Effects of Slash Burning on Some Soils of the Douglas-Fir RegionROBERT F. TARRANT

Reprinted from SOIL SCIENCE SOCIETY OF AMERICA PROCEEDINGSVol. 20, No. 3, July 1956

Pages 408-411

k

Effects of Slash Burning on Some Soils of the Douglas-Fir Region'ROBERT F. TARRANT=

ABSTRACTThe studies summarized in this paper indicate that effects of

slash burning vary with different soils and locations in theDouglas-fir region and also within an individual clear-cut andslash-burned area. Although severe burning damages some soilproperties, the soil surface affected by severe burning usuallyrepresents only a small portion of the total area logged andburned. As low-grade materials are utilized more fully, a reduc-tion can be expected in the volume of heavy tuck left afterlogging. This will decrease even more the small amount of soilsurface damaged by severe burning.

In weighing the effects of slash burning on soil properties,consideration must first be given to the relative amounts of light-ly burned, severely burned, and unburned soil surface. This isnecessary to avoid the possibility of assigning to an entire area,the effects of only the most severe burning treatment.

It is also emphasized that findings of these studies apply onlyto intentional slash burns and should not be construed as apply-ing to areas affected by repeated wildfires.

MHOUSANDS of acres of logging slash are burned annu-1 ally in the Douglas-fir region to reduce the hazard

of accidental fires. That broadcast burning does reducethe volume and hazard of fuels on clear-cut areas isseldom questioned. However, under the increasingapplication of sustained yield forestry, which demandsprotection and improvement of the basic asset forestsoil many people have questioned the advisability ofslash burning. They see a possibility of soil damage.

Theoretically, a case can be made against the prac-tice of slash burning. Theory suggests that broadcastburning can destroy soil organic matter, and alongwith it the micro-organisms that contribute to goodsoil health. Destruction of incorporated organic mattercan be shown to reduce soil permeability, increasingchances for erosion, decreasing aeration and moisture-holding properties important to tree growth, and alter-ing generally the chemical nature of the soil.

Until rather recently, such theory has not been testedunder Douglas-fir region conditions. Consequently,many generalizations have been made. The subject ofslash burning, as it affects the soil, is one in which con-troversy has always been present. Since 1948, however,studies of the effects of slash burning on soil propertiesand the resulting impact on tree growth and watershed

'Presented before Div. V-A, Soil Science Society of America.Davis, Calif., Aug. 17, 1955. Received Aug. 8, 1955.

'Soil Scientist, Pacific Northwest Forest and Range ExperimentStation, Forest Service, U.S.D.A., Portland, Oreg.

conditions have been under way in the Douglas-firregion.

Studies have included both field and laboratory soiltests as well as related observations of germination andgrowth of seedlings in the greenhouse and undernatural conditions. Information has been gathered onboth physical and chemical soil properties under vari-ous degrees of burning and on germination and growthof seedlings in relation to soil properties. All of thework has been aimed at determining the ultimateeffects of slash burning on future tree growth andwatershed values. Also, some progress has been madein answering two questions that, in the author's opin-ion, represent the heart of the problem: (1) What arethe effects of different degrees of burning? and (2) Whatportions of a clear-cut area are burned to varyingdegrees in a slash fire? This paper summarizes some ofthese studies.

Intensity of Slash Fires

Throughout this series of studies, three classes ofsurface soil condition were recognized.

Unburned condition is that surface condition whichfor one reason or another escaped the fire. Some areasare unburned because logging disturbance has re-moved all fuel including surface litter. Others mayhave too small an amount of litter to carry fire. Localweather conditions may also stop fire spread.

Light burn is defined as that surface condition inwhich fire has charred the surface of organic litter buthas not removed all litter from the soil surface. Thiscondition occurs where large fuel is absent but a heavysurface litter is present to carry fire. Clear-cut areas thathave burned rapidly and well usually show large areasof blackened surface; these charred areas sometimes aresaid to be "hard burned," but the term is in error, sofar as soils are concerned, according to results of thesestudies.

Severe burn is defined as that surface condition inwhich fire has removed all organic litter from theground surface and in addition has baked the mineralsoil to a highly colored, crusty state. This type of burnoccurs beneath large pieces or accumulations of fuelwhich burn at high temperatures for comparativelylong periods of time.

PERCOLATION RATE

SANDY PUM ccCLAY LOAM SANDY LOAM

BULK ENS T

SANDY pumicyCLAY LOAM SANDY LOAM

1.00

0.50

0

TARRANT: EFFECTS OF SLASH BURNING ON SOILS OF THE DOUGLAS-FIR REGION 409

The intensity of slash burning is far from uniformover any single cutting area, and it is on this pointthat confusion arises concerning the effects of slashburning. We hear or read statements such as: "Slashburning destroys humus and nitrogen." "Slash burn-ing destroys soil aggregation." The fallacy in suchstatements stems from the application of a generality,without qualification as to the proportion of an areaon which the fire has an effect.

An intensive line sampling of 10 representative clear-cut and burned areas gave these results:

Surface condition Percent of total areaUnburned 47.1Light burn 46.9Severe burn 2.8Non-seedbed (rock, etc.) 3.2

Other workers have made a comprehensive study ofslash burning throughout the Douglas-fir region. Theirfindings, as yet not released, agree closely with the fore-going figures. We can conclude that the extent ofsevere burn rarely exceeds 5% of the area and in mostcases is less. This maximum figure of 5% of severelyburned soil surface is supported also by recent studiesin California (11).

These figures are important because they provide adefinite base on which to apply findings of soil studies.Citing the worst effects of burning as a generality isprobably the greatest single reason why foresters todaycannot agree as to the true effects of slash burning onthe soil.

Physical PropertiesThe most important physical properties of forest

soils are those affecting soil moisture. Moisture rela-tions affect not only tree growth but also watershedquality. A study was made to evaluate the influence ofvarious degrees of burn on several important physicalsoil properties.

Two soils of different textures were selected forstudy, a pumicy sandy loam and a sandy clay loam.Plots were established on areas burned the previousyear, and undisturbed soil cores were taken in steelcylinders. Unburned, light burn, and severe burn sur-faces were intensively sampled. In the laboratory, de-terminations were made of percolation rate, macro-scopic and microscopic pore volume, and bulk den-sity (1).

Severe burning was clearly found to reduce macro-scopic pore volume and percolation rate below thelevels in unburned soil as shown in figure 1. Micro-scopic pore volume and bulk density, in contrast, wereboth increased by severe burning. Direction of thechange was the same in both soils; and the magnitudeof change, when expressed as a percent of unburnedsoil, was remarkably consistent. Total pore space wasreduced as a result of severe burning in the pumicysandy loam but unchanged in the sandy clay loam.

When changes in soil properties after burning areexpressed as a percentage of the corresponding valuefor unburned soil, it was found that decrease in perco-lation rate after severe burning about equals decreasein macroscopic pore volume; that increase in micro-scopic pore volume is almost identical under both lightand severe burn; and that change in bulk density (ex-cept for severely burned sandy clay loam) is aboutmatched by an equal but opposite change in total pore

SOIL POROSITY TOTAL PORE VOLUME

M I CROSCOP I C PORE AG ONE —

MACROSCOPIC PORE VOLUME

SANDY punTICy SANDY POM c ySAND pumicy

CLAY LOAM SANDY LOAM CLAY LOAM SANDY LOAM CLAY LOAM SANDY LOAM

LEGEND. 0 UNBURNED ED LIGHT BURN ED SEVERE BURN

FIG. 1.—Changes in physical soil properties after slash burning.

volume. In lightly burned soils, only the relationshipbetween percolation rate and macroscopic pore volumefails to conform to recognized principles of soil watermovement.

Although severe burning seriously lowered perco-lation rate in the two soils analyzed, the over-all in-fluence of slash burning on soil moisture properties ofthe cutting areas studied was of minor importance. Ifsevere burning were applied uniformly over a largearea, increased surface runoff and erosion would beexpected. However, since severe burn occurs only insmall, scattered patches, usually totaling less than 5%of the cutting area, the net effect is not consideredhighly detrimental. Concern is also lessened by thecontinuing improvement in wood utilization that hasbeen evident in recent years. Eventually loggers mayremove most of the large material which now providesfuel for severe burns.

Chemical PropertiesProbably the most discussed change in soil condi-

tions after burning is that of soil reaction. We knowfrom earlier studies that "Slash burning tends tochange, temporarily at least, the duff and surface soilfrom an acid to an alkaline condition, but how longthis change continues is not known (3)." Accordingly,a study was made in which the trend of pH during the4 years following slash burning was studied (7). Sev-eral relationships were noted (see table 1) as follows:

Significant differences in soil pH were found be-tween unburned and light burn, unburned and severeburn, and between light and severe burn. The amountof change in soil pH is evidently related to severity ofburn. This finding substantiates an earlier laboratorystudy (6). To minimize change in pH during slashburning, the fire should apparently be light enough sothat only part of the litter is consumed.

pH decreased significantly with an increase intime since burning. One year after burning, pH ofburned soil was neutral or slightly acid. Three yearsafter burning, the soils had not returned to their orig-inal level of acidity, but even severely burned soils were

well within the acid range found naturally in theDouglas-fir region. Perhaps a new humus layer willhave to form before severely burned soils will fullyregain their original pH value.

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410 SOIL SCIENCE SOCIETY PROCEEDINGS 1956

Table 1.-Average pH on clearcuttings in Douglas-firby time since slash burning and intensity of burn.

Timesince burning

Severeburn

Lightburn

Un-burned

Adjacenttimber

Cascade Head Experimental forestFresh burn 1948 7.2 7.1 4.4One year 1949 5.8 5.6 4.5Two years 1950 5.8 5.2 4.4 -Four years 1952 5.0 4.6 4.3

Wind River Experimental ForestFresh burn 1953 7.6 6.8 5.3 4.2One year. 1952 7.0 6.0 c.? 4.8Two years 1951 6.4 5.5 5.2 4.8Three years 1950 6.0 5.0 4.8 4.0

H. J. Andrews Experimental ForestFresh burn 1953 7.4 7.1 5.2 4.5One year 1952 7.] 6.5 5.2 4.9Two years 1951 6.1 5.5 5.0 5.0Three years 1950* 6.5 6.0 5.1 5.1

*This clearcutting was included in the study because it was the only 3-year-old burn in the locality. The soil is derived from andesite and is less acid thanthe agglomerate soils which characterize other areas studied on the H. J. Andrew'sExperimental Forest. Nevertheless, the basic relationship among the threeintensities of burn remains the same.

3. The rate of change in soil pH after burning isrelated to severity of the burn. With increasing timeafter burning, pH decreased more on lightly burnedthan on severely burned areas.

Another study of the effect of heat on soil pH wasmade in which soils were heated in muffle furnaces (6).From this study, it was found that the greatest changein pH takes place at temperatures below 900°F. It isalso at temperatures below 900°F. that duration ofexposure to heat exerts an effect on the increase in soilreaction. Isaac and Hopkins (4) found that at a depthof I inch below the forest floor (about the same depthas the soils were sampled in this study), the heat gen-erated under a heavy pile of Douglas-fir slash was608°F. Our later study shows that this temperaturewould be sufficient to make a significant change in soilreaction. The findings of Isaac and Hopkins lendadditional strength to the recommendation that slashfires should move rapidly through light concentrationsof fuel if changes in soil reaction are to be avoided.

Still another study, as yet not completed, was madeof the effects of slash burning on other chemical prop-erties of the soil. Preliminary indications are that:

I. Light burning stimulates nitrification, but severeburning strongly reduces nitrogen content of the soil.

Light burning increases the amount of acid sol-uble P205 and exchangeable potassium. Severe burn-ing increases even more greatly the available supplyof these nutrients.

Light burning has no appreciable effect on cation-exchange capacity, but severe burning reduces thisproperty strongly.

This study will eventually relate the chemical effectsof burning to seed germination and seedling growth,and will provide more direct evidence of effects of slashburning on forest productivit y than a consideration ofchemical data alone.

of tree seed on slash-burned forest land. One experi-ment (2) indicated that "...there was absolutely nogermination where ash was added to the unburnedsoil." Reasons given for this hindrance to germinationincluded increased pH of the soil, hardening of theash layer, and exclusion of oxygen and high concen-tration of the nutrient solution.

In contrast, experiments at the University of Wash-ington (10) indicated that Douglas-fir seed germinatedsuccessfully under controlled pH conditions rangingfrom strongly acid to strongly alkaline.

These contradictory reports prompted the inclusionof further tests in our program of research on the effectsof slash burning on soil and regeneration. To test theeffects both of high pH and wood ash, Douglas-firseed was germinated in three different media: arti-ficially heated soils, an ash-sand mixture of varyingconcentration, and soils from slash-burned areas.

Reaction of these three germinating media rangedfrom pH 4.5 to pH 9.8. Despite the great range ofpH, there was no evidence of a relation between soilreaction and germination rate of Douglas-fir seed (seefigure 2). Of course, the fact that a seed may germinateunder a great variety of pH conditions does not neces-sarily mean that further growth will also be satisfac-tory. These tests do indicate, however, that relativelyhigh pH, within limits found after slash burning (7),does not prevent germination of Douglas-fir seed (8).

Interest in the effects of slash burning on soil isfocused on the ultimate effects on tree growth. It isimportant, therefore, to relate seedling growth to in-tensity of burn. In such a study (9), more than 400natural Douglas-fir seedlings, including both 1- and2-year-old trees, were dug and their height and rootlength measured. This sample of seedlings was takenfrom unburned, lightly burned, and severely burnedsoil surfaces on two experimental forests.

At the Wind River Experimental Forest, no signifi-cant differences were found in either seedling height orroot growth between the various degrees of burn or

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70

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0.40

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2

3

4 5 6

7 8 9 ID

Germination and Growth of Douglas-fir Seedlings PH

Wood ash and its accompanying alkalinity have 2.-Relation of seed germination to pH its three differentsometimes been cited as being harmful to germination germinating media.

TARRANT: EFFECTS

NUMBER OF SEEDLINGS

OF SLASH BURNING ON SOILS OF THE DOUGLAS-FIR REGION 411

rates on some severely burned soils. Further tests ofthe relation between soil reaction and damping-offunder field conditions are now in progress.

A study was made in which natural Douglas-fir seed-lings growing on slash-horned areas were criticallyexamined for mycorrhizae. The following significantdifferences were observed:

One year after the burn; 65% of 1-year-old seedlingson unburned soil had external mycorrhizae. On burnedsoil, only 40% of 1-year-old seedlings were mycorrhizal.The second year after burning, 100% of 2-year-old seed-lings on unburned soil had mycorrhizae, but only 79%on burned soil.

Mycorrhizae were deeper in the burned soil in thecase of 1-year-old seedlings, but for 2-year-old seedlings,there were no differences in depth of mycorrhizae be-tween burned and unburned soils.

111111

8

616

K;

WIND RIVER H 3 ANDREWS EXPERIMENTAL FOREST EXPERIMENTAL FOREST

'61 El 2

AX

a 35 4

56

F.7O

0 Z 80 61

ct -1 9

10

11

12

UNBURNED 115331 5333 R UNBORN D IGHT SEBURN BURN BURN B N

50 23 36 33 36 10 71 30 50 30 20 30

1 TEAR

2 SEARS

FIG. 3.—Average height and root growth of natural Douglas-firseedlings after slash burning.

unburned soil (see figure 3). At the H. J. Andrews Ex-perimental Forest, height growth of 1-year-old seed-lings was significantly greater on severely burned soilthan on unburned soil. For 2-year-old seedlings, heightgrowth was significantly greater on lightly burned thanon unburned soil.

These findings indicate that growth of youngDouglas-fir seedlings, at least under field conditionsrepresented by the two study areas, was not inhibitedby slash burning. In making this statement, we empha-size that the data do not cover possible effects of slashburning on germination, seedling survival, and seed-ling growth after the first 2 years. These effects, whichcould be equally important, are the subject of a moreintensive study currently under way.

Biological Properties

An interesting sidelight of the germination studieswas the disclosure that seedlings in the media of higherpH suffered heavy damping-off losses a day or two afteremergence. It has long been established that near-neutral or alkaline soil reaction favors the develop-ment of damping-off organisms (5). Perhaps preemer-gence damping-off is responsible for low germination

Literature CitedForest Soils Committee of the Douglas Fir Region. Sampling

procedures and methods of analysis for forest soils. Univ.Wash., Seattle. Processed (1953).

Gayle, W. B., and Gilgan, W. W. The effect of slash burn-ing on germination and primary survival of lodgepolepine and Douglas-fir. Univ. British Columbia Forest ClubRes. Note No. 2. Processed (1951).

Isaac, Leo A. Reproductive habits of Douglas-fir. CharlesLathrop Pack Forestry Foundation, Washington, D. C.(1943). and Hopkins, Howard G. The forest soil of the

Douglas-fir region and changes wrought upon it by loggingand slash burning. Ecology 18:264-279 (1937).

Lutz, Harold J., and Chandler, Robert F., Jr. Forest soils.John Wiley and Sons, New York, N. Y. (1946).

Tarrant, Robert F. Effect of heat on soil color and pH oftwo forest soils. U. S. Forest Serv., Pacific Northwest Forestand Range Exp. Sta. Res. Note No. 90 (1953). . Effect of slash burning on soil pH. U. S. Forest Serv-

ice, Pacific Northwest Forest and Range Exp. Sta. Res.Note No. 102 (1954). . Soil reaction and germination of Douglas-fir seed.

U. S. Forest Service, Pacific Northwest Forest and RangeExp. Sta. Res. Note No. 105 (1954). and Wright, Ernest. Growth of Douglas-fir seedlings

after slash burning. U. S. Forest Service, Pacific NorthwestForest and Range Exp. Sta. Res. Note No. 115 (1955).

Thrupp, A. C. Effect of seed-ash characteristics and treatmentof seed and soils upon coniferous seed germination. Un-published doctoral dissertation, College of Forestry, Uni-versity of Washington (1939).

11. Vlamis, J., Schultz, A. M., and Biswell, H. H. Burning andsoil fertility. Calif. Agric. 9 (3):7 (1955).