Let's Talk About Some of the Methods for Measuring Fire History. These Are Characterized Using Natural and Human Archives
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Let’s talk about some of the methods for measuring fire history. These are characterized using natural and human archives. 1 What gets there and survives there will be there post-fire. 2 The temperature experienced by plant cells is the ultimate cause of death. Plant parts can be heated directly during combustion, or they can be indirectly affected by heat altering biochemical pathways. At any given temperature, the effects of heat depend upon the length of heat exposure and the hydration of the cell. Dehydrated cells (dormant) can endure more heat than hydrated and metabolically active cells. I would also like to mention here that not all plant tissues are equally important in determining the mortality of an individual. Due to the modular construction of plants they can tolerate a loss of a substantial amount of biomass and still survive. The meristematic tissues, which are those tissues from which new tissue grows, such as cambium and buds, are especially important for survival of plants. 3 This figure shows the effects of heating to plant tissue and the different pathways leading to types of injury. You will note that it is divided into direct and indirect effects. Note that chemical decomposition pathway can be reached by both complete combustion (often called consumption) and through transmitted heat (called scorch) and in many cases will be caused by a combination of both combustion of plant materials and the heating of materials. Another important pathway is the severing of vascular connection. This can occur when a stem is heated enough to prevent the transportation of water and nutrients and leads to starvation. As you can see from this list there are many other pathways which can also cause injury to plant tissue. 4 As we mentioned earlier the plant response to temperature is affected by the resistance to heat (hydration) and it’s protection from the heat source. The ability of individual cells to survive heating appears not to vary much among species or between tissue types. Most plant material will ultimately die when it reaches a temperature of 60 C (140 F). Therefore protection of key tissues is a key factor in determining plant survival. Living plant tissue is protected from fire in one of four ways. First, thick bark can shield meristematic tissue in the cambium from heat. Second, meristems may be deep enough in the soil where they are subjected to enough heating for long enough to reach lethal temperatures. Third, if the sensitive tissue is high enough above the flames and hot, dry air they can survive. Fourth, it is possible that the tissue isn’t exposed to heat because the fire didn’t come close enough. Thus, seeds might survive because they are inside cones that are not heated long enough for the heat to penetrate to tissues, they may be buried in the ground or held high in the canopy. Or, it might be in a patch that didn’t get burned. 5 The thickness and thermal thermal properties of bark are the major factors determining the length of heat exposure that trees can take before their cambial damage is killed because it reached lethal temperature. Larger diameter trees have thicker bark, so smaller trees are generally more more susceptible to heat than larger plants. Ponderosa pine and western larch have thicker bark that is better at insulating cambium than aspen bark. 6 The total amount of heat, and especially the duration of heating, influences tissue mortality. When there is more fuel available to burn, e.g when logs and duff are dry in the late summer and fall, fires may have a long residence time. Smoldering fires, such as occur from backing fires may have more impact on the living tissue in tree stems and roots, and on the seeds and buds that are the source of regrowth after fire for grasses, shrubs, and forbs. A rapidly moving high intensity fire may readily top-kill grasses, forbs, shrubs and small trees, but is not likely to damage tree boles and roots on large trees shielded by thick bark. Fire scars happen when there is sufficient duration of heating, usually from smoldering combustion, to overcome the insulating capacity of the bark. Often fire scars form on the uphill side of trees because fuels often accumulated there, and because the wind eddies around the uphill side of trees, maintaining glowing, smoldering combustion. Fire scars generally form where heating is sufficient to kill a portion of the cambium, typically on the uphill side of the tree due to fuel accumulations and from heating caused by eddies as the hot gases circle the trees. Another factor which can affect bark heating is the time between fires. Gill (1980) found that it took over 7 years for smooth-barked Eucalyptus to recover. Thus if fire was to occur before the plant had enough time to recover after a fire there is a higher probability that the tree’s cambium will experience lethal temperatures. 7 Vegetative insulation is the protection of meristems by other plant material. For example bunchgrasses have meristems protected by thick matted grass leaves as shown here in Arizona fescue. In addition to the protection of the grass leaves, recall that most grasses have meristematic tissues located belowground thus providing even more protection since heat rises. Leaf arrangement can protect the buds. For example longleaf pine, as shown here on the left, has a needle arrangement and tissue paper wrapped around the buds as well as larger buds which both help protect the tissue from heating. In many cases the needles will be scorched during fire while the tissue paper around the bud remains untouched by heat. In some cases the entire plant architecture may be orientated towards fire protection. For example, it is thought that the entire tree canopy in deflects heat away from the apical bud for some tree species in Africa. 8 Bark is not the only line of defense against high temperatures in shrubs and trees. Many plants sacrifice meristematic tissue in the canopy but nevertheless tolerate fire by sprouting from previously suppressed underground buds. Re-sprouting can occur from adventitious buds or from latent axillary buds. Adventitous buds can be found almost anywhere on the plant such as along the stem in eucalypts or from root buds as in aspen. Lignotubers are specialized root-crown structures which help protect the plant tissue from heating and also contain a large amount of starch reserves. Thus they provide a dual role, as they protect the plant tissue and they provide energy for sprouting. The ability of a plant to sprout from its lignotuber after a fire is dependent upon the duration of heating, fire frequency and the physiological status of the plant. For example, if buds are not heated to lethal temperature, they will likely be stimulated to grow by the changed post-fire environment – warmer soils, more light, fewer of the plant hormones that suppressed them. The season the fire will influence the growth and survivability of the regeneration after disturbance. In the left hand picture shown here you can see sprouting occurring from the root collar of a young shortleaf pine that was killed by fire. In the middle picture you can see hardwoods resproutign from lignotubers after a prescribed burn. And in the top right picture you can see resprouting from the stem due to injury from ice. 9 As height above the flame increases, we see a decrease in the temperature. Seeds andbdd buds in tllttall trees may surv ive fire w ithtbiithout being hea tdtlthlted to lethal tttemperature. Long leaf pine, shown here, is well adapted to fire. The seedlings stay in the “grass stage” for a long time while roots develop. The terminal bud is well protected from heat by the abundant foliage. Eventually the trees start to grow very rapidly in height. They are very vulnerable to surface fires until the tree is tall enough for the termin al bud to be abov e th e fl am es an d h ot air fr om fl am es. Un til th e seedlin g develops thick bark, it is the cambium is vulnerable if surface fires have sufficient residence time to overwhelm the insulating properties of the bark. Typically fires burn with high rates of spread and low residence time in these forests, particularly when the understory fuel is light, largely consisting of recent pine needle fall and wiregrass foliage. When longleaf pine trees are in the grass stage, most of the photosynthate is allocated to root growth. The growing bud is protected by proximity to the ground and a densely packed needle geometry which reduce the oxygen available for combustion in the immediate area. When height growth resumes, it is very rapid quickly lifting the bud above the flames. At this same time bark thickness also increases. The increase in bark thickness is important because any increases in height for protection are pointless unless the vascular tissue is also protected. Any advantages gained by height in forested areas are related to the distance from the flames. So despite a tree having cones at a height of 80 feet if ladder fuels are present and allow the flames to reach this height the protection is lost, and the plant must rely on some other strategy for survival. This is the concept behind raising crown base heights in some forest operations. The increased distance from the forest floor to the vulnerable meristematic tissue in the buds does not allow fire to reach the canopy and allows the height of seeds and buds to act as a protective measure.