Robert H. Rutri 3

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Effect of Slash Burning on SomePhysical Soil Properties

BYROBERT F. TARRANT

THOUSANDS OF ACRES of loggingslash are burned annually in the Douglas-fir region as a fire-protection measure.Whether or not such burning changesphysical soil properties has not been thor-oughly explored.

A series of studies has been conducted bythe Pacific Northwest Forest and RangeExperiment Station to determine the effectof slash burning on soil properties. Thepresent study was made during 1954 tolearn whether slash burning actuallychanges physical soil properties and howany changes might be influenced by theseverity of burning. Several related phaseshave been reported previously (Tarrant,1953, 1954a, and 1954b; Tarrant andWright, 1955) .

MethodsTwo different soils were studied. One,from the upper Wind River Valley ofsouthern Washington, was a pumicy sandyloam with a weakly developed mor humuslayer. The other, from the H. J. AndrewsExperimental Forest in the Cascade Rangeof Oregon, was a sandy clay loam with amull humus layer. At both locations, plotswere established in 1954 on areas that hadbeen clearcut and subsequently burned inOctober 1953.

On each clearcut area, 33 undisturbedsoil cores were collected from each of the

following surface conditions:Unburned No evidence of disturb-

ance during logging andno visible effect of fire.

Light burn Where fire charred thesurface of the forest floorbut did not remove all lit-ter from the soil.

Severe burn Where fire removed allorganic litter from thesurface and in additionleft the mineral soil high-ly colored.

Undisturbed soil cores were collected insteel cylinders of 350 cc. capacity and 3-inch depth. In the laboratory, percolation'tests were made on all cores after satura-tion. The cores were then subjected to atension of 60 centimeters to determinemacroscopic and microscopic pore space.Bulk density was calculated after ovendrying. Cores were analyzed in random

The author is Soil Scientist, Pacific North-west Forest and Range Experiment Station,Forest Service, U. S. Dept. of Agriculture,Portland, Oregon.

1Percolation refers to the passage of waterthrough the soil profile, whereas infiltrationrefers to the entry of water into the surfacehalf inch of soil. Since the effect of severeburning usually extends below the half-inchdepth, the term percolation is considered tobe applicable.

Reprinted from FOREST SCIENCE, Volume 2, Number 1, March, 1956

TABLE 1. Differences between treatment means compared with differences re-quired for significance at the 95-percent level of fiducial probability.

Differences between treatment means'Soil property and

Unburned and

Unburned and

Light burn andunit of measure light burn severe burn severe burn

Pumicy Sandy LoamPercolation rate 22.15 16.87 39.02

(cu. in./hr.) (10.63) (10.63) (11.20)Macroscopic pore space

(percent by volume)Microscopic pore space

(percent by volume)Total pore volume

(percent by volume)Bulk density

(gm./cc.)

Percolation rate(cu. in./hr.)

5.46 15.93 10.47( 3.07) ( 3.24) ( 3.07)

11.33 11.06 0.27( 3.78) ( 3.59) ( 3.59)

5.88 4.86 10.74( 2.95) ( 2.95) ( 3.11)

0.10 0.07 0.17( .06) ( .06) ( .06)

Sandy Clay Loam2.49 43.96 46.45

(18.70) (18.70) (19.70)Macroscopic pore space 14.15 18.22 4.07

(percent by volume) ( 2.46) ( 2.59) ( 2.46)Microscopic pore space 18.89 18.78 0.11

(percent by volume) ( 2.37) ( 2.25) ( 2.25)Total pore space 4.74 0.56 4.18

(percent by volume) ( 3.18) ( 3.01) ( 3.01)Bulk density 0.06 0.05 0.12

(gm./cc.) '( .05) ( .05) ( .06)

Figures in parentheses are differences required for significance at the 95-percent level of fiducial probability.

order to avoid the possibility of concentrat-ing any technique error within any onetreatment. All anal yses followed proce-dures suggested by the Forest Soils Com-mittee of the Douglas-fir Region (1953).

Effects of Severe Burning

Bayer (1940) stated: "Permeability of thesoil for water is unquestionably a functionof the amount of the larger pores." Inthis study, severe burning was clearly foundto reduce' macroscopic pore s pace and per-colation rate below levels for the same

2All references to reductions or increases insoil property measurements imply significanceat the 95-percent level of fiducial probability.See Table I for statistical comparisons.

properties in unburned condition (Figs. 1and 2). Microscopic pore space and bulkdensity, in contrast, were both increasedby severe burning (Figs. 1 and 3). Di-rection of the change was the same in bothsoils; and the magnitude of change, whenexpressed as a percent of unburned soil(Table 2), was remarkably consistent.Macroscopic pore space was reduced toabout 25 percent of its volume in unburnedsoil, and percolation rate to about 30 per-cent.

Total pore space was reduced as a resultof severe burning in the pumicy sandy-loam, but unchanged in the sandy clayloam.

The practical effect of a reduction inpercolation rate is an increase in surface

volume 2, number 1, 1956 19

70

1 GO

so2

f,O o_

30

0 tO

qv toAmt

SANDY CLAY LOAMPUMICY SANDY LOAM

1 IN Unburned

Light bur n

Severe bur n

Unburned

Light burn

Severe burn

PUMICY SANDY LOAM SANDY CLAY LOAM7o

0_ PO

TABLE 2. Change in physical soil properties after light and severe burning(expressed as percent of same value for unburned soil).

Light burn Severe burnPhysical soil

propertyPumicy

sandy loamSandy

clay loamPumicy

sandy loamSandy

clay loam

Percolation rate +93 + 4 —71 —72Macroscopic pore space —26 +58 —77 —74Microscopic pore space +32 +55 +31 +54-Total pore space +10 + 8 — 9 + 1Bulk density —11 — 8 + 8 + 7

runoff of water during wet periods, causingsoil erosion and a reduction in the amountof water taken into the soil . Only beneaththe heaviest concentrations of large fuels,however, do slash fires generate enoughheat to burn organic matter within themineral soil. Thus, severe burning occursonly in small and discontinuous patches. Astudy of some 75 field plots in the Douglas-fir region showed that less than 5 percentof the total surface of a logged and slash-burned area was severely burned. A simi-lar figure on the proportion of severelyburned soil surface after slash burning wasreported from California (Vlamis et al.,1955 ) .

Although severe burning definitely dam-ages physical soil properties, it affects only

0.0000 AficroscOr, TOTAL M OO oscopic Micresccpl TOTAL

TYPE OF PORE SPACE

FIGURE 1. Relation between soil po-rosity and surface condition afterslash burning.

a very small portion of the total area loggedand slash burned. Therefore, it is con-cluded that severe burning exerts a minorinfluence under current slash-burning prac-tice. Increased utilization of low-gradelogs and small size stems will tend furtherto reduce the volume of heavy fuels leftafter logging, thus minimizing the extentof severe burns and soil damage duringslash disposal.

Effects of Light BurningAlthough the effects of severe burning areclearly defined, an anomaly is apparentwhen the results of light burning arestudied. In the pumicy sandy loam, perco-lation rate (Fig. 2) on lightly burned areaswas greater than that of the unburned

FIGURE 2. Relation between percola-tion rate and surface condition afterslash burning.

20 / Forest Science

condition, although macroscopic pore space(Fig. 1) was less. In the sandy clay loam,there was no difference between percola-tion rates in unburned and lightly burnedsoils, although again, macroscopic porespace was less. Microscopic pore space andtotal pore space were greater in both lightlyburned soils than in their unburned coun-terparts. Bulk density of both lightlyburned soils was less than that of the un-burned soils (Fig. 3).

There is no ready explanation for thefailure of percolation rate and macroscopicpore space to follow the usual relationship.Since the sequence of soil samples in labora-tory tests was determined by random selec-tion, there is no reason to suspect a con-centration of analytical technique error inthe lightly burned soils. Moreover, thechanges in total pore space and hulk densityin the two soils were highly consistent inboth direction and amount (Table 2), al-though these soils were distinctly differentin texture and were collected from cuttingareas more than 100 miles apart. The in-consistencies in percolation rate and macro-scopic pore space after light burning may bedue in part to the highly aggregated char-acter of the surface soils. Low values forhulk density (even after severe burning)are typical of Douglas-fir region soils, asevidenced by some 1,200 measurementsnow available for comparison.

Discussion

Some interesting associations are evidentwhen changes in soil properties after bothlight and severe burning are expressed asa percent of the same value for unburnedsoil (Table 2). In both soils, the decreasein percolation rate after severe burning isvery nearly balanced by the decrease inmacroscopic pore space. For each soil, theincrease in microscopic pore space follow-ing both light and severe burns is almostidentical. Change in hulk density, exceptfor severely burned sandy clay loam, ismatched almost exactly by a change in totalpore space in the opposite direction. Only

E

FIGURE 3. Relation between soil bulkdensity and surface condition af terslash burning.

the relationship between percolation rateand macroscopic pore space in lightlyburned soils fails to conform to establishedprinciples of soil water movement. •

ConclusionsSevere burning seriously lowered the rateof moisture movement in the two soilsstudied. Light burning did not hamper themovement of water within the surface 3inches of soil, although macroscopic porespace was reduced.

Severe burn occurred only in small,scattered patches that, combined, amountedto only a very small portion of the totalsurface of the slash-burned areas. For thisreason, it is concluded that the over-all in-fluence of slash burning on the moistureproperties of the soils studied was of minorconsequence. These findings are applicableonly to intentional slash burning and shouldnot be applied to areas in which repeatedwild fires have occurred.

Literature Cited

B AYER, L. D. 1940. Soil physics. Wiley,New York. 370 pp.

FOREST SOILS C OMMITTEE OF THE

D OUGLAS FIR R EGION. 1953. Samplingprocedures and methods of analysis forforest soils. Univ. Wash., Seattle. 38 pp.Processed.

volume 2, number 1, 1956 21

TARRANT, ROBERT F. 1953. Effect ofheat on soil color and pH of two forestsoils. U. S. Forest Serv., Pacif. North-west For. Range Exp. Sta. Res. NoteNo. 90, 5 pp. . 1954a. Effect of slash burn-

ing on soil pH. U. S. Forest Serv., Pacif.Northwest For. Range Exp. Sta. Res.Note No. 102, 5 pp. . 1954b. Soil reaction and

germination of Douglas-fir seed. U. S.

Forest- Serv., Pacif. Northwest For.Range Exp. Sta. Res. Note No. 105, 4-pp. , and ERNEST WRIGHT.

1955. Growth of Douglas-fir seedlingsafter slash burning. U. S. Forest Serv.,Pacif. Northwest For. Range Exp. Sta.Res. Note No. 115, 3 pp.

VLAMIS, J., A. M. SCHULTZ, and H. H.B ISWELL. 1955. Burning and soil fer-tility. Calif. Agric. 9 (3) :7.

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