RexfordDaubenmire
Route 1, Box 265 A
Sorrento,Florida 32776
MountainTopographyand Vegetationpatterns
Abslracl
In rhe northeroRocky.Mountains,ropographic
controlover microciimates
allowsvegaationthat
rs chardcrerrsric_of
subaipine
environmenrs
to descend
locallyro verylow altirudes.
w6ereas
srepoe
vegeration
canbe foundlotallyat ratherhigh elevations.
fheretori,rhe siqniticance
"f.].;r(i;;
abovesealevelhas!ery linle ecologic
s;gnificaDLe
in rhisarea.
Introduction
The progressivechange in plant life obsewable wherever one ascendsthe mountains
long ago stimulated atempts !o divide montane vegetation into altitudinal zones,with
quantitative altitudinal limits often suggested.So long as only a few very broadly conceived zonesbasedon recoonaissaoce
level observationswere recogoized,figures referring to their alti.udinal limits were commonly accepred.But experiencehas shown that
as the synecologyof at uea is carefully investigated,rhe concept of alritudinal zones
weakensprogressively.Somespecificexamplesof this point, basedon srudiesin flotthero
Idaho and eastern NTashington, are here presented.
Methods
The field work was basedon analyzinglandscapesfrom the standpoint of the type of
potential (i.e., climax) vegetation thar should develop in the absenceof disrurbance.
Since most of the forest land has been logged, burned, or heavily grazed in the past
ceorury,the presentdevelopmentalstagesof the vegetadoooften bear little resemblance
to the type to be expected when srability is attained. Such disturbance types usually have
boundariesbearing limited relation to rhe physical envilonment, whereasclimax types
are closelycoupled with a relatively fixed pattern of physical factors.Unless vegeration
dynamicsare emphasized,the order rhat exists in the field is obscuredby the closeresemblancethat a disturbancetype in one habitat type may bear to the climax
rype elsewhere.
Popularion sftucture is the key to understandinghere. Seedlingsor suppressedtrees
in rhe forest indicate rhe furure domioantsregardlessof the characterof the presenroverstory. Climax equilibrium can be recognizedonly when the compositionsof the canopy
and young trees are similar.
The PalouseRange
A striking illustration of the way topographic featuresmay complicarethe influence of
altitude on vegetationmay be observedon the PalouseRange in the western foothills of
the Bitteroor Mountains. A few km ooth of Moscow, Idaho (Fig. 1), the south face
of this Range rises abruptly from the basal plain to a clest more thart 600 m above. This
Raoge, as well as othe! mountains ro rhe norrh and easr, is forested, whereas sreppe ex_
tends to the horizon on rhe sourh ano wesr.
146
Northwest Science,Vol. 54, No. 2, 1980
i
O NORDMAN
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--J-t-r'
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P41^\.1---4--..J
RIVER
I--------
o MOSCOW
CLEARWATE;
6v<
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"N..-i
Figure 1. Nortbern Idaho showing the locations of cities and rivers mentioned
Mountain Topography and Vegetation Parterns
147
By walking from the crest of the main ridge southward down the side ridges and
valley bottomsto the edgeof rhe basalplain, the altitudinal limits of the forest types have
been obtained vrith an aoaeriodaltimeter. Such altitudinal determinationsare ooly approximations owing to the necessityof depending on rhis iostrumenr for many hours
betweenopportunitiesto recalibrateit at a known benchmark.
Figure 2 is an elevationaldiagram in which no great attempr has been made to keep
horizontal distancebetween topographic featuresproportionate, and only the principal
valleys and cootiguousridges have beeo showo. The branching of the valleys is coosiderablymore intricate than cao be suggestedon an elevationaldiagram, but this simplificatioo does no violence to rhe basic pattern.
Two more or less independentzooal patterns are evident in Figure 1. By ignoring
the valleys aod confining aftention to the interfluves that are fully exposedto sun and
wind, one can recognizean easr-westseriesof altitudinal belts which have a decided
cant, with the ends rcwald the sreppe lying at relarively high altitudes. Since these altitudinal belts rise as the massof the ridge diminishes,rhe situation constirutesa local reversal of the Massenerhebungeffect, in which the altitudes of macroclimatesand biota
oormally increase as mounrain mass increases.The explanatioo of the anomaly is
obvious. On approachilg the arid steppe,each forest belt finds suitable conditions at
higher and higher altitudes, with lowered remperatulescompetrsaringas precipitation
drops off.
A secondevident systemof zones is one that is repeatedin the deep valleys that
cleasethe face of the Range.Each deep valley containsa fusiform island of distinctive
vegetatioo, orieoted at right angles ro the somewhar horizontal system of zones_Since
these d€ep valleys open sourhward, sunlight penetrates to the bottom for only a short
period of time centeredon noon, so the valley-dependeorsysremis reladvely mesophytic.
Starting very simply near rhe western extremiry of the ridge, the vegetationof the fusiform islandsbecomesincreasinglymore complex to the east,until it consistsof several
conceotlic types,arrangedin a seriessrith the most mesophyticmember deepestin the
valley.An additional elementof complexity,not shown,is thar alire of Piceaengelmannii
follows the creek banks io each suip ol Thu.ja fotest.r'fhe Picea,having seedlingsthat
F j a u r e 2 . F l e v a u o n a l a i a g ^ r a ms h o w i n g r h c . p a r r e r n o ( . p o t e n t i a l v e g c r a r i o nr ) p e , o n r h e s o u r h t a L e
or rne rarouseKange, Dorth ot Mosco\\. tdaho.
rFor descriptionsof forest communitiesnamed, seeDaubenmire and Daubenmire (1968).
r48
Daubenmire
require a more constantlymoist soil surface,does oor extend ro either the uppef or the
lower extremity oI the Thuja islands. For example, in the island opposite the highest
poiot of the Range (i.e., Moscow Mountaio) , Picea occrrs as a seral streamsidespecies
betweenthe elevatiols of 900 aod 945 m, whereasthe Thuja island extendsfrom 863
to 1158 m.
If one startsat the lower exffemities of the deepervalleysand travels up the creeks,
he encountersvegetationrypes in order from dry to moist, as is generallytrue wharever
courseorre takes to leave the steppe aod ascendthe Bitterroots irr eastern rVashington and
adjacent Idaho. But above the middle of the valley the order becomes reversed, and the
higher he climbs the more xerophyric rhe vegetationbecomes.The combined effects of
increasingexposuleto suo and wirld coupled$/ith the blo$/-overeffect on precipitation.
and good soil draioage,outweigh rhe drought-amelioratioginflueflce of loweriog air
temperature.
Sometemperaturemeasurementsin one of thesevalleysare significanr.At the lowest
end of the Thuja island, which is situated in the westernmosr valley to contain such forest, daily maximum and minimum temperatuleswere measuredat an elevation of 1 m
above the soil surface. One starion \r'as h rhe Thuja srrip along rhe creek, with another
lr the Pinwt ponderosaforest on the ridge above,90 m higher. During the month of
study (22 June'2O July), there was a themal inversion 93 percent of the nights, as
well as 68 percent of the days.The ridgetop averagedwarmer by 1.1'C during the day,
and 8.2oCduring the nighr.
Since srudies of the soil moisture regime in this vegetation mosaic (McMion,
1952; Daubenmire, 1968) have revealeda consistentffeod of increasiflgdrynessfrom
Tbuja to Abie: grawtrit to Pseud,ottugato Pin J forcst, it would appear that cold air
drainage,often continuing during the day as well as lhe night, is an important factor
maintaining the strong moisture gradieot from valley bottoms to ridgetops.
In placeswhere the crest of the main ridge has a slight dip with a proiecting k[ob
or lateral spur immediately west of this dip, the suips of relatively mesophyticvegetation extend up to afld acrossthe summit to become continuous with a more exteosive
body o{ the same forest type on the norrh slope of rhe Range. In these casesthe south
slope units are analogousto pelinsulas rarhel rhao islands,although complerclyisolated
fragmentsof all four major vegetationtypes occr:r elsewhereon the south slope of the
Range.The farther from the west end of the Range,the greater the tendencyfor valley
systemsto be like peninsulasinsteadof islands.
\(here the ridge crest is not marked by a protected dip, there is a very abrupt ecotone between a lelatively xerophyric commuliry on the sourh slope and a more mesophytic community on the north stope. The ody exceptioo is on the highest point (Moscow Mouotain): Abies forest e[circles the peak, although its expressionon rhe south
side is very impoverishedA fourth noteworthy feature of the south slope is the nature of the vegetation jusr
below the summits of each of rhe major prominencesalong the crest of the Range,
especially West Twio, East Twin, and Moscow Mountain. Each of these places is
occuPiedby vegetationwhich ha"swide distribution only at considerablylower altitudes.
On \7est aod East Twin the vegetationislandsconsistof a small savanna-likestand of
a.Pinus-Pseudotwgaintergrade.The pines rhar are in the open forestsof theseresuicred
areas suffer from wind breakage, but they are not noticeably dwarfed.
Mountain Topographyand Vegeation Pattetns
r49
The xerophytic islaod ar the top of Moscow Mountain consistsof a spot of grassland which resemblesvery closelythe F elracd-SJtmp
boicatpoJ associationon the adjacent
basalplain, and is partly surroundedby an irregular zoneol Slmphoricarpor shrubbery.
The slender frin'ge of Psetdotsugawhich borders this park is open and savanna-like,
with short trees having rapid taper. The environrnent here is so ill-sdted. to Pinus
Ponderorathat ody ooe small specimenocculs at the upper margio of the park.
In no casedoes the vegetation of these exposedsituations exhibit evidenceof previous occupancyof the site by more mesophytictypes. Nor is there any evidencethat
succession
to a more mesophytictype is takiog place at present.Theseislaod-like stands,
as $/ell as the special strips of valley vegetation,are thelefofe to be classedas topographic climaxes io equilibrium with a set of enviroomeotal factors iohereot in their
exPosufe of protectiol.
Every vegetation type from steppe to Tlruid forest exists at elevatiols between 1000
and 1500 m on the south slope of the PalouseRange,and this vegetationmosaic is not
ao isolated extfeme. For example, io rhe Clearwarer Valley about 40 km to the south,
similar topographic patterns in the vegetation can be found, arrd on a much larger scale.
Frost PocketEftect
The vegetationpattern describedabove illustratesclearly how topographic control over
the eovironmental warer balance, improving it in some places and impairiog it elsewhere, cao over-ride the broad-scaleinfluence of macroclimatein determioing the plaorenvitonment potential locally.Another type of environmentalcontrol, operadng through
temperatule and apparently rather free of water balance,involves cold air drainage. Two
examplesfollow.
The north-facing slopesof the PalouseRiver Valley approximately 15 km east of
Potlatch, Idaho, support mainly an alteroarioo of Abies grandit/Pachiaima and Thuja/
Pachioima habitats. The latter are restricted to valleys that clease the slope arrd exrend
up to an elevationof about 880 m, the former dominating the ridges between.This pattern extends down the valley sides to the river rerrace forming the flat valley floot at
about 780 m, where both habitat types eod abruptly. The valley floor belongs to the
Abiet latiocarya/Pachisima habittat type, alrhough clearing and grazing have left little
mole than Abies lasiocarpa ard Picea engeltnarnii ftom the natufa.l flofa.
Owiog to cold-at drainage,the valley floor is an elongatedstrip of lanci subject to
frost any night in the year,aod this frost apparentlyexceedsthe toleranceof uees primarily adapted to oceaoic climate, s\lch as Ab;es grdad.fuar.d T huja, allowiog continental speies that are more frost-toleraot to retain domioance. The pennant-shaped islands of Thuja,
which broaden downslope, abruptly and remarkably terrinate at the very edge of the
llvet telrace.
A much more common example of frost allowing ffees characteristicof the highest
forestedridges to descendto los/ ahi$de is the frost-pockerpheoomenon.(The Palouse
"pocket,"
Valley is too long a strip to merit the term
although the vegetatio[ phenomenon, if not the topography,is idendcal with thar of the pocket.) A frost pocket is the
result of a consffictioo in a valley where opposed spurs along the sides draw close
togethe!. Cold air draioing into the valley flows down its gradieot more lapidly than
it can filter through the oarrow gap between the spurs, so that each nighr a pool of
cold air accumulatesimmediately above the bottle-neck.
1i0
Daubenmire
A fioe exampleof a frost pocket catr be observedapproximately25 km eastof Moscow, Idaho, where the highway to Deary drops briefly into the valley of Dry Creek.
Abier grandit/Pachirrima forcst is the potential climax cover of north-facing slopes
m+baceuJin this role on south-facingslopes.Behere, with Pseudotsuga/Physocttrpal
fore the valley floor was nearly all clearedfor agriculture,it was coveredwith Abies lav
iocarpa/Pachioi,oa forest. Howevet, oot even barley proved successfulon this frosty
surface,so unmanagedPoa prdteuir pasture has become the principal land-use.The
elevatioo of this frost pocket is approximately820 m. Twenty-five km to the west, at
Moscow,Idaho, Fertuca/Slnzphaicatpol steppecoversthe hilly topography to this elevarion! Here again rhere is a sharp distinction between continental cee specieson the
valley floor, and the oceanicAbies grandit and Ldrix occidentalis(seral) on the valley
sides.Pi,ced,commonly a seral tree towald the wetter margin of the Abiet grandis totest,
here extendsa short distanceup mioor ravines off the frost pocket.
Frost pockets closely similaf to the above ale cornmon io the forests of northerrr
Idaho and easternVashington, and usuallythere is no evidenceto indicate that the frost"
toletant Abier latiocatpa ^\d Picea are susceptibleto eventual displacementby competitively superior trees. The sole exceptioo thus far ooted is as follows.
IThere Granite Creek flows from Washington into Idaho at a point approximately
11 km northvrestof Nordman, Idaho, opposedspurs of the valley sidescome so close
togetherthar rhe creek once occupiedneady all the nallow gap. Even the constructiooof
a one-laoeroad necessitatedblasting away o{ the northern spuf, and much more of the
samewas larer neededto widen the road to two-lane status.
Immediately below the bottle-neck,Tsuga heterophylla/Pachittimaforest occupies
all slopes within sight of the road, extending to the very margios of Granite CreekAbiet lariocarpaand Picea engelmanniiare seral in rhis habirat tt?e, and at preselrtare
of rare occurtence
below the constricrion.
Immediately above the bottle-neck,large trees of the Abiet ^r'd,Picea are abundant,
with smaller trces of T rtuga and the seral Thu ja plicata well represeoteclamorg them. As
ooe plogressesalong the road up the creek,Abiet ar.d Picea thi.rLout, afld beyood about
6.5 km from the constriction the Tsuga/Pachi*ima habitat type prevails even dowo to
the creek margil. Since this point is approximately60 m above the creek at the constriction,the pool of cold air that is ecologicallycritical may be (ol was,Prior to blasting)
about 60 m deep at the coostrictioo,if its surfaceis level. The envirooment in this frost
pocket appears to have allowed Abiet af'd Picea to gair dominance soon after the last
dwastating fire accentuatedfrostiness, with Tvga ard Thuja then benefiting from
microclimateamelioration by the primary invadersto dominate reproduction later. Since
Thuja i.s seral where it occurs on drained soils in this valLey,T saga/Pachitt;//ra iorcst
appearsto be the potential climax in this frost pocket, as well as above and below it in
the valley. The Granite Creek basin which collectsthe cold air exceeds1000 ha in area,
but the floor of the basin is tilted sufficiently that cold air accumulatesin only a restricted area above the constliction.
Discussion
Interdigitation of vegetationtypes in the Rocky Mountains, with low-altitude types extending highest on sourh-facingslopesand ridges, whereashigh-altitude types descend
farthestalong north-facing slopesand creek margirs, haslong been recognized(Dauben-
Mountain Topographyand VegetationPatterns
r )i
mirc, 1943). The specific examplesdescribedbriefly here emphasizethe magnitude of
ovedap in their altitudinal raoges,and point out some local variations involving reversals
of the gerreral pattern. Cleady topographic influences on microclimate are so stroog aod
pefvasive io these forests rhar references to elevation above sea level have very low value
for indicating the nature of the environmenr of an organism or a community. It is not
difficult to chancte ze the absolute altitudinal limits of each specific community type,
but their rangesoverlap so gready that their detemination sefvesno useful purpose.
On the other hand, most of the montane communities in the Rocky Mountains can
be aligned in an ecologically significant series re{lecting an enviroomental gradieot
from warm-and-dry to cold-and-wer,irrespective of altitude. Although this series is
often referred to as an altitudinal selies, the expression musr oot be taken literally. It is
this interdigitation of vegetation types which brings an ecologically wide variety of
communities into closeproximity, and it reducesthe precision with which paleontologists
can ioterpret the oature of past vegetation, especially from fossil pollens.
LiteralureCited
Daubenmire, R. 1943. Vegetatiooal zotradoo jn the Rocky Mountains. kt- F.ev- 9:)26-)93.
_--r
and J. B. Daub€nmire. 1968. Forest Vegetation of Easrero rVashington and Northern
Idaho. \trash. Agric. Exp. Sta. Tech. Bul. 60. 104 pp.
McMinn, R. G. 1952.'lhe role of soil drought in the distributior of vegetation in the northern
Rocky Mountains. Erol. 33 :1-15.
Recei,ued
December1, 197I
Acceptedlor publi.cationMarcb 2, 1979
r52
Daubenmire