twigs are also killed, abscission cannot take place, and the leaf is retained for months. The damage is most severe if the leaves and twigs are charred or even consumed. Sometimes all levels of severity can be found on a single long crown, with the lowest foliage consumed and the topmost foliage still green.

Damage to branches does not become obvious until the bark cracks and falls away, several months after the fire. Severity of scorching of foliage provides a much earlier guide to severity of damage to branches. Limbs more than 10 cm in diameter tend to survive all but the most severe types of scorching.

Loss of foliage is rarely lethal by itself. It is the accompanying loss of buds that can be critical. If some foliage remains green, some buds will also have survived, even if the buds are unprotected; the tree will probably survive, unless it has been girdled. If all the foliage is browned or consumed, the unprotected buds will also have been killed and the tree can survive only if it possesses buds (or tissue able to produce buds) well protected within thick bark. Radiata pine and most other conifers do not have such protected buds and are killed by any complete scorching. The tops of most acacias are not much better off, but most eucalypts are.

In some situations, even trees with surviving bud tissues cannot recover from a complete scorching. This is probably because they lack the food reserves to start off new shoot growth. Such inability to recover is more likely after a complete scorching in autumn than in spring, and is most likely on sites where growth is rapid.

Many trees and shrubs have the ultimate in protection for some of their buds -insulation by soil. They can be killed right back to ground level but remain able to sprout again from buds located on stumps, lignotubers or roots well within the soil.

Loss of foliage will tend to result in a corresponding loss of growth, till the foliage is restored. This tendency may be offset by an increased availability of nutrients and water. Normally, a severe scorching with much delay in crown recovery will reduce growth for several years. The loss of limbs or part of the trunk may degrade the stem permanently.

Scarring and girdling of stems

Girdling here means that the bark is killed to the wood surface around the whole circumference of the stem. This has the same effect as ringbarking, i.e. removing a girdle or ring of bark. The food made by the leaves can no longer reach the roots, and these will starve unless other surviving trees are connected to the same root system, or new shoots sprout from below the girdle, either from the stem or the roots. Even if the roots continue to be fed from elsewhere, the top of the girdled tree will die after some months or years because water conduction past the girdle eventually fails.

Fire scars and dry sides at the butts of trees are major causes of degrade in wood, and of the eventual decline and collapse of trees. Often a fire does not kill any cambium but only reduces the effective thickness of the bark. Recovery to full bark thickness may take 3-20 years after the fire, depending on the amount of original bark lost. Frequent severe fires therefore make trees more and more vulnerable to scarring and girdling, even in species that are normally highly resistant. This is important to remember where fuels recover quickly after burning, e.g. in areas with dense bracken or bladey grass.

Preventing fire damage

In many environments it is rarely possible to ensure that a particular area will never be burnt. Usually a balanced judgment needs to be made on how to manage the fire risk; it is necessary for compromises to be made. Strategies may range from attempting 'total exclusion' of fires to deliberate burning every few years. In the wet forests of mountain ash (Eucalyptus regnans) with dense woody undergrowth, for instance, natural fires tend to occur only rarely, less than once a century, but to be extremely destructive when they do occur. The aim here usually has to be total exclusion of fires. In dry forests of jarrah (Eucalyptus marginata), on the other hand, the climate and nature of the undergrowth are such that fires could occur almost every 3 or 4 years, but the bigger trees are well adapted to withstand brief fires of low to moderate intensity. In this situation, the more destructive fires can be avoided by reducing fuel accumulation through deliberate burning every 5-10 years. But remember, young trees, with stems less than about 10 cm in diameter, can rarely, if ever, be burnt even by the mildest fire without suffering excessive damage.

The following discussion is about reducing the potential destructiveness of fires in farm situations by managing the fuels to reduce the intensity and duration of fires and by managing the trees to resist fire damage. Carrying out controlled burns or preventing and suppressing fires are separate major topics covered in other books, such as (Luke and McArthur 1978).

Managing the fuels

The main aim is to avoid having fuels that produce tall fires that could scorch the foliage of desirable trees, or cause prolonged fires in which the bark might be killed.

Areas that are particularly hazardous should be avoided for tree planting or should be planted with fire-resistant species and treated with special care. Variation in the natural frequency of fires is a major reason for variations in type and quality of forests in Australia. The forest types, in turn, strongly influence the likelihood of fires. These are the main hazards:

- Uncontrollable ground vegetation that produces highly flammable fuels, e.g. dense bracken, or tall dry grass.
- A likelihood of ignition (e.g. by careless people, machinery)
- A location downwind and upslope of likely points of ignition, especially on sites exposed to fire-weather winds (e.g. W to NW in southeast Australia). A doubling of windspeed tends to treble the rate of fire travel. Strong winds have the main influence on speed and direction of fire movement, but slope is also important. On slopes of 10-20 degrees, fires travel respectively two and four times faster than on level ground, if the slope faces the wind. Conversely, vegetation located in fire 'shadows' burns relatively rarely.
- Sites with difficult access (rocky, steep), which makes fuel management and fire suppression difficult.

Fuels can be reduced by grazing, mowing, cultivating, weedicide application, controlled burning or any practice that influences what grows on the site, including choice of crops or weeds and fertilising. Halving the amount of fuel in a pasture (5-10 t/ha on good to very good sites) reduces the intensity of the fire by about 50%, but may not affect its rate of spread.

The main qualities that affect flammability are moisture content, oil or resin content, salt content, and thickness of fuel 'particles'. Drier fuels ignite and burn more readily because less energy is needed to heat and vaporise the water. Therefore, green plants are safer than dry ones. Many plants will, however, burn when green, because of high oil content (e.g. Eucalyptus sp., Melaleuca spp.), provided the fire is supported by dry fuel. The green foliage of many conifers burns because of high resin content. High salt content (e.g. saltbush, tamarisk), on the other hand, reduces flammability. Thinner 'particles' are more flammable because they dry faster, are heated to ignition point more easily, and are more accessible to oxygen. Dead grasses readily change in moisture content, and hence flammability, as the humidity of the air changes. Pastures may burn when they contain at least 50% dead material but are rarely dangerous before at least 90% of the material is dead.

Arrangement of fuels

Combustion is inhibited if the fuel particles are spaced so widely that they cannot heat and ignite each other, or if they are spaced so closely that the fire is starved for oxygen. A dense stand of grasses can burn explosively if left standing, or mildly if it is compacted by mowing or rolling.

Discontinuity in the arrangement of fuels is most important, both horizontally (as in fire-breaks) and vertically. Crown fires are rarely possible unless tall flames from ground fires support them. Trim off the lower parts of crowns, especially any dead parts (foliage, branches, bark) and be sure to remove heavy fuels from near tree stems, especially thin-barked ones.

Compaction of fuels by rolling, trampling, or mowing not only reduces their flammability but also hastens their decay.

Roads should be kept free of flammable fuels to 3 or 5 m widths. In the case of plantations on gentle topography the average compartment should be about 20 ha. There should be more roads on more difficult terrain. External roads, especially, should be located with fire control in mind.

Managing the trees

Choice of species

Look for the following virtues, but do not expect them all in any one species. In most situations, however, productivity is a more important criterion for choice of species than flammability or resistance to fire.

- Live foliage should be difficult to burn, i.e. it should be green and moist, and it should preferably be high in salt and low in oil or resin content.
- Dead foliage, branches and loose bark should be readily shed so that fire cannot climb into the crown and produce sparks and hence spot fires.
- Foliage should preferably be shed only at safe times and decay rapidly: the foliage of temperate deciduous species falls in autumn and is mostly decayed by next summer, while the foliage of hard-leaved evergreens, such as the eucalypts, falls mainly during the fire season, decays slowly, and thus tends to accumulate, together with fallen bark.
- The bark should be thick and difficult to burn, and the species should be able to recover vigorously after loss of foliage by sprouting from buds well protected within the bark.
- The tree should be able to suppress any flammable undergrowth.
- The species should be able to survive drought and other stresses, and grow rapidly, so that thick bark and high crowns can be achieved quickly.

Rapid

early growth The sooner the bark is thick and the foliage high above the ground the better. Thinning, weeding, mulching, irrigating and fertilising promote these objectives. Pruning off the lower limbs is useful, especially if they carry dead material.

Protection strategies

Recently planted trees

If the chance of ignition is very high, do not use flammable mulches and keep the area virtually clear of fuels by cultivating, mowing or weedicide treatment. Remove flammable debris left from previous clearing operations.

Windbreaks

Choose species of low flammability and high fire resistance. Provide a firebreak upwind (road, cultivation, green grass). Minimise the amount of fuels on the ground and in the lower crowns by mowing and pruning.

Plantations

When choosing a species for a plantation, productivity is usually the prime criterion, but fire resistance should also be considered, particularly on hazardous sites. Promote rapid early growth to shorten the most vulnerable stages. Reduce the amount of ground fuels by cultivation or grazing. Pruning serves to improve access and decrease the quantity of aerial fuels. Trees at edges of plantations should probably not be pruned, to keep the wind out. In the short term, pruning increases amounts of ground fuels, but this hazard can be reduced by stacking the debris away from trunks, or by grazing. Fresh needles of radiata pine are often palatable and nutritious for sheep. Alternatively, run a slasher over the prunings to mulch them. The fuel produced by frequent light pruning is less hazardous than that from rare heavy pruning. Culls and tops produced by thinning should be delimbed, so the slash decays rapidly. Once the stem bark is thick and the foliage high above the ground, fuel reduction by deliberate burning can also be considered. Maintain appropriate external and internal firebreaks.

Original source

This article was adapted with the kind permission of the author and sponsoring organization (CSIRO) from:

Cremer, K.W. 1990. 'Fire' In: Cremer, K.W. (Ed), Trees for Rural Australia, Inkata Press, Melbourne, Australia.

About the author

Kurt Cremer is a forest scientist. He graduated in 1957 from the Australian Forestry School in Canberra and gained a Master of Science degree from the University of Tasmania. He started his research in the Tasmanian forest service and finished in 1987 as Principal Research Scientist with CSIRO in Canberra. He published over 50 papers in silviculture, forest ecology, fire ecology, storm damage and, in recent years, on the natural spread of willows in Australia's streams. He can be contacted by email at kcremer@snowy.net.au .