Nited,State E Artrenvo,Tt Ricultureop,V,Vit S Ervice EFFECTS of FIRE on SOIL
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nited,State e artRenvo,tt ricultureop,v,vit s ervice EFFECTS OF FIRE ON SOIL A State-of-Knowledge Review Prepared for the Forest Service National Fire Effects Workshop, Denver, Colo., April 10-14, 1978 by Carol G. Wells Project Leader, Research Triangle Park, N.C. Ralph E. Campbell Soil Scientist, Flagstaff, Ariz. Leonard F. DeBano Project Leader, Tempe, Ariz. Clifford E. Lewis Range Scientist, Marianna, Fla. Richard L. Fredriksen Soil Scientist, Corvallis, Oreg. E. Carlyle Franklin Program Manager, Charleston, S.C. Ronald C. Froelich Plant Pathologist, Gulfport, Miss. Paul H. Dunn Microbiologist, Glendora, Calif. (All with the Forest Service, U.S. Department of Agriculture.) U.S. Department of Agriculture Forest Service General Technical Report WO-7 March 1979 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 PREFACE Recent changes in Forest Service fire approach needed to make the results as use- management policy make it clear that re- ful as possible to land managers. source managers today need a great deal The National Fire Effects Workshop was more information on the physical, biological, held April 10 through 14, 1978, as a first step and ecological effects of fire. They will need in responding to the most recent changes in information on fire behavior and fire effects policies, laws, regulations, and initiatives. as a basis for analyzing the benefits, dam- One of the major Workshop objectives was ages, and values of various fire manage- to prepare a report indicating the current ment alternatives. Managers must be able state-of-knowledge about effects of fire on to place a value on all resources if they are various resources. These reports formed the going to incorporate fire and its effects into basis for pinpointing knowledge gaps. Using land management plans. The Forest Service this information and input from land man- is committed to the concept that fire man- agers, priorities for research needed on the agement planning has to be a fundamental effects of fire were established. part of all our planning. Six work groups were established to pre- Recent laws and regulations also give ad- pare the state-of-knowledge reports on the ditional guidance for the Forest Service to following subjects: soil, water, air,. flora, use in developing land management plans fauna, and fuels. Work group members were for each unit of the National Forest System. mainly Forest Service research scientists, These plans must coordinate outdoor recrea- but individuals from National Forest Sys- tion, range, timber, watershed, wildlife and tems, Bureau of Land Management, Nation- fish, and wilderness resources. Interdiscipli- al Park Service, Fish and Wildlife Service, nary planning is vital, and research must and Bureau of Indian Affairs also partici- cover the same universe as our planning— pated. therefore interdisciplinary research is a We hope these state-of-knowledge reports must. will prove useful to researchers and re- The effects of fire have been studied .since search planners as well as land and fire the beginning of organized Forest Service management planners. Each report will be research, but the results are scattered over published as an individual document. A sep- a wide range of outlets. In addition, re- arate bibliography also will be included in search is conducted on the effects of fire this series in an effort to provide a source under several appropriation line items, and document for most of the literature dealing in some instances lacks the interdisciplinary with the effects of fire. ii CONTENTS Page Preface i i Introduction 1 Soil temperature and heating 1 Chemical properties and nutrient cycling 7 Soil microflora 14 Soil physical properties 16 Erosion 20 Effect of fire on range soil 24 Knowledge gaps, research scope, and priorities 24 Conclusions 26 Summary 26 Literature cited 28 iii EFFECTS OF FIRE ON SOIL A State-of-Knowledge Review INTRODUCTION It is important to understand the effects protective forest floor, volatilizes large of fire on forest and range soils. This knowl- amounts of nitrogen and smaller amounts of edge provides a basis for developing guides other elements, and transforms less volatile to the effective use of prescribed fire and for elements to soluble mineral forms that are determining the situations where wildfires more easily absorbed by plants or are lost can be minimized or prevented by using by leaching. Heating the underlying soil prescribed fires. layers also alters the physical, chemical, and The catastrophic effects of uncontrolled biological properties of the soil dependent fires on the soil have been observed fre- upon soil organic matter. These general re- quently following wildfires. There is a need lationships would lead one to believe the for evaluating those burning situations effects of burning are predictable; however, where fire effects are less dramatic than to the contrary, the effects reported in the during wildfires so the tradeoffs between literature are highly variable. These varia- prescribed burning and/or other means of tions are attributed to fire intensity, tem- vegetation manipulation can be compared. perature, vegetation type and amount, soil, For example, burning volatilizes nitrogen, moisture, and other speculative factors. In an essential element for plant growth; how- spite of the seemingly drastic changes asso- ever, the loss of some nitrogen by burning ciated with fire, most of the effects on the might become a more acceptable alternative soil are relatively minor. It is the purpose of than large soil losses occurring after the use this report to summarize the information of heavy mechanical equipment. Likewise, available in the literature about the effects several light or moderately controlled burns of fire on soil properties and provide more possibly have less impact on the soil than a specific data for certain important forests, single severe wildfire that may result from brush, and range types. This information a large fuel accumulation. will provide the basis for evaluating altera- Fire destroys soil organic matter, or resi- tions in fire management. dues that eventually become soil organic matter. The amount and location of the or- ganic matter lost depends on the intensity) 1 Fire intensity is defined as the rate of heat release of the prescribed fire. Burning the surface per unit of ground surface area and is proportional to organic matter removes or decreases the flame height and rate of spread. SOIL TEMPERATURE AND HEATING Many of the changes in soil chemical, energy released by the combustion of plant physical, and biological properties that oc- biomass and forest floor during a prescribed cur during a fire are related to the degree burn or wildfire is lost upward. However, and duration of soil heating. Most of the the energy transmitted downward heats and 1 alters the underlying litter and mineral soil. the soil, the temperature at any particular If the surface organic layer is moist and depth does riot exceed 100° C (212° F) until thick such as occurs in some forests, little the water has evaporated or moved into soil heating occurs (Agee 1973). However, if lower layers (Scotter 1970, DeBano et al. the litter layer is dry and partially or wholly 1976). Heating moist soil under grass has consumed in the fire, or if the litter layer is been analyzed by the diffusion equation al- thin as in chapparal, the underlying soil can though heat transfer by moisture fluxes be heated substantially and large tempera- Were not considered (Scotter 1970). However, tures are generated in the underlying soil. later Scotters data along with additional Ignition of the surface litter and heating of laboratory data were used to develop a more the underlying mineral soil alters all soil sophisticated model coupling the transfer of physical, chemical, and biological properties water, heat, and water vapor (Aston and dependent on soil organic matter. Gill 1976). Although this later model suc- The degree of soil heating during any par- cessfully predicted soil temperature profiles, ticular fire is highly variable and depends moisture profiles, ground heat flux, and upon the type of fuel (grass, brush, trees), evaporation under simulated surface fire fire intensity (wildfire, prescribed burns, conditions in grasslands, it has not been etc.), nature of the litter layer (thickness, tested for forests and brushlands. Also, the packing, moisture content, etc.), and the soil soil surface temperature data, required as properties (organic matter, soil water, tex- input data for this model, have not been ture, etc.). This section summarizes our related directly to fire intensity as defined knowledge of soil heating in relation to var- in terms of traditional fire intensity such as ious fire, soil, and fuel parameters. It also rate of spread or energy released per unit reviews basic heat transfer in soils along area. with a discussion of the various methods of Measuring soil temperatures.—Soil temper- characterizing soil temperatures and fire atures are commonly reported in the litera- intensity as related to soil heating and min- ture in two ways—maximum or continuous. eral soil exposure. Maximum temperatures are measured with heat sensitive materials (paints, tablets, Characterizing Soil•Heating, Soil etc.) that melt at known temperatures (De- Temperature, and Fire Intensity Bano and Conrad in press, Bentley and Fenner 1958, Tothill and Shaw 1968, De- Before considering the various factors Bano et al. in press). Temperatures can be affecting soil heating we need to review (1) measured at both the soil surface and down- the nature of the heat flow in soils during ward in the soil by placing these materials fire, (2) the methods of characterizing soil at the desired depths. Maximum tempera- temperatures, and (3) the attempts that tures can also be measured with high tem- have been made to characterize fire intensi- perature maximum thermometers. ty in terms of soil heating.