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CONSTRUCTION of forest accessroads often increases soil erosion

and stream sedimentation. In steepterrain, road construction may dis-turb 6 to 8 percent of the area undermanagement, and the bare roadsideslopes are especially vulnerable toerosion during the first few months af-ter construction. Fredricksen (3 ), instudying the effects of the construc-tion of 1.65 miles of road in a 250-acre watershed in the Oregon Cas-cades, reported that stream sedimentincreased by 250 times during the firstrainstorms after construction butdropped almost to preconstructionlevels 2 months later.

To minimize soil movement, somesort of cover is needed on roadsideslopes as soon after construction aspossible. Unfortunately, natural re-vegetation is extremely slow on these

C. T. Dyrness is program leader at theInstitute of Northern Forestry, PacificNorthwest Forest and Range ExperimentStation, Forest Service, U. S. Departmentof Agriculture, Fairbanks, Alaska 99701.The author thanks R. S. MacLauchlan, W.H. Billings, and S. L. Swanson of the SoilConservation Service Plant Materials Center,Corvallis, Oregon, for their help in. suggest-ing and obtaining plant species for evalua-tion. He also thanks A. B. Levno and R. C.Mersereau, U. S. Forest Service, for help inobtaining field plot measurements.

sites because of low soil fertility andunfavorable microclimate conditions.

The Forest Service initiated a road-side grass seeding program in westernOregon and Washington about 18years ago. Seed mixtures, fertilizationtreatments, and methods of applica-tion are not standardized within theagency; rather, methods vary fromforest to forest and even amongranger districts.

Seedings along forest access roadsoften have been unsatisfactory. Inmany cases seeded areas were not sta-bilized at all or erosion was only re-duced temporarily. Some of the dif-ficulties encountered include poorgermination, rolling of seed to the bot-tom of cut-and-fill slopes before ger-mination, deterioration of good grassstands after 2 or 3 years, and highwinter mortality due to late fall seed-ing.

Some basic recommendations thathave been made to aid seeding suc-cess include the following:

Seeding should be completed inlate summer or early fall, immediatelyafter road construction.

Fertilizer applications should hedetermined by soil sample analyses.

3. Species seeded should be selectedto fit the site and to produce fast ini-tial growth for quick stabilization.

Reprinted from the Journal of Soil and Water ConservationJuly•August 1975, Volume 30, Number 4

Copyright , :!7) 1975, SCSA

4. Inclusion of legumes with grassesfor soil stabilization may improve thenitrogen economy of the soil sufficient-ly to produce a denser, longer lastingstand of vegetation. Preliminary leg-ume trials (1) indicated that whiteDutch clover, New Zealand white clo-ver, big trefoil, and birdsfoot trefoilwere suited to climatic and soil condi-tions in the mountains of westernOregon.

Despite the seriousness of the prob-lem, there has been little formal re-search on roadside soil stabilization inthe Pacific Northwest. Most informa-tion has come from experience andcasual observation. This report sum-marizcs results of exploratory fieldstudies I began in 1964. Portions ofthe information presented here ap-peared in two progress reports (1, 2).

Methods

To assess the effectiveness of grass-legume mixtures in controlling soilerosion, two sets of plots were estab-lished on roadside backslopes. Thefirst set was installed in September1965 on a 5-year-old backslope, whichwas almost devoid of vegetation andobviously had been eroding. Thesecond set of plots was established inSeptember 1967 on a newly con-structed backslope in order that thestabilizing effects of roadside treat-ments could be followed during thefirst rainy season after construction.

The experimental design (random-ized block) and measurement tech-niques were the same for both setsof plots. Backslopes at both locationswere approximately 1:1 ( about 45 de-grees), constructed in soils derivedfrom tuffs and breccias. The 1965plots were located at an elevation of2,450 feet, the 1967 plots at 3,100 feet.Both sets of plots are located in theBlue River District of the WillametteNational Forest.

Five seeding mixtures plus a. con-trol were replicated twice at both lo-cations for a total of 12 plots at eachsite. Plots were 6 feet wide and ex-tended up and down slope for theentire length of the backslope ( 20 to25 feet). The five treatments appliedwere (1) straw mulch and fertilizer,with no seed applied; ( 2) Blue RiverDistrict seeding mix, a mixture offour grasses and white Dutch clover;(3) Oregon Highway Departmentmix, consisting of three grass speciesplus white Dutch clover; and (4 )

Grass-legume mixtures forerosion control along forestroads in westeren Oregon

Purchased by the Forest Service for official use

C.T. DYRNESS

ABSTRACT—Attempts were made to identify legume species that would becompatible with grasses in roadside seeding mixtures for use in the mountainsof western Oregon. Despite successful germination and early establishment,legumes were unable to compete with grasses and largely disappeared frommost roadside stands after I year. Grass-legume seed, fertilizer, and strawmulch applied to road backslopes for the most part successfully halted ero-sion. Rates of erosion from unmulched and bare control plots during the firstyear after seeding were substantially higher on newly constructed backslopesthan on a backslope that was several years old at the time of plot installation.Results indicated that a mulch, as well as seed and fertilizer, should be placedon roadside slopes if soil erosion is to be —minimized during the first rainy sea-son following road construction. In addition, infertile subsoils failed to main-tain a viable vegetative cover and required ref ertilization 7 years after plotswere initially seeded and fertilized. Bare, unprotected roadside slopes con-tinued to erode at a rather constant rate throughout the course of the study.The loss amounted to about 0.2 inch of soil per year.

and (5) two experimental mixtures ofadditional grass and legume species(Table 1). With the exception of theBlue River District mix, all treat-ments received straw mulch at therate of 2 tons per acre.

The 1965 plots were installed in lateSeptember, the 1967 plots in late Au-gust. All treated plots were ferti-lized with ammonium phosphate atthe rate of 400 pounds per acre, bothat the time of seeding and the follow-ing spring. Control plots, of course,were untreated; and although theywere virtually bare at the outset, na-tive vegetation was allowed to invadethese plots during the experiment.Figure 1 shows the 1965 plots.

Soil movement on the plots wasmonitored by periodic measurementsof slope elevation. This involvedmeasuring the distance to the soilsurface from points on a wire cablestretched between stable, referencedpositions at the top and bottom of theslopes. Use of this method minimizeddisturbance on the measured slopesection. Elevations of 45 points (15at 1-foot intervals along each of 3transects) were measured in eachplot. Change in slope elevation sincethe last measurement was computedfor each point, then averaged amongthe 45 points to provide a single in-dex of soil loss or gain during themeasurement period.

Vegetative cover on each plot alsowas estimated at frequent intervals.In addition, species composition wasnoted and recorded.

Vegetative Cover on Plots

Vegetative treatment of roadsideslopes must provide fast initial growth

•

Figure 1. Roadside seeding plots on a5-year-old backslope 6 months after seed-ing.

and quick cover to minimize soilmovement. This is especially true ofnew slopes where the loose soil isparticularly vulnerable to erosion dur-ing the first heavy rainstorms.

As shown in figure 2, plant coveron seeded plots during the first winterfollowing seeding ranged from 15 to45 percent. It is questionable wheth-er such levels of plant cover would, bythemselves, effectively curtail erosion.Plant cover during the winter wasappreciably greater on the 1965 plotsthan on the 1967 plots ( Figure 2).This was probably due to the eleva-tion difference between the two sites.

On both sets of plots, the BlueRiver District mixture provided theleast plant cover during the first win-ter after seeding. This apparentlywas caused by the lack of strawmulch. The mulch applied with theOregon Highway Department mixture

and the two experimental mixturesappeared to reduce downslope seedmovement and frost-heaving mortal-ity, thus insuring an even distributionof vegetation.

During the first fall and winter,ryegrasses provided the dominant cov-er on seeded plots. Annual ryegrassin experimental mixtures 1 and 2clearly outperformed perennial rye-grass in the Oregon Highway De-partment and Blue River Districtmixtures. The annual ryegrass germi-nated and became established morequickly than did the perennial ryeand was about twice as tall by the endof the winter period.

With the exception of the mulch-only and Blue River mixture treat-ments in the 1965 set of plots, coveron treated plots was satisfactory bythe end of the first growing season( Figure 2). Vegetative cover on the1967 set of plots averaged almost 90percent - a surprisingly high value.Even the mulch plots established in1967 averaged over 70 percent cover.Most cover was contributed by rye-grass because ryegrass clippings wereused as the straw mulch.

Although legume species were muchin evidence during the first spring,after seeding, they largely disap-peared from most plots by the end ofthe summer. Virtually the only leg-umes remaining at the end of the firstyear were scattered white cloverplants near the base of the backslope.The legumes included in these mix-tures apparently were unable to com-pete successfully with the vigorouslygrowing grass. Additional legumespecies should be tried. Lower ratesof nitrogen fertilization might also fa-

Table 1. Grass•legume treatments applied on two sets of plots.

ltlulch andFertilizer

Blue RiverDistrictMixture

Oregonil igh watj

DepartmentMixture

ExperimentalMixtureNo. 1

ExperimentalAlixtureNo. 2

Species 1965 1967 1965 1967 1965 1967 1965 1967 1965 1967

lbs acreper0 0 6.25 6.25 0 0 3.0 3.0 3.0 3.0Colonial bentgrass (Agrostis tenuis Sibth.)

Creeping red fescue (Festuca rubra L.) 0 0 5.00 5.00 18.0 18.0 8.0 8.0 8.0 8.0Perennial ryegrass ( Lolium perenne L.) 0 0 3.75 3.75 4.0 4.0 0 0 0 0Tall fescue [Festuca arundinacea (Schreb.) Wimm.) 0 0 8.75 8.75 0 0 20.0 20.0 0 20.0Chewings fescue (Festuca rubra var. commutata Gaud.) 0 0 0 0 12.0 12.0 0 0 0 0Annual ryegrass (Lolium multiflorum Lam.) 0 0 0 0 0 0 8.0 8.0 8.0 8.0Orchardgrass (Dactylis glomerate L.)White Dutch clover (Trifolium repens L.)

00

00

01.25

01.25

06.0

06.0

00

00

20.00

00

New Zealand white clover ( "Trifolium repens L.) 0 0 0 0 0 0 2.0 2.0 2.0 0Big trefoil ( Lotus uliginosus Schkuhr.)Birdsfoot trefoil (Lotus corniculatus L.)

00

00

00

00

00

00

2.00

t)2.0

2.00

00

Red clover (Trifolium pretense L.) 0 0 0 0 0 0 0 0 0 3.0Alsike clover (Trifolium hybridum L.) 0 0 0 0 0 0 0 0 0 3.0'Lana' woollypod vetch ( Vicia dasycarpa Ten.) 0 0 0 0 0 0 0 0 0 7.0

Total applied 0 0 25.0 25.0 40.0 40.0 43.0 43.0 43.0 52.016-20-0 fertilizer 400 400 400 400 400 400 400 400 400 400Straw mulch 4,000 4,000 0 0 4,000 4,000 4,000 4,000 4,000 4,000

170 JOURNAL OF SOIL AND WATER CONSERVATION

PLOTS SEEDED IN 1965

0 3 MONTHS AFTER SEEDING (DEC 19651O 7 MONTHS AFTER SEEDING (APR 19661

O 1 2 MONTHS AFTER SEEDING (SEP 1966)

PLOTS SEEDED IN 067

®7 MONTHS AFTER SEEDING (APR 1968)0 9 MONTHS AFTER SEEDING (JUN 19681El 12 MONTHS AFTER SEEDING (SEP 19681

(00

80tr

8 60

z40

cc0.

20

7

5: E

Figure 2. Vegetative cover trends on seededduring the first year after treatment.

0zLi' -Frit'.

backslope

oQ

=

7

x

plots

e,;eiter,

•

5. Luxuriant grassafter refertilization.these plots resembled the seededfigure 4.

Figure1 yearzation,plot in

on seeded plotsBefore refertili-

Li 100C9

0

tL 60— 0

—CD

CC 40— Q4.1Lti0

20-zUcc

a. 00

1

80 H

0

O

JJCONTROL PLOTS

2 3 4

5 6

7 8

(1965) 0966) ((967) (1968) (1969)

(1970) (1971)

(1972) (1973)YEARS SINCE ESTABLISHMENT

Figure 3. Trends in coverage of live foliage for 8 years after plotestablishment. In 1972 one block of plots was refertilized, whilethe other block remained unfertilized.

I

1 1 I Slot/ of Declining Vigor

0 ;Q.11

N 1

Ct

I

vor legume growth by reducing grassvigor.

Fast-spreading, sod-forming grassesdominated the seeded plots by theend of the first year. Bunch-typegrasses, such as tall fescue and or-chard grass, were present only as scat-tered individuals. Colonial bentgrassdominated on plots treated with theBlue River District and experimental1 and 2 mixtures. Creeping red fes-cue dominated on the Oregon High-way Department plots. End-of-the-year plot descriptions also indicated agood distribution of ryegrass. Afterthe first year, however, ryegrass grad-ually decreased in importance.

Although plots appeared lush andgreen for the first 2 years after estab-lishment, the effects of decreasing soilfertility became apparent by thethird growing season. The grassesexhibited less vigor and a chloroticappearance. As a result, live coveron the experimental plots decreaseddramatically after the third year fol-lowing establishment ( Figure 3).Plots seeded in 1965 went from anaverage of 91 percent live cover dur-ing the third year to 10 percent livecover 5 years later (8 years after theplots were installed). Despite suchdepauperate growth, dead grass(thatch) provided sizeable amountsof litter on the plots. Bare soil aver-aged 10 to 15 percent.

In 1972, treated plots in one blockwere refertilized with ammoniumphosphate at a rate of 400 poundsper acre, while plots in the adjacentblock were left unfertilized. Resultsof this refertilization were astounding

Figure 4. Control and experimental mix2 plot with no refertilization, 8 years afterseeding. Note depauperate grps stand onseeded plot.

(Figures 4 and 5). Within 1 year,foliage on both the seeded and mulch-only plots recovered to an average of95 percent cover (Figure 3).

Soil Movement on Plots

Slope profile measurements indi-cated that, as expected, successfulgrass establishment on road back-slopes did indeed curtail soil erosion.Soil stabilization was especially effec-tive on the older backslope plots (Fig-ure 6).

On the basis of 7 years of soil move-ment data, apparently the only plotsto lose appreciable amounts of soilwere the unvegetated control plots.These plots lost a cumulative total of1.44 inches of soil over the 7-yearperiod (Figure 6). This roughlyequals a soil loss of 144 tons per acre.

Despite some variation in plant cov-er characteristics, all five treatmentshalted soil erosion about equally well.The fact that some plots showed anet gain in slope elevation probablywas due to deposition of soil that rav-elled down from the top of the back-slope onto the measured slope sec-tion.

In view of the declining vigor ofgrass stands on the 1965 plots (Figure3), it is surprising that measurementsafter 1969 did not indicate increasedrates of soil loss (Figure 6). As pre-viously mentioned, despite extremelydepauperate growth, treated plotsmaintained considerable litter. Baresoil areas were, for the most part,small and discontinuous. The resultwas localized erosion and deposition,but no detectable net loss of soil from

JULY-AUGUST 1975 171

JUN972

SEP AUG SEP SEP JUN JUN JUN1 966 1967 1968 1969 1970 1971

DATE OF MEASUREMENT

Figure 6. Cumulative soil loss or gainfrom plots with five vegetative treatmentsand a bare control on a 5-year-old back-slope (1965 plots).

1 50AUG1967

I _J

JUN JUN1971 1972

SEP SEP OCT1968 1969 1970

CONTROLMULCH

— — BLUE RIVEROREGON HIWAY

— EXP MIX I— — EXP MIX 2

t 050

(,)

0.50

I 00

00

— "1— --f"

I050-

0O-

------------

CONTROL MULCH— — — BLUE RIVER

- OREGON HIWAY— EXP MIX

— — —EXP MIX 2I -1

0<A0

the slope. As the grass stands con-tinue to deteriorate, however, baresoil areas inevitably will grow andcoalesce, leading to channelized ero-sion and soil deposition at the base ofthe slope.

Measurements of soil movement onplots established on the newly con-structed backslope showed theseslopes to be somewhat more resistantto stabilization (Figure 7). Althoughthe vegetative treatments certainlyreduced soil erosion, long-term trendsindicated that even the treated plotsapparently lost some soil. Measuredlosses for treated plots ranged from0.14 to 0.38 inches of soil for a 5-yearperiod after establishment (Figure 7).Apparently the -most ineffective treat-ment from the standpoint of soil sta-bilization was the Blue River Districtmix, the only treatment that did notinclude a straw mulch. The remain-ing four treatments (Oregon HighwayDepartment, mulch, and experimentalmixes 1 and 2) appeared to be aboutequally effective in reducing erosionlosses. As expected on the newly con-structed slopes, the most severe soillosses occurred during the first yearafter establishment (Figure 7). Evensoil losses from the bare control plotsdecreased substantially after the firstyear.

First-year soil movement trends forplots on the recently constructedbackslope showed the important sta-bilizing effects of straw mulch (Fig-ure 8). During this period, the plotsnot protected by a straw mulch (con-trol and Blue River District treat-ment) lost five to seven times as muchsoil as the mulched plots. By June,a nearly complete covering of grassesand legumes on all plots, includingBlue River District mixture (Figure2), had substantially reduced soilmovement. As a result, the controlplots were the only ones to showcontinued high rates of soil loss dur-ing the June through September pe-riod (Figure 8).

These results suggest that mulchingbackslopes may be essential to mini-mize soil losses during the first fewcritical months following construction.Especially at higher elevations, theamount of grass-legume growth thatoccurs during the fall apparently pro-vides insufficient cover to halt ero-sion. Contrary to appearances, aluxuriant growth of grasses and leg-umes during the first growing season

DATE OF MEASUREMENT

Figure 7. Cumulative soil loss from plotswith five vegetative treatments and a barecontrol on a newly constructed backslope(1967 plots).

100, 1 1 1 I I I 1 1 1 1 1Newly Constructed Bockslope (1967 p/ols)-

— — — 5yr o/d Bocks/ope (1965 plots)

1000 I 2 3 4 5 6 7 8 9 !O a 12

SEP OCT NOV DEC JAN FEB MAR APR MAY JUN JUL AUG SEP

MONTHS SINCE INSTALLATION

Figure 8. Soil movement during the firstyear after treatment on a 5-year-old back-slope, compared with results on a newlyconstructed backslope.

following fall treatment is not conclu-sive evidence that soil loss was negli-gible during the preceding wintermonths.

Conclusions

Even though the results reportedhere are based on limited exploratorystudies, certain tentative conclusionsappear warranted. These conclusions,

although preliminary and subject toadditional experimental verification,are as follows:

The search for vigorous, fast-spreading legume species suitable foruse in grass-legume mixtures on sub-soils in western Oregon should beexpanded. Although our test seed-ings identified big trefoil and birds-foot trefoil as promising species, theydid not perform well in the roadsideplots, probably because of severegrass competition. Better culturalpractices may also be needed to im-prove survival and growth of legumesin grass-legume mixtures. For exam-ple, less nitrogen fertilizer should fa-vor legume survival and growth.

Soil erosion is substantially high-er on newly constructed backslopesthan on those that are several yearsold. Mulch as well as seed and fer-tilizer should be placed on fresh road-side slopes if erosion is to be mini-mized during the first rainy season af-ter road construction. In many areasvegetative growth during the fall andearly winter is insufficient to curtailerosion. This is especially true athigher elevations, where low temper-atures limit rates of grass and legumegrowth during the fall and early win-ter.

Bare, unprotected roadside slopeserode at a substantial rate over a pe-riod of years if vegetative cover is notestablished. Constantly ravelling soil,coupled with generally low soil fertil-ity, delays vegetative stabilization ofslopes — sometimes for many years.In this study hare backslopes erodedat a rate of about 0.2 inch of soil peryear.

Even a partial cover of grassesand associated litter effectively cur-tails erosion from backslopes. Al-though localized erosion may occur inisolated patches of exposed soil,eroded material is deposited abovedownslope vegetation or litter. Thisresults in little net loss of soil fromthe slope.

A grass mulch plus fertilizertreatment may curtail erosion as effec-tively as a treatment that also in-cludes seeding with a variety ofgrasses and legumes. To he effectivewithout seeding, the mulch should' beone that already includes abundantseeds, such as ryegrass hay.

Roadside seedings may requirerefertilization every 4 to 7 years.Without refertilization, some stands

172 JOURNAL OF SOIL AND WATER CONSERVATION

may deteriorate to the extent that anew cycle of erosion is initiated. Myexperience indicated that even standsthat had been declining in vigor for4 years responded quickly and satis-factorily to refertilization.

REFERENCES CITED1. Dyrness, C. T. 1967. Grass-legume

mixtures for roadside soil stabilization.Forest Seiv. Res. Note PNW-71. Pac.

N.W. Forest and Range Exp. Sta., Port-land, Ore. 19 pp.Dyrness, C. T. 1970. Stabilization ofnewly constructed road backslopes bymulch and grass-legume treatments. For-est Serv. Res. Note PNW-123. Pac. N.W.Forest and Range Exp. Sta., Portland,Ore. 5 pp.Fredriksen, R. L. 1965. Sedimentationafter logging road construction in a smallwestern Oregon watershed. Misc. Pub.No. 970. U. S. Dept. Agr., Washington,D. C. pp. 56-59.

JULY-AUGUST 1975 173