Let s talk about some of the methods for measuring fire history. These are characterized using natural and human archives.

Transcription

1 Let s talk about some of the methods for measuring fire history. These are characterized using natural and human archives. 1

2 What gets there and survives there will be there post-fire. 2

3 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

4 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

5 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

6 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

7 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

8 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

9 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

10 As height above the flame increases, we see a decrease in the temperature. Seeds and buds in tall trees may survive fire without t being heated to lethal l temperature. t 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 terminal bud to be above the flames and hot air from flames. Until the seedling 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. 10

11 Flowering is one stage of plant reproduction that is particularly susceptible to fire. The relationship of season of burning to the plants flowering time can be important, because fire too early would kill flowering buds or developing flowers and thus kill a year s worth of seeds. Plants which sprout are also highly susceptible as it may take several years to recover enough to support flowering again. A few key situations in which the affect of fire on flowering plants are during years where the current flowering will contribute a high proportion of the accumulated, dormant seed bank and where a mast flowering year occurs only infrequently and happens to be preceded by a fire. So the effects of fire are related to the seed longevity in the seed bank and the variation between seed production each year. 11

12 The seed is a state of the plant life cycle which has some built-in advantages to surviving fire. Seeds contain cells in a dormant and often dehydrated state, both of which help provide tolerance to high temperatures. For example the domesticated seeds of peas, sunflowers and wheat have been shown to survive four hours of dry 70 to 90 degree Celsius temperatures. However seeds that are in the leaf litter during a fire can experience temperatures above 110 degrees and therefore would not survive the fire. So the two major means of protection for a seed are burial in the soil and enclosures within the plant s canopy. Gill developed a model of seed mortality and germination where the depth of burial helps seed survival, but if seeds are buried too deep, fewer will germinate. This model is shown here along the right hand side of the slide. You will notice that as fire intensity increases (from panel a to b) seeds are less likely to survive. This model can be used to predict seed germination in relative terms under different fire intensities. For example we would expect reduced germination under a slash pile than in broadcast burn assuming all other factors were equal. 12

13 Seeds can be protected simply by their location within a canopy or by being insulated by fruits. As an example, we could look at longleaf pine forests in the southeastern US. These ecosystems were characterized by frequent surface fires which did not have a major effect on the forest canopy where the seed crop is protected in large cones. Other species of plants such as eucalyptus also have protective fruit structures which can help protect the seed from heating. 13

14 Most of us are familiar with the idea of a seed bank existing within the soil layers, however seed banks can also exist in the canopy of forests. Seeds stored within the canopy in cones are often referred to as bradyspory or (more commonly in the US) serotiny. The classic example of a serotinous species is lodgepole pine (Pinus contorta). Mature seeds in serotinous cones are not released but are retained in cones within the canopy. The seeds can remain viable and alive due to intact vascular connections for many years. When intense wildfires occur, tree canopies may burn, and heat from the fire melts the resin sealing cones so that the cones can then open to release the seed. Once released, the seeds fall into a nutrient-rich ash seedbed that has abundant light and space favorable for germination and establishment. Not all cones within a tree or a stand of lodgepole pine are serotinous, so some cones open as soon as seeds are ripe. Jim Lotan, a scientist who studied lodgepole pine ecology observed that the rate of cone serotiny was very low on when lodgepole pine were growing on pumice soils in south central Oregon and where fires are relatively frequent and not likely to be stand-replacing crown fires. He thought that stands that evolved with fire regimes dominated by stand-replacing fires were more likely to carry serotinous cones, and that the degree of serotiny increased with time since last fire. Do you think that other trees with serotinous cones, such as jack pine, have a similar fire ecology? 14

15 In general the percentage of trees bearing serotinous cones varies greatly by region, elevation, stand age and fire history. Most young trees produce open cones and serotinous cones will not be produced until the trees are 20 to 30 years old. For example in the Canadian Rockies, 80 to 90 % of lodgepole pines bear serotinous cones, while in the northern and central Rockies the percentage of trees bearing serotinous cones in a given stand ranged from 0 to 85 percent and was on average less than 50 percent. Fire history has also been shown to be a major factor in determining the amount of serotiny in a stand. For example in a study near West Yellowstone, Montana, stands initiated by high intensity crown fires (a process which selects for closed-cone traits) had a higher percentage of serotinous trees than stands which were initiated from a non-fire related disturbance. 15

16 Off-site colonizers are plants with seeds that are readily dispersed by wind and animals into burned areas. Fireweed can be found in abundance after fires and other disturbances. The seeds of this plant are only viable for around 18 months so the presence of this species in post-fire ecosystems is due to distribution of the seeds from areas outside the burn. This type of trait may or may not be a specific fire adaptation, since many weedy and pioneering species have high seed mobility. (Information from Fire Effects Information System) Although this trait is not necessarily fire-related, it does lead to the point of local and long range seed dispersion as a source of colonization after fire. So the genetic traits of the plants after fire will be based on the ability of plants to either survive the fire or colonize the burned area. 16

17 Many species will have multiple adaptations to fire. For exampl,e redwoods have both fire resistant bark plus sprouting from the root crown and stem. Other examples include many of the shrubs common in chaparral ecosystem most produce seeds at an early age, have seeds that can survive in soil seedbanks and tolerate high heat and then germinate in abundance post-fire, and have the ability to sprout from lignotubers. Can you think of other species which appear to have multiple adaptations to fire? 17

18 As we end this lecture I want you to think about the strategies different plants use to survive the flaming front of the fire and to colonize burned areas. These strategies are only part of the problem as now the plants have to deal with altered post fire conditions. In the next lecture we will talk about changes in post-fire conditions and both population and community responses to fire. 18