Some Relationships of Plant Cover
Infiltration
on Granitic
Run-oil, Erosion, and
Soils
W. M. Johnsonand C. H. Niederhof
RockyMountainForestand RangeExperimentStation•
The fact that plant coverinfluencesrun-oil, erosion,and infiltration has been firmly established
by past research. Only under a few specialconditions,however,has the degreeof this influence
been determinedquantitatively. The study reported in this article was undertaken to secure a
usablemeasureof this quantitativerelationship,and to provide data for watershedmanagement
in the mountainsof Colorado. With the use of small plots, results have been securedwhich
evaluateplant coveras a watershedfactor and at the sametime demonstrate
that under certain
circumstances,
soil conditionsmay overshadowand nullify the eilects of vegetation.
LANTspecies,
types,
anddensities
which The predominantplant cover is ponderosa
largeopenparkson both
regulaterun-offand erosion
mostsatis- pine,with numerous
factorilyon a givenwatershed
can be de- the slopesand alongthe valleybottoms.The
terminedonly by detailedresearchover a con- more favorablesitesin the valleyshave at one
siderableperiod of time. Indicativeresultscan time been under cultivation,but at presentare
and revertingto a coverof mixed
be obtainedrather quickly,however,by useof abandoned
smalltemporaryplotsto whichmoisturemay be weeds,grass,and browse. The predominant
on theabandoned
fieldsareaster(•4ster
appliedartificially. While the resultsfrom these species
plotscannotbe considered
absolutein termsof sp.), sunflower(Helinanthuspetiolaris),and
grass(Stiparobusta).Theareasurroundlarge watersheds,
theycan be usedvery effective- sleepy
fieldsis occupied
by native
ly in quantitativelycomparingplant species, ing the abandoned
bunchgrass
andis designated
in thisstudyasthe
types,anddensities
aswatershed
factors.
•type. The predominant
speAs a preliminarystep in a comprehensivevalleybunchgrass
muhly(Muhlenwatershed
management
researchprogramon the ciesin thistypearemountain
headwaters of South Platte River in Colorado, bergiamontana),Arizonarescue(Festucaari(•4rtemisia
]rigithe smallplot approach
wasadoptedasdesirable zonica),andfringedsagebrush
in themountain
bunchgrass
and necessary
to secureimmediatequantitative da). Thevegetation
results. Beginningin 1936, therefore,a series typewhichoccupies
the parkson the steeper
of studiesusingsmallplotswasstarted.These slopes
is similar
in composition
to thevalley
studieswere designedto measuredirectlythe in- bunchgrass
type,withthesamegrassandbrowse
fluenceof plant type and rainfall intensityon species
predominating.All studieswere conrate of surface run-off and erosion and the effect
finedto thesenaturalpark-likeareasin theabanand mountain
of individual plant specieson the infiltration donedfield, valley bunchgrass,
types.
capacityof thesoil. The resultsobtained,though bunchgrass
inconclusive
in somerespects,
are convincingeviIn orderto helpminimizethe effectof factors
dencethat certainplantspecies
and associationsotherthanvegetation,
onlyplotshavingthesame
have more effectthan otherson infiltration, run-
degree
of slopewereselected
withina giventype.
field and valleybunchoff and erosion,and that the effectiveness
of any In both the abandoned
speciesor type is greatlyinfluencedby soil con- grasstypes,slopes
of 10 percentwerestudied
ditions.
sincetheymostnearlyrepresent
theaverage
slope
condition,
whileslopes
of 40 percent
werestudied
AREA STUDIED
The studies were conducted near Woodland
Park, Colorado, at an elevation of about 7,600
feet. Precipitationin thislocalityaverages
about
18 inchesperyear. It occurslargelyin theform
of rain, with approximately
three-fourths
of the
total moisturefalling betweenApril 1 and September30. Snowfallis occasionally
important
as a sourceof springrun-off.
•Maintained by Forest Service, Department of Agriculture, in cooperationwith ColoradoState Collegeof
Agriculture and Mechanic Arts in Fort Collins, Colo.
854
in the mountainbunchgrass
type.
The soils'of the area are derivedchieflyfrom
granite,butremnants
of limestone
andsandstone
formations have contributed some of the finer
soilparticles.Thesoilsarehighlyerodible,very
porous,
andunusually
coarse
textured.Thesand
andgravelcomponents
(particles
over0.05mm.)
comprise
66 percentof thesoilin theabandoned
field type,76 percentin the valleybunchgrass
type,and83 percent
in themountain
bunchgrass
type.
SOME RELATIONSHIPS
METHODSANDEQUIPMENT
SurJacerun-offanderosion.---Todeterminethe
influenceof plant covertypeson surfacerun-off
and erosion,equipmentwas usedwhich simulatednaturalrainfallon 1/200-acreplots?Metal
baffle plates delineatedthe boundariesof the
plot (6.6 by 33 feet) and at the lowerendof each
a collectortroughconducted
surfacerun-offto a
tankwhereerodedmaterialwasdeposited.Run-
OF PLANT
COVER
855
individual speciesto be studied was selected
within a radius of 10 feet from each coordinate
intersection.The densityof the plantspecies
on
eachplot wasestimated
by the "point-observation" method. Within a givenplanttype,plots
occupiedby bare soil were selectedin a similar
mannerfor comparison
withvegetated
plots.
The equipment
usedfor studyinginfiltration
consisted
of a metalplot frame one foot square,
fitted
with
a feederpipe alongthe insideof the
offwasmeasured
by a recording
tipping-bucket
upper
wall
and with a collectortroughforming
gage. Rainfall was •pplied by sprinklerpipes
the
opposite
wall. From a reservoirattachedto
on eachside of the plot and measuredby four
the
feeder
pipe,
waterwasappliedto eachplotfor
standardrain gagesand one tipping-bucket
re30
minutes
in
quantities
sufficient
to produceruncordingrain gage. A rainfall rate of two inches
off.
Both
water
applied
and
run-off
were meaper hour wasappliedto approximately
one-half
of the plotsselected
in eachtype. The remaining plots receiveda rainfall rate of four inches
per hour. On all plotsthe waterwasappliedfor
a 30-minuteperiod.
An estimateof plant cover densitywas made
on eachplotby the"point-observation"
method,
a
and a soil samplewastakenon eachsideof the
plot for subsequent
mechanicalanalysis.These
samples
consisted
of the surfaceinchof soiltaken
from three holes within a radius of five feet to
form one compositesample. Material eroded
during each test was collected,air dried, and
weighedto determinethe amountof erosionon
eachplot.
In the analysisof surfacerun-off and erosion
the dataweregroupedby rainfallintensities
and
plantcovertypes. The averagerun-offin cubic
feetandthe averageerosionin gramsper cubic
footof run-offweredetermined
for eachtypeand
intensityof rainfall. Both surfacerun-off and
erosionwerethenanalyzed
by thevariancemethod4 usingtwo rainfallintensities
and threeplant
covertypes. An analysisof the soil wasmadeto
determinethe percentof sand and gravel,silt
and clay, and colloids.
Infiltration.--Theinfluence
of individualplant
specieson the infiltration of water into the soil
was studiedon plotslocatedby a systemof 50foot rectangular
coordinates
in eachof the three
plant types. A square-foot
plot containingthe
-•Pearse,C. Kenneth. Specificationsfor the construction and operation of a portable apparatus for measuring surficial run-off and erosion. Intermountain Forest and Range Experiment Station, Mimeo. March,
1936.
sured at 3-minute intervals, the difference be-
tweenthe two representing
infiltration.
The infiltrationfor each3-minuteperiodwas
totaled for all plots characterizedby a given
plant speciesand the averageinfiltration rate
calculatedfor each period and condition. The
infiltration rates for the ten 3-minuteperiods
wereplottedand a free-handcurvefitted to the
plottedpoints. In nearly all casesthe curveflattenedoff afterabout21 minutes,showingthat in
the last 9 minutes the infiltration
rate remained
aboutuniform. Infiltration in inchesper hour
for the last 3-minute period was used in the
analysisto represent
the constantminimuminfiltration capacity.
The infiltration data were tested statistically
to determinethe significance
of differences
betweenthe followingconditions:individualplant
species
versusbare soil within covertypes,bare
soil betweenyears,and individualplant species
between
years. In addition,a simplecorrelation
analysiswas rhadeto ascertainthe relationof
plantdensityto theinfiltrationcapacityof soil.
RESULTS AND DISCUSSION
Run-off.•The volumeof surfacerun-off differed considerably
betweenthe two rainfall intensitiesand the three plant covertypes (Table
1). Run-offwasgreateston the abandoned
field
type,intermediate
on the valleybunchgrass,
and
leaston the mountainbunchgrass
type (Fig. 1).
An increasein rainfall intensityfrom two to four
inchesper hour more than tripled the surface
run-offfor all threetypes. The varianceanalysis
showed
thesedifferences
to be highlysignificant
8Stewart,Georgeand S.S. Hutchins. The point-ob- in all cases.
servation plot method of vegetation survey. Jour.
Amer. Soc. Agron. 28. 1936.
•Snedecor, G. W. Statistical methods. Collegiate
Press, Inc. Ames, Iowa. 1938.
The fact that surface run-off from the moun-
tain bunchgrass
plotson a 40 percentslopewas
lessthan from the othertwo typeson 10 percent
856
JOURNAL OF FORESTRY
slopescanbe explained
by characteristics
of the mediateeffecton erosionin comparison
with the
soil. The plotson the steeperslopeshad sig- plantcoverin the othertwo types.
Of the three types,valley bunchgrass
had the
nificantly
moresandandgravelandlesssilt,clay,
lowest
erosion
rate
per
cubic
foot
of
run-off.
Alandcolloids
thantheplotson thegentlerslopes
though
the
soil
is
finer
than
that
of
the
mountain
(Table 1). Sincethe coarserparticlesin this
soilshould
tendto increase
infiltration,
thelogi- bunchgrasstype, it did not erode as rapidly;
cal assumptionis that soil conditionsobscured probablydue to the reducedvelocityof run-off
on thegentlerslopes,andpossiblyby the slightly
theeffects
ofslope.Thisassumption
isstrength-higher plant density. In comparisonwith the
enedby a porositystudywhichrevealed
that on abandoned
field type,the valleybunchgrass
type
the steeper
slopes
the non-capillary
porespace hasa coarsersoil andgreaterplantdensity;both
was7.55percent
compared
to only1.35on the of thesefactorsapparentlyaccountfor the lower
gentlerslopes
in valleybunchgrass.
rate of erosionin the valleybunchgrass
type.
Plantdensity
influenced
run-offin thisstudy Infiltration.--The infiltration rates of soils
onlywhereabandoned
fieldsandvalleybunch- supporting14 dominantplant speciesand their
grassplotswerecompared.Whenthe mountain comparison
with similar and adjacentbare soils
bunchgrass
plotswereintroduced
it wasagain are presentedin Table 2. Of the ten plant speapparentthat conditionsof the soil overshadowed ciesstudiedin the abandonedfield type, only the
not onlythe effectsof slopebut alsothe effects soil supportingPentstemon(Pentsteraonuniof plantdensity.Therelationof plantdensitieslateralis) showeda significantlyhigher infiltrato run-offwithintypeswasthesubjectof a sepa- tion capacitythan bare soil. In contrast,soil
rate analysiswhichfailedto revealany correla- supportingprostrateknotweed(Polygonurnaviction. Apparently
therangeof density
whichwas ulare) showed significantly lower absorption
possible
to includein this studywasnot suffi- ratesthan comparablebare areas. The effectof
cientlypronounced
to overcome
the naturalsoil the remainingeight specieswas not significant
variations
between
plotsin thesametype.
in either direction.
Erosion.--Erosion
wasfoundto bemostrapid
on the abandonedfields, intermediate on the
mountainbunchgrass,
and least on the valley
bunchgrass
type. Varianceanalysis
showed
these
differences
in erosionratesbetweentypesto be
highly significant. The total volumeof eroded
material naturally increasedwith increasedintensityof rainfall, but the amountof erodedmaterial carriedper cubicfoot of run-offshowedno
significantchange.
The highestratesof erosionon the abandoned
field type were probablydue to two reasons:
first,the natureof the soil,whichis composed
of
a largerquantityof easilyerodiblefineparticles
suchassilt andclay;second,
thedensityof the
plantcoverwhichwassomewhat
lessthan in the
othertwo types (Fig. 1).
The mountain bunchgrasstype, which produced the least surface run-off, was intermediate
in the rate of erosionper cubicfoot of surface
run-off. Althoughthe soil in this type is very
coarseand highly porous,the relativelygreater
erosionwhen comparedwith the valley bunchgrasstypemay be accounted
for by highervelocities of overlandflow causedby the steepness
of
the slope. The densityof plant coveroccupies
an intermediatepositionwhen comparedwith Fig. 1.--Run-off and erosion statisticsfor three plant
types.
the othertwotypes.,
andprobablyexertsan inter-
SOME
RELATIONSHIPS
OF
PLANT
COVER
857
TABLE ].--COMPARISON OF SURFACE RUN-OFF AND EROSION WITH DIFFERENT RAINFALL
TENSITIES AND SOIL FACTORS ON THREE PLANT COVER TYPES
Erosion
Cover
type
Rain
Number
intensity of plots
In. perhr.
No.
Soil components-
Surface percu.ft.
run-off of run-off
Cu.]t.
Grams
2
20
7.8
233.3
4
16
24.1
172.6
2
Sand
andgravel
Percent
Silt
andday
Percent
66
34
76
24
10
9.2
83
17
7
7.7
Abandoned
field
24
6.5
64.8
Valley
bunchgrass 4
25
19.4
66.7
2
24
2.0
111.3
bunchgrass 4
23
8.2
105.2
Mountain
TABLE 2.--INFILTRATION
IN-
Colloids
Percent
17
Plant
density
Percent
4.8
RATES FOR VARIOUS PLANT SPECIES AND THEIR COMPARISON WITH
BARE SOIL
Infil-
Condition
Plant'"
tration
Standard
speciesvs.
Year
Plots
No.
capacity
ln./hr.
errod
ln./hr.
baresoil
ln./hr.
Hordeurn jubatum.........................
Agropyronpauciflorum...............
1937
1937
18
18
1.66
1.45
0.18
0.22
--0.15
--0.36
0.575
1.270
Stipa robusta ......................................
Artemisia canadensis ..................
1937
1937
18
18
1.61
2.00
0.13
0.18
--0.20
+0.19
0.910
0.740
+0.12
0.364
Abandoned
fields
Artemisia frigida .............................
1937
18
1.93
0.30
Bare
1937
18
1.81
0.18
1938
1938
30
30
2.03
4.32
0.13
0.31
--0.64
+1.65
2.3314
4.1675
Helianthus petiolaris.....................
1938
30
2.36
0.21
--0.31
0.950
Cirsium
1938
30
2.51
0.25
--0.16
0.458
Astragalussp..............................
1938
30
2.42
0.27
--0.25
0.671
Bare
1938
30
2.67
0.24
1937
1937
36
31
2.36
1.88
0.12
0.15
+0.56
+0.08
2.906•
0.124
1937
31
1.80
0.15
1937
30
1.81
0.12
--0.46
1937
30
2.27
0.13
..................
................
soil ..........................................
Polygonurnaviculare ......................
Pentstemonunilateralis................
undulatum .............................
soil
.......................................
Valley bunchgrass
Muhlenbergia montana ......................
Festuca arizonica .........................
Bare soil ..................................
Grama (sub-type)
Bouteloua gracilis
.......................
Bare soil .............................................
Mountain bunchgrass
Arctostaphylos uva-ursi ................
Festuca arizonica
.............................
Muhlenbergiamontana .................
•Standard error of the mean infiltration
1938
17
9.67
1.0l
1938
30
6.00
0.70
1938
31
3.16
0.28
..................
2.618•
........
rate.
"This comparisonis expressedas the differencebetweeninfiltration capacitiesof soil occupiedby the plant
speciesand bare soil.
aDifferences
were testedfor their significanceby Student's"t" test.
4Significant.
•Highly significant.
858
JOURNAL OF FORESTRY
In thevalleybunchgrass
type,mountain
muhly The study was made on the watershedof the
causeda significantincreasein the infiltration South Platte River in central Colorado. Artificapacityof the soil, while Arizonarescueshowed cial rainfall was appliedto plots,1/200 of an
no appreciable
influence.Blue gramasignifi- acre in size,to studysurfacerun-offand erosion.
cantlydecreased
absorption
rates. Apparently Smallerplots were usedto determinethe effect
plantcoversuchasbluegramasodwhenpres- of individualplantspecieson the infiltrationcaenton soilswithnaturallyhighinfiltrationrates pacityof the soil.
tends to create conditions which decrease the
For the threeplantcovertypesstudiedit was
rate at which water is able to enter the soil.
Infiltration data for the soil in the mountain
found that:
1. The abandoned
field type producedthe
bunchgrass
type show very high absorptionmost run-off, the valley bunchgrass
an inter-
rates, supportingresultsfrom studiesof surface mediateamount,and the mountainbunchgrass
run-offby larger plots. Soil supportingMan- the least. This was true of rainfall intensities
zanita(•drctostaphylos
uva-ursi)hadthehighest of bothtwo inchesandfour inchesper hour.
rate of infiltrationof any speciestested;more
2. Within each of the three types studied,
than twice as great as the highestrate in the plantdensityseemedto haveno definiteeffecton
abandoned
field type. It is very probablethat surfacerun-off or infiltration. This was probthehigherinfiltrationratesin thistypearenot ably due to the extremecoarseness
of the soil,
dueto the effectof the plantcover,but princi- which in effect far overshadowed the influence
pally to the greaterporosityof the soil. No of plant cover.
studies
weremadeof the infiltrationcapacityof
3. The volumeof erosionper cubic foot of
baresoil in this type.
run-offwasgreateston the abandoned
field,interData from all the plots were combinedin a mediateon the mountainbunchgrassand least
correlationanalysiswhich indicatedthat, under on the valley bunchgrass
type. When the raintheconditions
of thisstudy,therewasno signifi- fall intensitywasraisedfrom two to four inches
cant relationshipbetweenplant densityand the per hour the run-off increased,therebyincreasinfiltrationcapacityof the soil. The lack of re- ing the total amount of erosionalthoughthe
lationship
maybedueto thesmallrangeof plant erodedmaterialcarriedper cubicfoot of run-off
densitiesencountered
within the types,together did not significantly increase. Porous soil,
with the extremelycoarse,poroussoil which gentleslopes,and a high plantcoverdensityall
tendsto minimizethe effectof plant cover on combine to decrease erosion such as in the valinfiltration.
ley bunchgrass
type wherethe leasterosionoccurred and wherethe soil is coarse,slopesare
SUMMARY
gentle,and densityof the plant coverwas the
Beginningin 1936 a studywasmadeof sur- highestof the threetypesstudied.
face run-off and erosionas affectedby rainfall
4. Of the 14 plantspecies
compared
to bare
intensityand plant types, and of the effect of soil,two species
increased
the infiltrationcapacindividualplant specieson the infiltration ca- ity of the soil, two species
decreased
it, and ten
pacityof the soil.
species
had no significanteffect.
Pennsylvan'.mGame Commission Big Landowner
The PennsylvaniaGame Commission
announcedon July 16 that it now owns
664,171 acresof land, and has undercontractfor purchase45,000 acresmore. Both
stategamefundsand federalaid fundsare beingusedfor thesepurchases.