My name is Zack Holden, co-instructor for this course. Many people associate severe fire with catastrophic, stand-destroying forest fires.

Transcription

1 My name is Zack Holden, co-instructor for this course. Many people associate severe fire with catastrophic, stand-destroying forest fires. But fire can have profound effects on soils and the below ground processes that are critical for ecosystem structure and function. In this lecture, we will explore some of the major effects that wildfires can have on soils. 1

2 The soils that we see today are the product of thousands of years of organic matter decomposition and accumulation. In many ecosystems, particularly arid environments, organic matter decomposition is very slow. Soils mediate the aboveground characteristics that we see. Soil structure is determined by organic matter content and mineral composition, which in turn influences it s water holding capacity. These factors influence nutrient dynamics and below ground microbial communities of the soil system 2

3 Disturbances like fire influence both the aboveground and below ground characteristics of a site. When we evaluate the effects of fire on soil characteristics, both of these become important. The most significant aboveground change that can occur post-fire is the removal of overstory vegetation. Think about what happens when we remove overstory trees. More solar radiation reaches the ground and wind speed increases, increasing soil temperatures and drying rates. Without canopy interception of rainfall, more rainfall reaches the ground which can increase soil erosion. At the ground surface and below, fire heats soil directly, which can volatilize nutrients, destroy soil microbes and change the way water interacts with the soil surface. 3

4 It is useful to think of fire effects on soils in terms of direct and indirect effects, that together will determine the total effects of fire on soil characteristics. Direct heating of the soil by fire kills plant roots and seeds, exposes the mineral soil layer and volatilizes organic matter and nutrients in the soil. These effects, combined with the altered biophysical site characteristics that result from overstory vegetation removal determine the immediate and short-term effects of fire on the belowground environment. 4

5 Together, the direct effects and indirect effects of fire combined with the frequency and intensity of rain events several years after the fire will determine the soil erosion response of a site, and hence it s long-term vegetation response. 5

6 Many factor can influence the severity of fire effects on soils. These include the length of time between fires, and the resulting fuel accumulation. The properties of fuels, prior to combustion, including size, type and moisture content of fuels available for burning. The effects of these fuels on resulting fire behavior, in turn, influences how these fuels burn, and hence how they heat soils. 6

7 Heat is transferred into the soil via 4 mechanisms. You should be familiar with basic mechanisms of heat transfer from introductory fire behavior or fire ecology courses. Radiation and convection are the principle modes of heat transfer by which overstory vegetation is damaged during a wildfire. Active flames radiate heat that combined with hot air and gases can scorch or burn plant matter quickly. The opposite is true of soil heating, where the insulating effects of soil prevent rapid movement of heat downward into the soil. It is estimated that only 10 to 15 percent of the heat generated by a wildfire is transferred directly downward into the soil. The key factor that determines how hot and how deeply that heat travels is duration, or residence time. Dry soil is a poor conductor, but hot air and gases can move through dry soil via convection. In moist soils, the primary mechanism for heat transfer is conduction. Think of a heavy metal pan placed on a hot stove. Heat from the stove can t directly heat the cooking side of the pan. It takes time for that heat to travel through the pan by conduction. The process of heating is similar in soils, where the heat from the fire must travel, layer by layer, molecule by molecule, deeper into the soil. Another mechanism can also contribute to soil heating, however. Moist soil, when heated, releases heated gaseous water vapor that is capable of moving through small pores in the soil. This super heated air can increase the rate of soil heating due to the high conductivity of water vapor. 7

8 Mineral soil is itself an excellent insulator and a poor conductor, and the physical properties of the soil will strongly influence the rate and depth of soil heating. The size of soil particles, the types of minerals in the soil and it s organic matter content influence the structure and chemical composition of the soil, which in turn determines how heat moves through the soil. Water conducts heat more easily than mineral soil. It is a much better conductor of heat than air. Therefore, given two areas of the same soil type, one moist and one dry, the moist soil will become heated more quickly than the dry soil. 8

9 The range of fire effects on soil resources can be expected to vary directly with the depth of burn, which is expected to vary as a function of the amount duff consumed, and the degree of large woody fuel consumption. As you can see from this graph, soil temperatures quickly drop with increasing depth in the soil profile. However, many of important ecological components in soil break down at fairly low temperatures. Organic matter begins to break down at around 200 degrees centigrade, and Nitrogen begins to volatilize at close to that emperature. Seeds stored in the soil and plant roots are destroyed at even lower temperatures. 9

10 The severity of fire effects on soils depends on duration of and depth of soil heating. This figure illustrates the range of fire severities for overstory and understory effects and the site level and environmental factors that influence them. At one extreme, it is possible to have severe stand replacing crown fire that leaves the understory and soil unburned. At the other extreme, in some ecosystems we see creeping surface and ground fires that are extremely severe in terms of understory and soil effects but do little damage to the overstory vegetation. These effects vary both in temperature and duration, with crown fires often burning rapidly and at extremely high temperatures, and ground fires burning at lower temperatures for hours or even days. 10

11 The severity of above ground and below ground fire effects should determine the recovery of vegetation post-fire. Unburned areas can serve as refugia within burned areas. Where fire overstory fire effects are severe, plants must regenerate from seeds that survive in seedbanks, or from seed sources that colonize off site. Where fire effects on soils are severe, for example, where soil heating kills roots and eliminates seeds from seedbanks, colonizing plants can come either from surviving overstory canopy plants, the edges of less severely burned areas, or from off site seed sources. The combination of overstory and understory effects should determine the biophysical and chemical properties of the site, and hence the long-term post fire vegetation recovery. 11

12 This photograph from the 2002 Hayman fire in Colorado shows a low severity surface fire. Notice that much of the litter and fine fuels remain unburned, and only some of the ground surface appears scorched. 12

13 In this moderately burned area, more of the ground litter and fuels have been consumed. Ash and exposed mineral soil are more abundant. Notice that the brown needles, most of which are scorched needles that have dropped from scorched trees cover much of the soil surface. This is a key characteristic that separates moderate and severe post-fire effects. Scorched needle cast can protect the soil surface from post-fire rain splash and subsequent erosion. 13

14 This site from the same fire was very severely burned. Almost all overstory vegetation was destroyed, and on the ground, litter, fuel and duff was almost entirely consumed. A lot of mineral soil is now exposed, and black and white ash are much more abundant. 14

15 The severity of fire effects on soils is determined primarily by the amount of heat energy that is radiated downward. The only way to directly quantify this would be to calculate the fires residence time and temperature at a particular location, then integrate the area under this curve. One of the only ways to to estimate this is by the appearance of the soil, litter and duff. We can draw inferences about changes in post-fire site characteristics and long term watershed responses based on what we observe in the field. It is important to keep in mind that scale is also important. The size and location of severely burned patches will influence both the short and longterm ecological responses to soil fire. 15

16 Fire effects on soils can be estimated in the field by evaluating depth of burn, and can generally be related to surface temperatures and depth of soil heating. Depth of burn is considered low when litter is charred or scorched but unconsumed, and duff is intact. Mineral soil will generally be left unchanged and temperatures below the soil surface will remain low. 16

17 With moderate depth of burn, small fuels are consumed and duff is deeply charred or partially consumed. More white ash is present on site, indicating complete combustion of surface fuels and charring and comsumption of large diameter coarse woody debris is evident. Temperatures will be high in the upper layers of mineral soil and heat will begin to penetrate deeper into the soil. 17

18 With high depth of burn, nearly all surface fuels, large and small will be consumed. The mineral soil surface is exposed and shows signs of oxidation. Organic matter beneath the soil surface will show signs of consumption, and a black char layer is sometimes evident below the soil surface. Temperatures at 5 centimeters and below can be quite high. 18

19 The severity of fire effects on soils can vary spatially, with mosiac patterns evident at different scales. In those photograph, we see effects that vary from unburned areas with understory vegetation still present to complete litter and duff consumption. 19

20 This variability can occur at a range of scales. We can think of a variety of theoretical pre-fire characteristics that could influence variability in fire effects on soils. Large coarse woody debris that burns in place for long periods of time often leaves patches of severely burned soil like the one in this picture. Fuel bed characteristics and fuel moistures likely also determine how fuel burns and it s resulting effects on soil heating. 20

21 The field estimates we ve discussed, such as depth of burn and the appearance of post-fire fuel and soil characteristics are incorporated into the Composite Burn Index that we discussed previously. These include rating factors of the amount of fine and large woody fuel consumption and estimates of changes in post-fire soil color and soil erosion. There are currently no real quantifiable measures of the severity of post-fire soil effects, so keep in mind that such estimates can be quite subjective and will depend on familiarity with the area and prior experience in making these estimates. 21

22 Another major effect that wildfires can have on soils is the formation of hydrophobic or water repellent soils. During the fire, surface litter, duff and overstory vegetation volatilize at temperatures above 175 degrees centigrade. These compounds can coat soil particles in the upper layers of the soil. These compounds can reduce the infiltration capacity of the soil, reducing it s infiltration capacity. This in turn can increase erosion rates and in some cases increases the likelihood of debris flows. 22

23 Several methods are now used to test for water repellency in soils. An older method called the Water drop penetration test, measures the amount of time need for a drop of water to penetrate and be absorbed by mineral soil. A second method, called the minidisc infiltrometer measures the vollume of water infiltrating the soil over a 1 minute period. 23

24 Both of these methods are point measurements, and can be used together, although they are both highly correlated with one another. Developers of these techniques feel that the information provided by infiltrometer is more useful than the water drop penetration test. 24

25 These two methods are some of the only QUANTITATIVE measures available for detecting fire effects in soils. However, as we ve seen there is a great deal of spatial variability in the severity of fire effects on soils. These measures can be used to detect water repellent soils. However, there is a great deal of variability in these data. Measures can vary dramatically just a few feet from eachother. In addition, because these are point measures, it is difficult to scale these measures up to larger areas typical of most widlfires. 25

26 As we ve discussed in previous lectures, there is need for remote sensing assessments of post-fire effects. After large severe wildfires it is important to identify areas with potential for erosion and debris flows. Ongoing research is attempting to develop relationships between field measures and spectral measures from remote sensing platforms. 26