Remote Sensing for Fire Management. FOR 435: Remote Sensing for Fire Management. FOR 435: Fire Effects on Plants

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1 Remote Sensing for Fire Management FOR 435: Remote Sensing for Fire Management 4. Fire Effects on Plants Ecological Effects Spectral Changes Source: Lecture adapted from FOR 434 with permission from Hoffman and Holden FOR 435: Fire Effects on Plants In order to understand what remote sensing methods will be effective at mapping changes due to fire, we must understand how fire affects plants and soil. 1

2 The ability of an individual plant to survive a fire is based on the heat generated by the fire, the post-fire environment, and the physiological / anatomical characteristics of the plant. Plant Characteristics: The ability of the plant to survive the fire itself The ability of the plant to survive in the post-fire environment. The heat regime of a given fire is a function of both the heat released and the duration that the heat is released over. Each of these is driven by how far the plant is from the heat source and the intensity and residence time of the fires. The ability of individual plant cells to survive heating does not vary greatly between different plant species. The majority of plants will due above temperatures of 140 F (50-55 C). The lethal temperature for most plants is reached if they attain at least 60 C for 1 minute in duration. Some plants have defenses. Bark is an effective insulator that can help shield the vulnerable insides of trees from lethal temperatures. The likelihood of attaining a lethal temperature is also dependent on the physiological condition of the cells and the duration of exposure to the heat. In forest fires, air temperature vary greatly. Flame temperatures can exceed 1200 C (2192 F). Air temperatures during fires vary greatly in response to fuel and environmental characteristics as well as with proximity to other objects such as a tree bole. 2

3 Maximum temperatures during fires generally occur immediately adjacent to tree trunks compared to open areas, due to radiation from heated bark surfaces and induced convection currents close to the boles. (Fahnstock et al 1961). These convection currents can lead to several hundred C differences, with higher temperatures recorded on the lee-side versus the windward side of the trees. Source: Dale Wade, Rx Fire Doctor, Damage to plant tissue is either caused by direct effects related to heating and combustion or indirect effects related to physiological changes in the plant productivity. Question: Given many fires exceed 60 C for more than a minute, how do any plants survive? Answers: 1. Not all plant tissue is equally important in determining mortality 2. Some plants have evolved with adaptations that help them survive fires Many plants can tolerate a substantial loss of biomass and still survive. The most critical tissues in plants are the meristamatic tissues as these have the ability to divide and thus help in the plant s growth. In terms of fire effects on plant survival it is important to consider how the fire impacts the cambium under the bark along the roots and tree boles, the buds that give rise to new tissue, and the seeds. Sources: Hoffman and Holden, Whelan (1995) 3

4 Many studies have found that bark thickness and thermal properties are the major factors determining the length of heat exposure required to reach lethal temperatures. Bark thickness is correlated with DBH. Therefore, smaller plants of all species are generally more vulnerable to fire than larger plants. This relationship has also been shown shrubs. Bark thickness is not uniform around the tree. Also the characteristics of the bark such as presence of fissures, will experience lower temperatures during a fire, but often will experience cambium kill because of lees protective bark. The effectiveness of bark as an insulator varies among different tree species as well as the ambient temperature. Bark heating is also affected by the relative flammability of the bark. A further driving affect is air temperature. Byram (1953) showed that plants at 50 F were able to endure twice as much heat as those at 95 F. Another factor is time since last fire. For example, Gill (1980) found that it took over 7 years for smoothbarked Eucalyptus to recover. Sources: Hoffman and Holden, Whelan (1995), Hare (1965) Clearly fire intensity (heat output) is a major factor affecting bark heating. Head fires are generally of higher intensity but low residence time, while backfires are lower intensity but higher residence times. This difference makes it possible for back fires to have greater heating effects than a head fire (as its easy to sustain > 60 C in a backing fire). When mortality does not occur, stem heating can lead to fire scars. Fire scars generally form where heating is sufficient to kill only a portion of the cambium. This is 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. 4

5 The effect of fire on meristematic tissue has been widely studied. Meristems are often protected by other foliage. For example, bunchgrasses have meristems protected by thick leaves. Most grasses have them located near the soil surface where they are often unaffected by fires, provided the fires have low residence times. The effect of fire on meristematic tissue has been widely studied. Meristems are often protected by other foliage. Height can also be very effective in protecting meristems as fire temperatures generally decrease with height. Newly established longleaf pines have a grass stage, where the growing bud is protected by proximity to the ground and densely packed needles that reduce the oxygen available for combustion in the area around the bud. Once growth starts, it is rapid and the bud is quickly lifted away from the ground surface. This is coupled with an increase in bark thickness. Many plants sacrifice meristematic tissue in the canopy to tolerate fire by sprouting from previously suppressed underground buds. Re-sprouting can occur from adventitious buds or from latent auxiliary buds. Adventitious buds can occur from 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. They protect the plant tissue and second they provide the energy requirements for sprouting to occur. 5

6 FOR 435: Spectral Changes When plants are combusted, they generally go through two chemical changes. Initially carbon-hydrogen bonds are broken down to produce char. On higher residence time fires, the carbon-oxygen bonds in this char is broken down further and only silica (mineral ash) remains. DECREASE in Visible-NIR (in general) 6

7 FOR 435: Thermal Infrared Changes Three main physical factors occurs to produce a rise in surface temperature: (i) decrease in vegetation cover (so reduced evapotranspiration), (ii) increase in soil cover (increase absorption), and increase in char cover (increase VIS-NIR absorption as its black). In grasslands and savannahs, the elevated temperatures can persist for about 6-12 months. In some forested ecosystems (e.g., boreal), these elevated surface temperatures can persist for decades. These changes make it very easy to map the area burned. Most burned area mapping methods solely make use of the reflectance changes as many surface temperature effects occur due to other reasons. Before During After 7

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