Plant Succession Due to Overgrazing in the Agropyron Bunchgrass Prairie of Southeastern
Washington
Author(s): Rexford F. Daubenmire
Reviewed work(s):
Source: Ecology, Vol. 21, No. 1 (Jan., 1940), pp. 55-64
Published by: Ecological Society of America
Stable URL: http://www.jstor.org/stable/1930618 .
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PLANT SUCCESSION DUE TO OVERGRAZING IN THE
AGROPYRON
BUNCHGRASS
PRAIRIE OF
SOUTHEASTERN
WASHINGTON
REXFORD
F.
DAUBENMIRE
University
of Idaho
matic climax in essentiallyvirgincondiseveral stages of retrogression,
tion,
and
During the period when southeastern
in
end
some
cases
the
point
in
this
biotic
Washingtonwas settledby whiteman the
bunchgrassprairies,which covered large successionappears to have been attained.
areas in thisregion,were used chieflyfor
CLIMATE
grazingcattle. This industryhad scarcely
become establishedwhen it was demonPrecipitationrecordswithinthearea of
stratedthat the deep loessal soils charac- studyconsistof observationsfrom 1899
teristicof the region were excellentfor to 1903 at Hooper, Washington (U. S.
growingwheat. Startingabout 1880 this D. A. Wea. Bur., '36). During this
new typeof land use began to replacethe periodthe mean annual precipitation
was
earlier,and by about 1910 all areas ex- 31.2 cm.,whichincludesa snowfallaverceptthoseon whichcultivationwas either aging 26.7 cm. each year. Calculations
impractical or impossible had been based on considerablylonger records at
broughtinto cropland. The remaining Lacrosse and Lind, whichare just outside
areas, the so-called "scablands," where the area studied,indicatethatthe average
the soil is shallow,or stony,or the sur- precipitationat Hooper is approximately
face is too frequently
interrupted
by such 29 cm.
outcrops,have remainedas grazinglands
Considerablyover half the annual preand probablywill be used in this manner cipitationoccurs duringthe winter. Alin the future. Within the last few mostexactlyhalf of the annual sum falls
decades the livestockindustryin this re- during the four months of November,
gion has undergonea secondgreatchange December,January,and February.
due to the replacementof the cattle by
At Lacrosse theaveragemaximumtemperatureof the hottestmonth (July) is
sheep over mostof the rangelands.
Whereverthe soil of the non-agricul- 31.50 C. withtheaverageminimumof the
tural land was deeperthan about 1.5 dm. coldestmonth(February) - 60 C. The
and was not sandy, the original bunch- average annual extremesof temperature
grass cover was apparentlyidenticalwith are 430 C. and - 330 C.
Some evaporationfiguresfor the sumthat of the deeper soils. As a resultof
long continuedheavy grazing of these mer monthsare available (U. S. D. A.
grasslands,the appearanceand the forage Wea. Bur., '36) for Lind, Washington,
value of the primevalprairie have been which is situated about 43 kilometers
considerablychanged. The bunchgrass,a northwestof the center of the area of
climaticclimax, has undergonea retro- study. These data make possiblethe calgressionwhichin many cases has culmi- culation of the precipitation/evaporation
nated in a biotic climax. The latter is ratiosforthe summermonthsat thatstaas permanentas the presentsystem of tion (table I), and such ratios are probspring-fallsheep grazing.
ably very similarto those in the region
An excellentopportunityof studying wherethe successionalstudieswere made.
The growingseason ends in earlysumthis vegetationalretrogressionexists today in northeastern
FranklinCountyand mer at which time the soil moisturebeadjacentWhitmanCounty. Here, in dif- comes practicallyexhausted. During the
and early fall the sheep
ferentareas, exist fragmentsof the cli- drymid-summer
INTRODUCTION
55
56
REXFORD
Ecology,Vol. 21,No. 1
F. DAUBENMIRE
insure a relativelyuniformclimateat all
stations.
In every case the topographyis quite
level.
Evapora- PrecipitaMonth
tion
tion
Prec./Evap. ratio
To demonstrate
thatthesoils at each of
in cm.
in cm.
the four studyareas are well withinthe
April
13.60
0.81
.059
range of texture (which is one of the
May
19.14
1.27
.066
mostimportant
aspectsof soils in thisreJune
21.74
2.03
.093
July
27.75
1.02
.037
gion) and depthwhichallow a good deAugust
22.97
1.17
.051
velopmentof the bunchgrassassociation,
September
13.85
2.46
.178
a comparableseries of wiltingcoefficient
determinations
was made at each station.
are taken to the mountainsfor grazing. The water-holdingpower of the soil as
In late fall whenthe rainyseason returns, indicated by the wilting coefficients
in
the perennialgrasses and forbsand a few table II, is in all cases well withinthe
annualssuch as Bronsustectorumput out
new leaves and again affordgreen forage
TABLE
II. Wiltingcoefficients
of soils as deindirectly
fromthehygroscopic
coefficients
untilthe colderpart of winter. In mid- termined
winter,whenno otherforageis available, (W.C. = H.C.10.68)
thedead shootsof Agropyronare grazed,
Decimeter Station Station Station Station
at which time theymay be eaten to the
horizons
1
2
3
4
groundline.
TABLE
I. Some average precipitation and
evaporation data at Lind, Washington: 1926
through1930 (U.S.D.A. Wea. Bur., '36)
METHODS
1st
2nd
3rd
4th
5th
6th
7.9%
9.5
8.1
8.0
7.0
6.6
8.4
8.9
10.3
9.2
9.7
7.9
8.9
9.2
9.0
8.3
8.9
8.2
9.0
8.5
7.6
8.5
8.7
Most studies of plant succession,including the present one, are made by
analysinga group of associationswhich
are thenarrangedin a sequenceillustratingtheinvestigator's
conceptof thestages range which is favorable to this comthroughwhich the final stage has gone. munityin our region. Otherobservations
Such a procedureis necessarychieflybe- by the writerhave shownthat this assocause of the greatexpanse of timewhich ciation is apparentlynormallydeveloped
would be requiredto observethe succes- in this region on soils as shallow as 1.5
sion frombeginningto end on the same dm., a soil depthwhichis well exceeded
plot. This methodopens a way for fal- on all the areas studied.
lacious conclusions,since one of the most
A checkon theuniformity
of the origifundamentalprinciplesof the scientific nal cover is the presenceof all of the
methodcan be so easily violated. Obvi- dominantor characteristicplants in the
ously,a studyof the effectsof variation immediatevicinityof all stations. Careof a singlefactorbecomesignificant
only ful attentionto this point indicatedthat
when all other factorsremain constant. the originalvegetationwas at least florisIn any studyof the grazing factor,one ticallycomparablein all cases.
musthave evidencethattheoriginalvegeThe area of bunchgrasswhichapproxitationwas essentiallyidenticalat all the matesthe virginungrazedconditionis on
stationswhichare analysed,and that the a spur projectinginto the Palouse River
environment,
except for the herbivores, Canyon. When the railroad was built
was uniform.
parallel to the canyon,the end of this
All stations occurred within an area spur was severedfromthe remainderof
having a diameterof 32 kilometersand the prairieby a steep-walledcut about 10
the elevation varied from 292 to 445 m. deep,and thushas remainedabsolutely
metersabove sea level. These features freefromgrazingfortwenty-seven
years.
January,1940
AGROPYRON
BUNCHGRASS
conOtherareas apparentlyin near-virgin
dition are very similar to this stand,
hence the assumptionof its undisturbed
natureseemsto be well supported.
The abundance and stature of the
Agropyronplants was taken to be an
indicatorof the relativeseverityof grazing. Actual records of the kinds and
numbersof animals and the periods of
if not imposrangeuse would be difficult
sible to ascertainand to evaluate in the
PRAIRIE
57
areas studied. The decrease of palatable
plantsand increaseof non-palatableones
bothpointto the validityof my assumptions of the proper sequential arrangementof the stations.
A statisticalcomparisonof thesegrazed
and ungrazed areas of grassland was
made by the frequencymethod. In each
of fourareas considered(figs. 1, 2, 3, and
4), a stationconsistingof 100 frequency
plots was studied. The plots were 2 X 5
FIG. 1. Station 1. An area whichwas lightlygrazed priorto 1911,and
typicalof the associationin virgincondition.The domiwhichis apparently
85 per cent of the annual forageoutput
nantAgropyronbunchescontribute
withthesmallerinterstitial
plantsof Bromusand Poa each
in thiscommunity,
5 per cent. The stake is markedoffinto decimeterunits. All
contributing
fourpicturesweretakenon May 21, 1938.
the smallestAgropyron
FIG. 2. Station2. Heavy grazinghas eliminated
bunchesand reducedthe size of thoseremaining.The groundis developinga
coverof smallplants,partof whichare annualsand partperennials.
58
REXFORD
Ecology,Vol. 21,No. 1
F. DAUBENMIRE
FIG 3 Station3 Agropyron bunchesare eaten back to the groundline
so thatthe shortweak shootswhichare put fortheach year can be distinguishedonlywithclose study.
.
.
. ....
:.;.....
.............
:.......
..... . ..| .. . ; .........
................................................
........
....:::
.~~~~~W
...;
.....
.....
..~~~~~~~~~~~~~~~~~~~~~~~
eliminated
is completely
FIG. 4. Station4. Agropyron
here,but the soil is
of small unpalatableplantswhichis
fairlywell coveredwith a community
dominatedby the perennialPoa secundaand a groupof annual dicots-apparentlya bioticclimax.
groups. Group one includes those species which decrease in abundanceas the
intensityof grazing increases. This decrease in some if not most cases seems
directlydue to stock eating the aerial
shoots. In perennials,suchas Agropyron,
organsbecomeweakened
theunderground
caused by the
due to undernourishment
tissue,
repeatedremovalof photosynthetic
RESULTS
and the declinein vigorof such plantsis
In table III the species encounteredin gradual until the plant succumbs. With
all 400 plots are classified into four an annual which is grazed, such as
dm. in dimensions,and were arrangedin
fiveparallelrows one meterapart. Each
plot of the400 was studiedat threecritical dates (April 7 or 8, April 30 or May
1, and May 21 or 22) which were so
timedon a basis of previousobservations
that the entire vascular flora (part of
whichis veryephemeral)couldbe studied.
January, 1940
AGROPYRON
BUNCHGRASS
Brotus tectoruns,
thefirstfewyearsduring whichthe stockfeed on the plantsso
heavilythatno seeds are maturedspell a
rathersudden extinctionof these plants
from the range. Scatteredgroups and
individuals of Broinus seem to escape
grazingand smutepidemiceach year and
TABLE
III.
Behavior
under
influenceof
heavy
grazing
59
PRAIRIE
thesematurea few seeds whichare scattered somewhatby the sheep, perpetuating the species in a verythinpopulation.
In this same group theremay be other
causes forthedisappearanceof theplants.
Some of those species which are not
grazed, such as Lithospermumi,
may owe
Frequency of plants as affectedby grazing. Presence in the community,althoughnot
frequentenoughto have beenincludedin one of theplots,is indicatedby +
Annual
(aa)
or
Bi- perenCli- Early Late
otic nial
rematic reclimax trogr. trogr.climax (P)
Per cent frequency
of occurrence
Species
*
Agropyronspicatum (Pursh) Scribn. & Smith
Bromus tectorum
L...
&
GayophytumramosissimumT.
6.t.................
Agoseris heterophylla(Nutt.) Greene.............
Achillea lanulosa Nutt.............
CalochortusmacrocarpusDougl ..................
Decreasing Lomatium macrocarpum(Nutt.) C. & R ..........
in
Astragalus spaldingii Gray.....................
Frequency Erodium cicutarium(L.) L'Her ..................
Gilia gracilis Hook .............................
Lagophylla ramosissimaNut....................
Festuca idahoensis Elmer.......................
Lomatium triternatum
(Pursh) C. & R ............
.......
SisymbriumlongipedicellatumFourn.......
LithospermumruderaleDougl ...................
70
100
58
16
6
4
3
2
1
1
1
+
+
1
+
Plantago purshii Nutt..........................
Plagiobothrystenellus(Nutt.) Gray..............
.53
.
Cryptantheflaccida(Dougl.) Greene
Pectocaryapenicillata (H. & A.) A. DC
var. elongatum(Rydb;) Thell.
Increasing in Lepidium densiflorum
Frequency Lappula redowskii(Hornem.) Greene............
Erigeronconcinnus (H. & A.) T. & G
Antennaria dimorpha(Nutt.) T. & G.
Agoseris glauca (Pursh) Steud...................
Stephanomeriapaniculata Nutt
84
12
Apparently
Characteristic of
Intermediate
Stages
53
3
17
9
3
22
31
4
9
+
6
+
+
+
+
100
28
56
3
2
2
25
LithophragmabulbiferaRydb....................
63
Festuca pacifica Piper..........................
.79
.
Alchemilla occidentalisNutt
L... .
Sisymbrium'altissimum
.27
Lomatiumfarinosum (Hook.) C. & R.
Madia exigua (Sm.) Gray......................
+
.
.
Delphinium nelsoni Greene
.
.......
Ranunculus glaberrimusHook
98
100
100
70
69
61
2
99
100
21
100
100
Poa secunddcPresl.............................
Frequency Draba vernaL ..................................
Scarcely Brodiaea douglasii Wats........................
Affected mosses (all Bryales).99
1
1
+
100
100
100
79
67
32
30
8
6
2
aa
aa
aa
aa
aa
aa
p
p
p
aa
36
8
p
aa
aa
aa
p
aa
p
p
2
10
+
1
100
2
+
+
p
aa
aa
p
p
p
p
p
aa
aa
aa
p
p
aa
P
+
2
32
2
1
100
+
2
2
100
100
66
100
100
100
23
100
p
aa
p
p
*
Includes A. inerme (Scribn. & Smith) Ryd. which is considered by the writeras one formof
A. spicatum (Daubenmire, '39). All have identical ecology in our region.
t This species and Epilobium paniculatumNutt. cannot be distinguishedby vegetative structures,
and since veryfewofthe plants flower,both species have been lumped underthe name ofthe one which
seems to be the most abundant.
60
REXFORD
F. DAUBENMIRE
theirextinctionto trampling. Still others
may have a more remotecause for decadence, namely,the removalof protection affordedby the largerand morepalatable plants. That this can be a potent
factoris well shown by the great reductionin statureof Poa in the bioticclimax
(or even in the spaces betweenthe Agropyronbunchesin the climaticclimax) as
compared to specimens close up under
Agropyronbunches. Those in the latter
situationare from 50 to 100 per cent
taller, apparentlydue to the protection
from insolation and desiccating winds
which the tall plants afford.
fromthe standpointof
It is significant
range managementthat 90 per cent by
dryweight,of the total annual outputof
shoots in the Agropyronbunchgrassassociationis producedby two membersof
this group of species (Agropyron85 per
cent and Bromus 5 per cent). The
amount of this forage available in the
bioticclimax is negligible.
A second group containsthose species
which seem to be favored by grazing.
These plants for the most part are not
palatable (except for the basal leaves of
LepidiumandAgoserisand smallamounts
of the young shoots of Cryptanthe),are
not seriouslyinjured by trampling,and
theirshootscan withstandexposureto the
full force of dryingwinds and intense
insolation. Apparentlythese species are
kept out of the climaticclimax by the
affordedby the dominantsof
competition
that community.This competitiveinfluence of other plants seems more detrimentalto thesespecies than the influence
of the animals.
Most of the plants in this group are
small annuals, and members of the
Boraginaceae are conspicuous among
them. Some may be avoided because
they are woolly (Plantago), others because theyare hispid (Lappula) or otherwise distastefulto stock. Certainof them
may escape only because theirfoliageappears at the same timeas thatof a more
desirablespecies. In the case of Festuca
pacifica the foliage is extremelyscanty
Ecology,Vol. 21,No. 1
and the stems wiry. Obviously these
plantsmake up mostof the bioticclimax.
A thirdgroup of speciesis made up of
plants which appear to be immediately
benefitedby the removalof the competitiveinfluenceof theclimaticclimaxdominants, but which are not very well
adapted to the biotic and aerial factors
whichaccompanythefinalstagesof range
deterioration.The conditionswhich favored group two also favor this group
during the early stages of overgrazing,
but some or all of the factorswhichoperate againstgroup one finallycome into
play so that these plants are ultimately
eliminated. Most of thesespeciesare primarilyseral,beingpracticallywithoutrepresentationin eitherclimax.
The fourthand last categoryis an assemblage of hardy unpalatable plants
whoseabundanceseemsscarcelyincreased
or reduced by the radical changes in
bioticand aerial factorswhichare brought
about by intensegrazing. All were present in the climaticclimax,have persisted
throughoutthe period of deterioration,
and are importantmembersof the biotic
climax.
One plant in this last group, Poa
secunda, deserves special mention. This
bluegrassformsabout 5 per cent of the
total shoots (by dry weight) produced
annuallyby the vegetationunderclimatic
climax conditions. The forage value of
thisspeciesovermostof its rangeis rated
as fair, and the average heightis about
3 dm. (U. S. D. A. For. Ser., '37). As
the protectiveinfluence of the larger
plantsin our regionis removed,the stature of Poa decreases until the plant is
considerablybelow this average, and the
leaves becomeso fineand wirythatthey
are scarcelyever eaten by any class of
livestock. Undoubtedly the nature of
thisplant'sresponseto the removalof the
larger and more palatable Agropyronis
the key factorin its persistenceinto the
bioticclimax. The climateof thisregion,
withits extremelydrysummers,probably
emphasizesthis phenomenon.
From the foregoingit is evidentthat
January,1940
AGROPYRON
BUNCHGRASS
plant successionleads from the climatic
climax of Agropyron-Poa-Bromus
to a
bioticclimaxdominatedby Poa and small
annual dicots among which membersof
the Boraginaceaeare conspicuous. This
process is not marked into recognizable
stages. Animal influenceresultsin bare
areas only where tramplingis excessive,
such as around a corral or bedding
ground.
Since Agropyronand Bromus together
form90 per cent of the climaticclimax
vegetationand nearly100 per centof the
valuable forage,any plan of range managementwhich maintainsthe vegetation
in a conditionmost nearlyapproximating
the virginstate would be most desirable.
Althoughno extensivestudyhas yetbeen
attempted
to determine
themostappropriate grazingsystemin thisregion,someobservationsby the writerare significant
in
this connection.
Several clippingexperiments
weremade
in easternFranklinCounty. When Agropyron was clipped to the ground at the
heightof its growingseasonin latespring,
mostof theclippedplantssuccumbed,and
the nextyear the few survivorsproduced
only small tufts of foliage with no inflorescences.On the otherhand,grazing
the Agropyronbunches to the ground
PRAIRIE
61
during the summerand fall appears to
have only slight detrimentalinfluence
upon theplants. The writerhas observed
a fence-corner
relic of this associationin
WhitmanCounty,whichhas remainedin
fair conditiondespite the fact that livestockwhichare turnedintothe fieldafter
the wheat is harvested often graze the
Agropyronto the ground during this
periodof vegetationalaestivation(figs. 5
and 6). These observationsindicatethat
a desirablegrazingsystemwould emphasize fall or winter utilizationof cured
shoots and minimizespring use of the
range.
In anotherexperiment,single bunches
of Agropyronwere released from the
competition afforded by surrounding
plantsby keepingthe latterclippedto the
ground. The experimental
plantsshowed
no great responsethe subsequentseason,
but by the thirdyear the increasein size
and vigor was very apparent,so that it
may be concludedthatcompetition
checks
the stature of even the most dominant
plantsin the association.
Withintheareas whereAgropyronwas
practicallyeliminatedby clipping,some of
the annuals, chieflyBromus, Erodium,
Lagophylla, Plagiobothrys and Sisymbriumlongipedicellatum
showed increase
April 7. Agropyronin the pastureto
FIG. 5. WhitmanCounty,Washington,
the left of the fencewas grazed to the groundduringthe precedingautumn.
Near-virginprairieto the rightof fence. The soil here is abouta meterdeep;
to theexcavationof a deep
thestoneson thesurfacewerethrownthereincidental
road cut to the rightof the photograph.
62
REXFORD
F. DAUBENMIRE
Ecology,Vol. 21,No.1
FIG. 6. Photographof the same area shownin figure5S taken on May 21
afterthe Agropyronwithinthe pasturehad attainednearlythe size of the ungrazedplantsto therightof the fence. The bunchesoutsidethefenceare denser
and lighterin color due to the presenceof old bleachedshoots. Sisymbrium
is conspicuouswithinthe pasture
altissimum
in size the firstyear,and increasein numbers on subsequentyears. Lappula has a
strongtendencyto invade these clipped
areas.
These clippingexperimentsshow that
there is considerablecompetitionamong
thedominantplants(Agropyronbunches)
as well as betweenthemand the subordinate annuals. When Agropyronis weakened or destroyedby injury during its
period of active growth,the lesseningof
competition
allows the annual florato expand so that the ground surfaceremains
well covered. Althoughvaluelessas forage, theseplantsof the bioticclimaxhave
distinctvalue as soil bindersin as much
as they are fairly efficientin retarding
erosionof the topsoil.
DIsCUSSION
The above studies,togetherwith other
observationsby the writer,bringout the
factthatthe ecologicresponsesof several
plants in Washingtonare different
from
theirresponsesto grazingin otherparts
of the western United States, as indicated by the literature. Four plants deserve special mentionin this connection:
Salsola kali L., Bromus tectorum,Agropyronspicatum,and Artemisiatridentata
Nutt.
In the recentsummaryof range conditionsin the westernUnited States (U. S.
D. A. For. Serv., '36), some generalreferencesare made to the behaviorof these
plants in regions where Agropyronspicaturn is dominant. These references
seem to be based on researchesin the regions to the south and east of Washington since no previous study of grazing
successionin this state has appeared. It
will be evidentfromthe followingdiscussion that in Washington,which includes
only a small part of the total Agropyron
rangeland, conditions apparently exist
whichare not typicalfor the range as a
whole.
Salsola, this source states, is widely
distributedin some parts of the bunchgrass range. In our particularregionit
is closelyconfinedto road right-of-ways,
and seems whollyincapable of invading
virginor grazed pasturesin this prairie
type.
Bromus tectorumw,
this accountalso remarks, "is now dominatinglarge areas
formerlyoccupiedby bunchgrass." This
is true in Washington,but the statement
needs qualificationhere. Bromus does
not dominate ranges which are being
heavilygrazed duringthe springseason.
When, after heavy grazing, herbivore
January, 1940
AGROPYRON
BUNCHGRASS
pressure is reduced, Bromus quickly
dominatesthe area, the other species of
the climax enteringmuch later. Patches
where the densityof this plant is very
high occur throughout
all grazed areas in
this region. These may possiblybe explainedby a lack of uniformity
in spring
grazing,coupled with the readinesswith
whichthe species will entergrazed lands.
If on two or more successive years, a
given area escaped grazing during the
short vernal season when Bromus is
highly palatable, this grass would undoubtedly dominate the area. These
island-likepatchesof Bromnus
increasein
abundance with distance from watering
places, a fact which is in harmonywith
this explanationof theirexistence. That
the Bromus colonies are not the simple
and directresultof overgrazingis indicated by the extensivegaps betweenthe
colonies,whichgaps are heavilyused by
the sheep.
Smut infectionsare not responsiblefor
the interstitital
areas because the Brornus
islands show strong conformitywith
fences whereverthese result in unequal
grazingintensityin adjacent pastures.
There also seem to be other factors
(perhaps cultivation,fire, etc.) which
promotedense standsof thisgrass,but it
is not to be consideredhere as a species
directlyfavoredby overgrazing,for it is
highlypalatable and is practicallyeliminatedby uniformly
heavyspringgrazing.
Again, it is statedthatin some regions
the Artemnisia
tridentateassociationmay
enterand dominatean area whichbefore
grazing contained little if any of this
shrub. This cannotbe said forWashington, however,for the writerhas seen no
evidenceof this phenomenonalthoughhe
has lookedforit forthreeseasons. Communitiesof Artemisiatridentataare commonin the regionwherethepresentstudy
was made,but theseare to be foundonly
on sandy soils of stream terraces and
never show the slightesttendencyto invade the surroundingfiner-textured
uplands from which most or all of the
bunchgrasshas been grazed.
PRAIRIE
63
Agropyronand Artemisiaseem to be
ratherthan competitive
complementary
in
centralWashington,whereasin southern
Idaho the competitionbetweenthe same
speciesis keen. In partsof the latterregion overgrazingtendsto resultin a pure
stand of the shrub,while completeprotectionfromgrazing allows the grass to
becomeat leastequallyas important
as the
shrub in the climaticclimax (Craddock
and Forsling,'38).
Any peculiaritieswhich may exist in
the behavior of plants in southwestern
to
Washingtonmaypossiblybe attributed
severalpeculiaritiesof the climatein this
region. In contrastto regions farther
south and east, the climatein this north
arm of the Columbia Plateau is characterizedby considerablygreatercloudiness
(Kincer, '22, p. 44) and less insolation
(Kincer, '28, p. 33), with a consequence
that the average relativehumidityat 2
P.M. is higherat all seasons (Kincer, '22,
p. 45).
Althoughthe total precipitationis not
significantlydifferentbetween the two
portionsof the Plateau (Kincer, '22, p.
30), thehighermoisturecontentof theair
resultsin a higherprecipitation/evaporation ratio for the frostless season in
Washingtonthanin southernIdaho (Livingstonand Shreve, '21, p. 342).
Winter temperaturesseem also to be
influenced by the humid air which
blanketsthis northernregion. Judging
fromthe numberof days with the minimumtemperature
below freezing(Kincer,
'28, p. 9), the wintertemperaturesare
less severein Washingtonthanin Oregon
and southernIdaho.
These climaticdata indicatethat sufficient differences
in climatemay exist to
accountfordifferent
ecologicresponsesof
Agropyron,Artemisia, and other associated plants,in Washingtonand in regions farthersouth.
ACKNOWLEDGMENTS
The writerwishes to express his indebtedness to the Northwest Scientific
64
REXFORD
F. DAUBENMIRE
Association for supportingthese studies
witha ResearchGrant,and to Mrs. Daubenmire for valuable assistance in the
field.
Ecology,Vol. 21,No. 1
midity. Atlas Amer.Agr. Part II, Sec. A:
1-48. U. S. Govt. Print.Office.
1928. Temperature, sunshine, and
wind. Atlas Amer.Agr. Part II, Sec. B:
1-34. U. S. Govt.Print.Office.
Livingston,B. E., and F. Shreve. 1921. The
LITERATURE CITED
distributionof vegetation in the United
States as related to climatic conditions.
Craddock,G. W., and C. L. Forsling. 1938.
Carn. Inst. Wash. Publ. 284: 1-590.
The influenceof climate and grazing on
spring-fallsheep range in southernIdaho. U. S. D. A. Forest Service. 1936. The western range. Senate Document199: 1-620.
U. S. D. A. Tech. Bull. 600: 1-42.
. 1937. Range plant handbook. U. S.
Daubenmire, R. F. 1939. The taxonomy
Govt.Print.Office.
and ecology of Agropyron spicatum and
A. inerme. Bull. Torrey Bot. Club 66: 327U. S. D. A. Weather Bureau. 1936. Cli329.
maticsummaryoftheUnitedStates. Sec.
Kincer, J. B. 1922. Precipitationand hu2: 1-40.