Forest Types of the Northern Rocky Mountains and Their Climatic Controls1

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FOREST TYPES OF THE NORTHERN ROCKY MOUNTAINS AND THEIR CLIMATIC CONTROLS1

J. A. LARSEN Iowa State College

Introduction The purpose in this report is to describe the natural forest types of the northernRocky Mountains in Montana and northernIdaho, to point out their natural distributionand chief silvical characteristics,and to show in what de- gree they are controlled by differencesin topography and climate. Such information may be useful in laying the foundation for later, more intensive silvical investigations and practices in a region rich in forest resources. Information on the character and distributionof the forests has been obtained from the silvical maps and reports prepared in the various forest supervisors' offices;by conversation and correspondence with the forest supervisors and members of the District I Office (Missoula, Mont.) of the Forest Serv- ice, and by the writer's own work in this region covering a period of 14 years. Some difficultywas experienced in assembling the forest type map (Fig. i), because most of the existing maps for the individual forests show the present " cover " types 2 rather than the climatic or climax types (associations). Also, two or more persons may differin type designations and inter- pretations. The author has, therefore,been forced to use a certain freedom in correlating and adjusting his material. In some instances where the subalpine forests were not clearly shown, the boundaries have been drawn according to topography and knowledge of the forest conditions in the region. In all cases the lines drawn around the types have followed natural divisions of permanent tree associations. Lists of the most common species of grasses, herbs and shrubs found in each forest type, obtained from various published and unpublished sources, are presented because this informationis of value in silviculture,grazing, and ecological studies. The climatic controls of the forest types are based entirely upon air temperature and precipitation records. Such indices of forest type requirements as the relation of air temperature to precipitation,or of the latter to evapora- 1 This paper includes the results' of work done for the Northern Rocky Mountain Forest Experiment Station of the U. S. Forest Service. The word " type" is virtuallythe equivalent of " association." Thus the western yellow pine type is the same as the Pinits ponderosa association. 2 The " cover " type is the growth at present on the ground, and may he a stage in the successional series or may be the climax association. 63I 632 J. A. LARSEN Ecology,Vol. XI, N1o.4 tion,have been purposelyavoided fortwo reasons. In the firstplace, evapora- tion recordsare as yet verylimited; and secondly,few botanistsagree on the true significanceof these relationships. The data on climatehave been obtainedfrom the printedreports of the United States Weather Bureau, and from some records gathered by the Forest Service of summermeteorological conditions on mountains. When- ever possibleat least 5 weatherstations have been chosenfor each foresttype and an efforthas been made to have one at eitheraltitudinal extreme. Rec- ords fromthe higherelevations have been very insufficient. In the summationsof temperatureand precipitationwhich are given,the monthlydata are stressed. These no doubt reflecta strongerlimiting influ- ence in manycases thando annual tabulations. Deficitsin the springrainfall restrictgrowth of conifersin the west. This is amplyshown by recordsof climateand heightgrowth of trees at the Priest River ExperimentStation (Brewster,'28) and otherplaces (Pearson, '28). The amountof summer rainfall,on the other hand, particularlythat which falls during July and August,will indicatethe relativedrought resistance of the species. Dates of the last killingfrost in the spring and the firstin the fall are taken fromthe averages for the stationswithin each type. It is well to bear in mindthat none of theconiferous forest types in Montanaor Idaho are free fromkilling frost at any time duringthe growingseason. For this reason frostdata are of littlesignificance as limitingfactors or as indicatorsof the lengthof the active growingseason. They reflect,nevertheless, important climaticdifferences. The data on snowfall representthe averages for the type stations. These figuresalso, at best, reflectonly the amountof winter precipitationand air temperature;unfortunately, the muchmore valuable figures on durationof snow cover and dates of disappearanceof snow are yet largelylacking.

Broad Topographic and Climatic Controls of Forest Types Montana and northernIdaho centeraround two great mountainsystems, the ContinentalDivide and the BitterrootRange. Betweenthese two lie the Bitterrootand Flatheadvalleys, which drain northwest into Lake Pend Oreille. From the crestof the BitterrootRange the forestedspurs and sharp canyons descendwestward and southwestwardto the borderof the treelessColumbia River Basalt Plateau. Centraland easternMontana slope graduallyfrom the elevatedregion along the ContinentalDivide to the Great Plains. The area under discussiontherefore is divided into three distincttopo- graphicdivisions: (a) Washingtonand Idaho to the west of the Bitterroot Mountains; (b) Flathead and Bitterrootvalleys between the BitterrootMoun- tains and the main ContinentalDivide; and (c) CentralMontana lyingto the east of the ContinentalDivide. These major divisionsare the foundations upon whichthe data and descriptionsrest, and theywill be adheredto through- out thisreport. October, I930 FOREST TYPES OF ROCKY MOUNTAINS 633

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In each of thesethree topographic units the forestsrange fromprairie at the lowerpoints to subalpineor alpine conditionsnear the mountainsummits, througha series of altitudinalzones characterizedby definiteassociations of trees,which are called foresttypes. These foresttypes are outlinedin Table I, and are mappedin Fig. i. Table II presentsthe area and volumecontained in each type.

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FIG. 2. Map showing National Forests in the northernRocky Mountains

Table III showsthat the western sections contain a muchgreater variety of trees than centraland easternMontana. In Idaho, in particular,there is a wealth of species. Conspicuousamong these are mesophytictrees such as westernred cedar, westernhemlock, western white pine, and lowland white fir,which are no~tfound east of the ContinentalDivide. Even the western larch,much less exactingin waterrequirements, does not appear in centralor easternMontana. Climate,as affectedby topographyand geographicalposition, plays a leading partinl affecting the variationsin the forestcover. The essentialelements of climatefor the threemajor sectionsare set forthin Tables IV to X. The October, I930 FOREST TYPES OF ROCKY MOUNTAINS 635

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TABLE II. Areas occupied by forest types, Montana and north Idaho * (Areas in thousand acres; stand in million board feet)

Per cent by species

Section estimated stand Open D. fir, D fi Cedar; Bar- U grass- W. y. lodge- . r, hem- Eng. Sub- rens can- land t pine pole W.c lock, spruce alpine and class- (b) pine larch W. w. P. alpine ified

Wash.- Area 5,247.3 10.0 I5.00 I7.5 14.70 I9.2 4.5 I7.4 1.7 I d a h o - ______Stand 26,594 7.IO 28.I 8.50 41.2 9.0 5.6

Western Area 8,540.3 3.4 11.70 33.2 23.90 4.1 4.4 15.9 3.2 Montana _ _ _ Stand I9,724 14.20 40.0 23.10 4.I 13.3 5.6

Central Area 8,592.9 15.7 3.IO 54.8 0.50 3.1 I 2.8 10.0 Montana _ - _ - Stand I6,826 0.04 83.5 0.02 5.1 I I.3

Total Area 22,380.5 Stand 63,144.0

* Data from figurescompiled by the U. S. Forest Service, Missoula, Montana. t Including non-reproducingburns.

TABLE III. Evergreen trees of Montana and northernIdaho

Arranged according to light requirement,the most tolerant of shade being placed last.

West of ContinentalDivide East of ContinentalDivide Juniperus occidentalis Sarg. Juniperusscopulorum Sarg. Western juniper Rocky Mountain Juniper Pinus albicaulis Engelm. Pinus albicaulis Whitebark pine Pinus flexilis James Pinus flexilis Limber pine

Pinus ponderosa Laws. Pinus ponderosa Western yellow pine Larix lYalli Parl. Mountain larch Larix occidentalis Nutt. Western larch Pinus contortavar. rnlrrayana Engelm. Pinus contorta Lodgepole pine Pseudotsuga taxifolia Pseudotsuga taxifolia (Poir) Britt.-Douglas fir Pinu-smonticola D. Don Western white pine October, 1930 FOREST TYPES OF ROCKY MOUNTAINS 637

TABLE III- (Conitinued) West of ContinentalDivide East of ContinentalDivide Picca enqelmanniiEngelm. Picea engelmannii Engelmann'spruce Babiesgrandis Lindl. Lowland white fir Babieslosiocarpa Nutt. Abies lasiocarpa Alpinefir Tsuga heterophyllaSarg. Westernhemlock Tsuga mertensianaSarg. Mountainhemlock Thuja plicataD. Don. Westernred cedar Taxus brevifoliaNutt. Yew

TABLE IV. Sunshine, wind and relative humidity Averagesof all recordsup to the timeof latestperiodic summary, fromU. S. WeatherBureau Bulletinand laterreports

Sunshine,per cent of Wind, miles Relative humidity possible per hour per cent 6:oo P.M. Station, region and type of vegetation Annual Aay- Aug. Annual May- Aug. Annual SMeapy Aug.

Spokane E. Washington 52 70 76 5.7 5.8 5.0 50 31.2a 25 W. yellowpine Lewiston N. Idaho (1Norecord) 4.3 4.4 4.3 48C 33.6C 31C Prairie

Kalispell N. w. Montana 48 62 68 4.8 5.3 5.1 57 42.0 37 Prairie Helena N. centralMon- tana 58 65 73 7.5 7.7 7.3 50 38.4 32 Prairie

Miles City S. e Montana (No record) 6.i 6.2 5.3 61 48.2 43 Prairie Yellowstone Nat. Park,Wyo. 56 64 7I 7.8 7.3 6.7 55 43.0 37 S. central Mon- tana

a Values for Spokane,5 P.M., local time. b Mean forthe five month period, May to Sept.,incl. e Noon values,no availablevalues for6 P.M. 638 J. A. LARSEN Ecology, Vol. XI, No. 4

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42 640 J. A. LARSEN Ecology, Vol. XI, No. 4 outstandingclimatic characteristics found in the westernand easternsections are: in the west,relatively mild winterswith heavy snowfall,and warm,dry summers;in the east, colder winterswith light snowfall and somewhatmore summerrains. Thus some parts of Idaho show an annual summerprecipita- tionof only5 inches,although the total for the year is 25 and over; whilecen-

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FIG. 3. Distribution of annual precipitationby months in eastern Washington- Idaho, western Montana, and central Montana tral Montana receivesan average of above 7 inchesduring the summer,the totalannual therebeing about i5 inchesfor the prairiesand 24 inchesfor the subalpineforests. Climate,as affectedby topographyand geographicalposition, plays a leading role in determiningthe variationsin forestcover. There are two out- standingmeteorological differences between the westernand easternsections October, I930 FOREST TYPES OF ROCKY MOUNTAINS 64I

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FIG. 4. Seasonal distributionof precipitationf or the principal types of vegetation in eastern Washington-Idaho, western Montana, and central Montana 642 J. A. LARSEN Ecology,Vol. XI, No. 4

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FIG. 6. Curves showing precipitationby months throughoutthe year for eastern Washington-Idaho, western Montana, and central Montana 644 J. A. LARSEN Ecology, Vol. XI, No. 4 of the NorthernRocky Mountainregion. In winter,Idaho has mildertem- peraturesbut more snow than easternMontana; in summer,it has consider- ably less rain. The climateof the intermountainvalleys is intermediatebe-

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FIG. 7. Air temperaturegradients for eastern Washington-Idaho, showing the relation of altitude to the number of days with a mean temperatureabove 430 F., and a mean temperaturebelow 320 F. tween that of the regionson eitherside. These characteristicsare brought out graphicallyin Figs. 3, 4, 5, and 6. The relationof altitudeto the length of thegrowing season, based on temperatureabove 430 F., is shownin Fig. 7.

Section Lying West of the BitterrootRange PRAIRIES OF PASTERN WASHINGTO.N The prairie lands overlyingthe Columbia Basalt Plateau, between the Cascade Mountainsin Washingtonand the BitterrootMountains in Idaho. October, I930 FOREST TYPES OF ROCKY MOUNTAINS 645 are for the most part flat. They are broken here and there by old water courses which have exposed basaltic cliffs,and round boulders or "nigger heads." Occasional depressionsof better,moister soil are occupiedby alfalfa ranchesand may containa few plantedpoplars. These brightgreen patches stand out in vivid contrastto the otherwiseparched landscape. The arid nature of this region is reflectedby the dominantxerophytic vegetationof Artemisiatridentata, A. tripartite,Chtrysotamnus ncuseosus, C. vescidiflorus,Eriogonum microthecuns, Sarcobatus verniculatus,Rhus toxi- codendron,Erigeron concinnus, Lupinus pusillus,Zygadenus paniculatus, and Rumnexvenosus (Weaver, 'I7, Gail, '24). These prairiesare chieflyof interestin this study because of their hot summerdays and the dryingwinds which.come fromthem, causing extreme droughtand dangerous fireweather in the forestedmountains to the east. These winds also carryparticles of soil fromthe semi-aridregion causing it to be depositedover the Palouse Plains and to some extentover the forestsin the Clearwaterdrainage basin (Larsen, '23).

THE PALOUSE REGION Betweenthe semi-aridColumbia Basin and the westernyellow pine forests is a billowy,treeless region. This is the richwheat-producing area in eastern Washingtonand adjacent Idaho, knownas the " Palouse country." The na- tureof the soil, the distributionof vegetation,and the climaticconditions have been well described by Weaver ('27). Climaticconditions, as representedby recordsfrom Moscow, Rosalia, and Pullman,show a mean annual air temperatureof 46.90 F., and a yearlypre- cipitation of 20.54 inches, of which 2.49 inches occur during July, August and September. The air temperature,as well as the amountof precipitationwhich falls here,appears to be suitablefor a forestof westernyellow pine. Probablythe principalreason why this area is not coveredwith timber is the extremelyad- versecondition prevailing in summer,due to extremedrought and high winds. It is not unlikelythat recurring prairie fires and shiftingsoil have also helped to tip the scales to the disadvantageof the trees. Measurementsmade by Gail ('24) show a reallyremarkable difference in dryingpower of the air be- tweenexposed and protectedaspects. He founda wind movementi9. times greateron the southwestthan on the northeastaspect, and a loss of water fromevergreen twigs 2.89 timesgreater on the exposed thanon the protected slopes. Consequentlyit is not surprisingthat pine is actuallyfound on the northside of SteptoeButte and along the Palouse River,and thatthe eastern or northernaspects of the smallerhummocks may readilybe reforested.

THE WESTERN YELLOW PINE FORESTS The lowerborder of the forestin the sectionwest of the Bitterrootrange consistsof thewestern yellow pine type. The treeline followsthe Clearwater 646 J. A. LARSEN Ecology, Vol. XI, No. 4 canyon fromGrangeville to Orofinoand Lapway; then closely parallels the NorthernPacific branchline to Moscow; then goes northalong the eastern

Wk! ^ 4 ,., ,% "," 27 FIG 8 Western yelloweasternWashingtonF pine Prnus ponderosa LawsinF

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FIG. 8. VWeS-ternyellow pine, Pinutsponderosa Laws in eastern Washington edge of the Palouse Plain, cuttingnorthwest about 20 miles south from Spokane, Washington. This type of forestis found generallybetween an elevationof 2,000 and 4,000 feet,but goes down to i,ooo feet in the Clear- October,1930 FOREST TYPES OF ROCKY MOUNTAINS 647 water canyon. In the Salmon River drainage on the Nezperce National Forest it reachesan elevationof 6,ooo to 7,000 feet. The upper line is very irregular,receding along streamsand protectedslopes and advancingupward on warm and dryaspects. Westernyellow pine is everywherethe climax species of this type and it usuallymaintains itself in pure stands,once the ideal forestcover of needles and lightgrass has been established. It is mainlyafter fires that Douglas fir and lodgepolepine seed in. On the bettersites, at lower elevations,as well as along the upper edge of the type other species are frequentlyassociated withthe yellowpine. The characteristicyellow pine forestshows a ratheropen, park-likestand, with lightvegetation (Fig. 7). This permitsof wind movementalong the surface,and more sunlightand evaporationthan occurs in the other forest belts. These conditions,combined with the scant humusand litter,result in a soil poor in loam and of low water-holdingcapacity. There is invariably muchrock material near the surfacewhich causes it to heat greatlyin summer, and bringsabout wider soil temperaturefluctuations than in the other forest types (Larsen, '23). The more outstandingspecies of shrubsand other vegetationcommonly associatedwith western yellow pine are listedbelow: (Much more shrubbery is encounteredon the mountainslopes than on the more level and even sur- faces near the lowertype border). ArctostaphylosUva-ursi (L.) Spreng...... Kinnickinnick Agropyronspicatum Pursh...... W...... Wheatgrass Apocynumandrosaemifolium L ...... Dog bane Balsamtorrhizasagittata Pursh...... Balsam root Berberisaquefolium Gray ...... Oregon grape Brornusmarginatus Nees ...... Brome grass Cala-magrostissuksdorfii Vasey ...... Reed grass Clarkia pulchellaPursh. Campanularotundifolia L ...... Bluebell Ceanothussanguineus Pursh ...... Mountainbalm Drymocallisglandulosa Linl...... Cinquefoil, five-finger Erythroniumgrandiflorum Pursh...... Dogtoothviolet Heuchera avalifolic Nutt...... Alum root Hieraceum albiflorumHook...... Hawkweed Holodiscus dumosus Nutt...... Oceanspray Lupinus ornatusDoug ...... Lupine Nacrea lanata A. Nels ...... Everlasting PhiladelphusLezmisii Pursh ...... Mock orange Physocarposmalveceus Kuntze ...... Nine bark Pteridium aquilinum ...... Brake fern Prunus demissa (Dietr.) ...... Chokecherry Pentstemonsp ...... Pentstemon Salix scouleriana(Barr.) ...... Willow Senicio-var.species ...... Groundsel Spiraea lucida Doug...... Spiraea 648 J. A. LARSEN Ecology, Vol. XI, No. 4

SedurnDouglasii Hood...... Stone crop Rosa nutkana...... Wild rose Zygadenus venenosus...... Death camas The weatherdata forthis type were obtainedfrom nine stations: Spokane, Coeur d'Alene, Potlatch,Port Hill, Bonners Ferry,Newport, Orofino, Cul- desac, and Kooskia. Orofino,which lies in the Clearwater Canyon at an elevationof only 1,027 feet, has the highestmean annual air temperature, 49.80 F. Newport,on the plateau south of the Selkirksat an elevationof 2,300 feet, has a mean temperature of only 43.60 F. From these figures it appears that 43 and 5o degrees representroughly the limitingmean annual temperaturesof theyellow pine typein the regionwest of the Bitterroots. It mustbe stressed,however, that these figuresare mere approximations,since it is quite probablethat at otherlocalities where there are no weatherstations the yellowpine exceeds this range. As regardsprecipitation, the lower limit of thistype would seem to fall betweenthe 17.47 inchesof annual rainfallof Spokane, and the i6.34 inchesof rainfallat Reardon whichlies just outside thistype. As forthe upperlimit, the 28.03 inchesof precipitationat Orofino was the highestencountered. It is not consideredthat maximumair temperaturesper se, or the extremesof air temperatureare sufficientto impose limitingconditions to the downwardextension of the westernyellow pine in this region,for Orofino, withexcellent yellow pine forests,exhibits extremes comparable to those ob- served over the open prairies. The limitationis found ratherin deficient precipitation. As to the upper altitudinallimit, the line betweenthe yellowpine and the whitepine type is drawnrather clearly at 43 or 44 degreesF. as a meanannual air temperature,and at a growingseason of 200 days duringwhich the mean averages above 43 degrees. It should be observedalso that at the Dalles, Oregon,elevation 112 feet,under conditionsof higherrelative humidity and less of the drywinds than obtainin easternWashington, the westernyellow pine thriveswith a mean air temperatureof 52.9 degrees F., and only 15.4 inches of precipitationannually. Evidentlyincreased atmospheric humidity compensatesfor decreased precipitation in thiscase. From the standpointof silvicultureit may be said that the bulk of this foresttype is characterizedby pure or nearlypure standsin groups of different age classes. These range fromlarge and old trees,sometimes 400 years in age, to patchesof reproduction.

WESTERN WHITE PINE TYPE The commercialvalue and great abundanceof the westernwhite pine has led to the adoptionof the name " westernwhite pine type,"though in reality thispine is not a climax species,since the moretolerant hemlock, western red cedar,and even whitefir ultimately replace it. October,1930 FOREST TYPES OF ROCKY MOUNTAINS 649

The westernwhite pine typeextends from southern Canada to the Locksa Divide betweenthe Selway and Locksa rivers. To the east it extendsto some of the bettersoils in the Purcell,Cabinet, and northernBitterroot mountains. On the west it is terminatedby the upper edge of the yellow pine type. It occurs also in scatteredpatches of littlecommercial importance outside of theseboundaries. It thrivesbest on deep, well-drained,moist soil, and is thereforefound on protectedslopes in brokentopography where the extremesof climateare con- siderablyreduced, not only by precipitationand cloudiness,but also by pro- tectionfrom sun and wind. Though muchof the forestarea is now occupied principallyby even-aged white pine forests,canyon bottoms still contain enormoustrees of live or dead-mostly dead-virgin cedar and hemlockin uneven-agedforest. T'he westernwhite pine type is richerin plant associates than any other foresttype in the region. The mostcharacteristic species are given below: Acer glabrum.Torr ...... Rockymountain maple Actaea arguta...... Bane berry Adenocaulonbicolor Hook ...... Silver green Adiantumpedatum L ...... Maidenhairfern Alnus tenuifoliaNutt ...... Alder Aralia nudicaulisL ...... Sarsaparilla Aspidium spW ...... Wood fern Asarum caudatumLindl...... Wild ginger Atrageneoccidentalis Hornem...... Clematis Betula fontinalisSarg...... Birch Circaea pacificaAsch, and Magn...... Circaea Claytonia;perfoliata Donn ...... Springbeauty Clematisoccidentalis Hornem. Hort...... Virgin bower Clintoniaunifolia (Schult.) Kunth ...... Clintonia Coptis occidentalis(Nutt.) T. & G...... Gold thread Cornus canadensisL...... Trailing dogwood Cornus stoloniferaMichx ...... Dogwood DryopterisFelix-nsas (L.) Schott...... Shield fern Epilobiumadenocaulon ...... Willow herb Fatsia horridaBent. & Hook...... Devil's club Galilmetriflorum Michx. and asperrin'zumtGray ...... Northernbedstraw Geraniumcarolinianum L ...... Geranium Geum imacrophyllumn...... Large leaved aven Linnaea borealisL ...... Twin flower Lonicera involucrata.Banks ...... Honeysuckle Lonicera ciliata Poir ...... Honeysuckle Lyco podium an-notinumand clavatum ...... Clubmoss Mertensiaciliata (Torr.) Don ...... Lungwort Mentha cancadensisL ...... Wild mint Mitella trifida.Graham and stauropetala...... Bishop's cap Montia spatulataacerifolia and siberica ...... Miner's lettuce Osmsorihzasp...... Sweet cicily Polygonumspergularaeforme Meis ...... Knot weed Polysdictumlonchitis L...... Holly fern 650 J. A. LARSEN Ecology,Vol. XI, No. 4

Pyrola secunda and bracteata...... Wintergreen Ribes viscossimumPursh ...... Gooseberry Rubus melanolasiusFocke ...... Red raspberry Rubus parviflorusNutt...... Thimbleberry Rubus sp...... Blackcap raspberry Sambucus melanocarpaA. Gray ...... Elderberry Samxbucusglauca Nutt...... Elderberry Smilacina sessilifaliaNutt ...... Solomon seal Spiraea,densiflora Nutt...... Spiraea Spiraea lucida Dougl...... Spiraea Stellaria longpipeslaeta Nutt...... Chickweed Streptopus amplexifolius ...... Twisted stalk Tiarella,unif oliata Hook...... Coolwort Thalictrumoccidental A. Gray ...... Meadow rue Trautvetteriagrandis Nutt. Trilliumpetiolatum Pursh ...... Trillium Valerianasitchensis ...... Valerian Vacciniumimembranaceum (Doug.) Hook...... Huckleberry Vacciniumoccidentale A. Gray ...... Huckleberry Vagnerica lilacea (Greene) Rydb...... False solomonseal Viola glabella Nutt...... Violet Viola blanda Wild ...... Violet (yellow) Zylosteon sp ...... Bush honeysuckle Dense virginstands containno grass and scant underbrushbecause very littlelight reaches the ground. There do appear a few characteristicplants, often in distinctcolonies, including Triarella unifoliata,Asarum caudatum, Aralia nudicaulis,Cornus canadensis,Linnaea borealis, Coptis occidentalis, several species of Lycopodiumand many mosses. Along the streamsis a great profusionof herbaceousplants among which are Adenocaulonbicolor, 0smiorihzanuda, Actaea argitta,Galiumt asperriiuinm, Ribes and Fatsia hor- rida. Some of thetypical shrubs are Amelanchieralnifolia, Alnus alternifolia, Ceanothusvelutinus, Acer glabrumand Cornus stolonifera. After fires,however, a great manyherbaceous species take possessionof the ground,including the majorityof the plants listed above (Larsen, '29). These cover whole hillsideswith luxuriantgrowth of Chama-enerionangusti- folium,Ribes zviscossimum,streptopus, etc. After five or six years these give way to shrubsof Ceanothusvelutinus, C. sanguineus,Acer glabrum,Ame- lanchieralnifolia, Symphoricarpos recemosus,,Alnus, Lonicera involucrata and L. utahensis,etc.3 The weatherstations representative of the westernwhite pine forestare Sandpoint, Lakeview, Priest River Experiment Station, Prichard, Avery, Murray,Wallace, Musselshell,Elk City,and Burke. Of these,Sandpoint is the lowest and warmest, with a mean annual temperature of 44.90 F., andh Burke the highestand coldestshowing a mean annual temperatureof 40.40. The range in temperaturerequirement for this type lies thereforebetween 3 Fires and forest succession in the Bitterroot Mountains is more fully treated in author's report,Larsen, '29. October,1930 FOREST TYPES OF ROCKY MOUNTAINS 651 about 400 and 45? F. The growingseason is from i65 to 200 days. The mean annual rainfallvaries between27 and 45 inches,with not less than 2 inchesduring July and August. It may trulybe said thatthe southwardand downwardextension of this type is limitedby insufficientmoisture, and the upward or northwardextension by low temperature. A minimumof 27 inchesper year is requiredin the formercase, and a mean annual temperature of 40? F. in the latter. One outstandingclimatic characteristic of this type is the abundance of snowfall. The contrastis very distinctbetween this type and the western yellowpine, where snow is shallow and intermittent.The deep snow cover is naturallya great protectionto seed and seedlings,as well as a source of moisturein springand earlysummer. On the moistersoils, especiallyalong water courses and on northaspects, westernred cedar, westernhemlock, and Engelmann spruce are abundant. On the driersouth and west aspects,and on theexposed, rocky slopes Douglas fir,western larch, and lowlandwhite fir are much in evidence,while western yellowpine is frequentlyfound. Lodgepole pine is foundon the poorersites, while alpine firis commonat the upper edge of the type. The whitepine is rarelyfound in pure stands,but not infrequentlyyoung stands have such a large proportionof thisspecies as to appear practicallypure at firstsight. Due to theinfluence of fires.western white pine standsare characteristically even aged. However,it frequentlyhappens that these fires do not completely destroythe originalforest, and in such case a two or more aged forestmay result. The more tolerantcedar, hemlock,spruce, and white firoften enter the standconsiderably after the dominantspecies. White pine forests(Fig. 8) are among the most beautifulin existence. The maturetrees are tall, straight,full bole and cylindrical,yielding a high amountof selectlumber and a muchheavier cut per acre thanany otherforest in this region: certainsections having averaged 50,ooo board feetper acre at 150 years,over and above the usual reservefor seed treesand fireinsurance. Many westernwhite pine trees attaina heightof 200 feet and diametersup to 50 inches. Westernred cedar also is a tree of no mean staturefor it fre- quentlymeasures 200 feetin heightand sometimes12 feet in diameter.

SUBALPINE FORESTS

The subalpineforests in the BitterrootMountains (Fig. IO) begin at an elevationof 5,500 feet,where the whitepine typeceases. They continueup- ward to mountaintops at six and seven thousandfeet elevation. In general this high countryis not precipitous,although there are conspicuousexcep- tions. The soil, especiallyon the broad ridgetops, is ratherdeep and fertile, fosteringexcellent stands of mountainhemlock which is here the climax species. The Engelmannspruce and alpine fir inhabitnorth slopes and moist situations,while the lodgepolepine, Douglas firand white bark pine prefer exposed and driersites. Vol. No. 652 J. A. LARSEN Ecology, XI, 4

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o October,1930 FOREST TYPES OF ROCKY MOUNTAINS 653

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IIM7 654 J. A. LARSEN Ecology, Vol. XI, No. 4

Under matureclimax hemlockforests the vegetationis composedprinci- pally of Vacciniuntmicro phyllurn, sedges, and broomgrasses. Under spruce and alpine firare foundMenzisia glabella,Vaccinium membranaceum, Sorbus scopulina,and Alnus tenuifolia. Aroundeach springand seep is muchlaurel, alder, elder,and a wealthof succulentspecies, but these have no bearingon forestdistribution. The completecorrelation of vegetationto foresttypes has not yet been ascertained. Recurringfires originating from lightning have greatlyreduced the areas properlybelonging to the climax species and have favoredreproduction and spread of lodgepolepine and to some extentDouglas fir,for the latterspecies withstanddrought and exposurein the seedlingstage muchbetter than hemlock,spruce and fir; but theyare not as valuable or effectiveas protectorsof the groundand conserversof moisture. Stands of lodgepolepine and Douglas firare abundanton the southand west aspects. Lodgepole pine is inavirablymost abundanttoward the upper part of the slope whileDouglas firis moreplentiful as a generalslope species. This forestmay be even aged over large areas, havingoriginated after a fire of considerablesize, or even aged in small irregulargroups. In the latter case it is the resultof groundfires in youngstands. Sometimesveterans of mucholder age thanthe surroundingtrees grow in draws.and protectedsitua- tions. The matureDouglas firtrees may varyin heightfrom i00 to i50 feet accordingto the qualityof the site,while lodgepolepine seldomexceeds 8o or go feetin height. The treesin this zone are rathertapering and limby. On the whole thereis littleunderbrush present on the upper part of such southerlyslopes, but much grass, much of which is bear grass. One very typicalplant association under the upper slope lodgepolepine is Xerophyllum tenax and Vacciniummicrophyllum. Sufficient natural reproduction does not take place and thereare very few seedlingsof any kind,or none, under the trees. White bark pine is a tree occurringmostly in openingson southand west aspects. It is therea shorttree seldomover 6o feetin height,more often30 or 40 feetwith a low, wide-spreadingcrown and taperingtrunk. Lyall larch is foundon isolatedhigher peaks above 7,500 feet. This speciesis not plenti- ful, but the stands found are even-agedand pure. It merelymarks the be- ginningof what would be a true alpine type had the mountainsbeen of greaterelevation. C. H. Shattuck('I5), in ascendingGraves Peak on the ClearwaterForest in I9I7, listedthe followingspecies:

Trees Babieslasiocarpa. (up to 8,ooo) Larix Iyalli (7,500 to 7,800) Picea Engelmanni(up to 8,ooo) Pinus albicaulis (timberline) October,1930 FOREST TYPES O;F ROCKY MOUNTAINS 655

Shrubs AInus sp. Cladothaimnuscam panulatus ...... Fowler, Flora of the Northwest Gaultheria humtifusa...... Coulter and Nelson, Gaultheriahumifusa ...... Coulter and Nelson, Rocky Mt. Flora Juniperusnana ...... Fowler Kalmia gland'ulosa...... Piper Ledum glandulosum...... Coulter and Nelson Phyllodoce impetriformis...... Coulter and Nelson Sambucus mnelanocarpus...... Coulter and Nelson Spiraea arbuscula...... Fowler Vacciniumoccidentale ...... Coulter and Nelson Vaccinium microphyllum...... Fowler

Ptcridofphytes Cryptogammafragilis ...... Fowler or Gray Manual of Botany Cryptogammnaacrosticoides ...... Fowler or Gray Cystoperisoccidentale ...... Fowler or Gray Isoetes sp. (in the lake) Herbaceous Plants Antennariaparviflora ...... Coulter and Nelson Antennariaamedia ...... Coulter and Nelson Arnica pedunculata...... Coulter and Nelson Aster palustris...... Fowler Boykiniamajor ...... Fowler Campanula rotundifolia...... Coulter and Nelson Campanula uniflora...... Coulter and Nelson Cicuta purpureata...... Fowler Clematis douglasii...... Coulter and Nelson Crepis elegans...... Coulter and Nelson Epilobiumspicatum ...... Coulter and Nelson Eryogonumjamesii ...... Coulter and Nelson Erythroniumngrandiflorum ...... Coulter and Nelson Gentianacalycosa ...... Coulter and Nelson Hypericumf ormosumn ...... Coulter and Nelson Media citriodora...... Fowler Mimulus lezwisii...... Coulter and Nelson Nartheciumamericanum ...... Pedicularis lunata..... Coulter and Nelson Phlox diffusa..... Coulter and Nelson Potentilla uniflora..... Coulter and Nelson Poligoniumviscosum ...... Fowler Rumex occidentalis.... Coulter and Nelson Valerianasitchensis .. . . . Coulter and Nelson Veronica scutilata..... Coulter and Nelson Zerophyllumtenax ..... Coulter and Nelson 43 656 J. A. LARSEN Ecology, Vol. XI, No. 4

Unfortunately,year-long weather records at these high elevations are limited. Roosevelt,Idaho, at 7,200 feet,south of the Salmon River in the Thunder Mountain region,was the nearest station. This is, of course, a good distancesouth of the westernwhite pine type,but much higher. Here a mean annual temperatureof 36.40 F. is recordedand a totalprecipitation of 26.59 inches. The site may be too cold and the precipitationin summerde- ficientfor the cedar-hemlock and whitefir type. No permanentsnow records are available,but casual observationsshow thatsnowfall here is muchgreater thananywhere else in the sectionwest of the BitterrootMountains. On Cook Mountain,6,575 feet elevation,the snow usually covers the buildingscom- pletelyand does not disappearentirely before July I. Observationsof weatherconditions on lookoutpoints in this regionduring July,August and September,i919, show that the air is cooler by day and warmerby nightat stationslower down, such as Oxford and Musselshell. The differencesare about I o F. The relativehumidity fluctuates less on the mountainthan in the valley in summer,and wind movementon the mountainis from3 to 4 timesas great and fairlyconstant both nightand day, while at low stationsthe air is fairly calm at night. One verynoticeable feature of the temperaturevariations at differentele- vationsis in thedifferences which appear in the springand fall. In the spring a greatergradient is observedfor differentaltitudes than in the fall. This is mostlikely because the snow coverlingers late in the mountainsin the spring, while duringthe fall the snow cover comes more nearlysimultaneously at all elevations. Flathead and Bitterroot Valleys In the intermontanebasins of Flathead and BitterrootValleys another fairly regular altitudinalsuccession of forest types occurs, ranging from prairieson the bottomlands and rollingplains at about 3,000 feet elevation, throughwestern yellow pine on the foothills,Douglas firand westernlarch on lowerand intermediateslopes, to subalpineand alpine forestson higherslopes and summits. Since the westernlarch does not go south into the Bitterroot Valley generally,the intermediatetype above the westernyellow pine southof Missoula consistschiefly of Douglas fir-lodgepolepine. This is in most re- spectssimilar to that foundin centralMontana and centralIdaho.

PRAIRIES OF THE FLATHEAD AND BITTERROOT VALLEYS The intermontanevalley floorsare in most cases ratherflat, originating from old lake bottoms. They are sometimesgently rolling, the result of wind-blowndeposits. The line betweenthe open grasslandand the westernyellow pine forests is in most places regular and distinct. On the west side of the valleys the thinlower border of yellowpine followsclosely the lowestpart of the slopes, October, 1930 FOREST TYPES OF ROCKY MOUNTAINS 657

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-.t c Jic October, I930 FOREST TYPES OF ROCKY MOUNTAINS 659 buton the east side the outlyingfoothills are bare of forestup to a distanceof 500 and sometimesI,ooo feetabove the valleyfloor. At the mouthof lateral canyonson the east side the demarcationis abruptand sharplydrawn, with foreston northaspects and prairieon those facingwest. The weatherstations which were chosen to representthis type are St. Ignatius,by far the most typical,Kalispell and Hamilton,somewhat on the borderlinebetween the prairieand the westernyellow pine. Deerlodgeis also a prairiestation but much higherand colder than Kalispell. These stations show a variationin mean air temperaturefrom 42.8 to 46.o0 F., Deerlodge being the coldestand Hamilton the warmest. Neitherof these recordsnor the extremeslisted in Table VII are sufficientto accountfor the absence of treegrowth, since the temperatureconditions are less extremethan those ob- served to the west of the BitterrootMountains. The general absence of forestmust be ascribedto deficientannual rainfallwhich averages only I3.50 inches,and in some years drops as low as IO inches. Here and there the cause is evidentlylack of drainage,alkalinity, and recurringfires; but these merelyexercise a local effect.

WESTERN YELLOW PINE FORESTS The forestsof westernyellow pine and the associated vegetationin the Flathead and Bitterrootvalleys are, in most respects,similar to the Idaho- Washingtonyellow pine forests. It is in effecta part of the same forest type,joined to it by narrow stripswhich follow the south and west slopes, often steep rockycliffs, along the Flathead and Kootenai rivers. Its best developmentis reachedin the southernportions of the BitterrootValley from Hamiltonto Como. The upper limitof the typeis near 4,000 feet in the northernportions of the Flathead National Forest, and near 6,ooo feet in southernparts of the Bitterroot. However, this type does not reach its lowest possible elevation herebut continuesdownward along the riverswhich drain into northern Idaho and Washington. The tree line is, nevertheless,rather generally associated withthe base of the mountains,giving way to prairiesas statedabove. The weatherstations selected to representthis typeare Libby,Thompson Falls, Superior,and ColumbiaFalls. Of these,Thompson Falls is the lowest and warmestwith a mean annual air temperatureof 45.o0 F., whichColumbia Falls, the coldest,has a mean of 42.80 F. The latteris apparentlyclose to the minimumfor the yellow pine here, as shown both by comparisonwith otherrecords in Idaho and the sudden transitionat Columbia Falls froma westernyellow pine to the larch-Douglasfir type. Nowhere in these valleysdo the air temperaturesreach such high points as in the Idaho yellowpine type. The maxima are lower here,and the absolute minimum is - 400 instead of - 30?. It should be noted that the average lengthof the growingseason in thistype has here only I79 days,and that it is over 200 days in the westernpart. The conclusionis reachedtherefore 66o J. A. LARSEN Ecology, Vol. XI, No. 4 that the yellow pine is not growing under optimum temperature conditions in the Flathead or in the northernparts of the Bitterroot Valley. The precipitation is about 20 inches, one station, Superior, within the type registeringonly I7.50. This compares well with the minima recorded in eastern Washington, Spokane, near the lower edge of the type, showing I7.47. The station Fortine near the very edge of this type has a precipitationof i8.io, and Kalispell, outside the type, I5.2I inches. But the summer months, April to September inclusive, register about 9 inches rainfall in the intermontane valleys and about 8 inches to the west. Most likely less severe drying conditions and more rainfall compensate somewhat for decreased air temperature. The conclusion is reached in this case also that the lower or downward extension of the western yellow pine type is limited by insufficientmoisture rather than by too high temperature.

DOUGLAS FIR-WESTERN LARCH FOREST'S Within the Kootenai and Flathead watersheds in northwestern Montana the position between the upper margin of the western yellow pine type and the subalpine forests is occupied by Douglas fir-westernlarch forests (Fig. 9). These occur at elevations from 3,ooo to 5,500 feet on benchland, foothills,and slopes and ridges generally. To be sure, there is also a great abundance of Engelmann spruce and lodgepole pine, the former mostly on north slopes, benches and along streams, the latter everywhere without reference to aspect. The larch and lodgepole pine occupy extensive areas as temporary forests because they have been favored in their distribution and quantity by past fires. The larch resists fire well on account of its thick bark; the lodgepole pine seeds in on burns very readily, and is favored by its early seed production, and the preservation of the seed within the tightlyshut cones. Both species show prompt germination and establishment on the ashes. Western larch, Douglas fir and lodgepole pine make up at least 90 per cent of the species within the type. Englemann spruce appears mainly along creeks and bordering swamps or low places. Certain pockets with more favored soil and moisture are occupied by the cedar-hemlock and white fir mixture representative of northern Idaho. The latter type has a scattering of western white pine; but the form, growth, and health of these mesophytic trees are far inferior to those growing in Idaho. Even-aged stands are the rule everywhere within the type, the age classes being well represented and having originated after more or less general fires. Since larch is very fire-resistant,veterans of this species exist everywhere, these having survived several very bad fires. Douglas fir, since it is more tolerant of shade than the larch, reproduces prolificallyunderneath the larch. The natural tendency is therefore for Douglas fir to supplant larch, and, were it not for recurring fires which destroy the Douglas fir and cause profuse larch restocking by exposure of fresh ashes as a seed bed, this type would naturally revert to Douglas fir and Engelmann spruce. October,I930 FOREST TYPES OF ROCKY MOUNTAINS 66I

tana.FI.Udrtor II. Foes fDulsfrPeudtsg of wetr ac ai ciesNt. taio LinFaha M.) BrtEnlmn VleM n spuePcaeglannem n oeapnfi Abe lasicra(okr ut 662 J. A. LARSEN Ecology,Vol. XI, No. 4

The vegetationwithin and underneaththe Douglas fir-Westernlarch timber is not so varied in species nor so tall or luxuriantas thatobserved in the cedar-hemlock-whitefir forests of Idaho. In this it reflectsthe driercondi- tions of Montana as comparedwith Idaho. Shrubs of Anielanchier,Cea- nothus,Shepherdia, Sorbus, Symphoricarpos,Lonicera, Kalmia, etc. are common in minoropenings or on double burns. Other typicalspecies are listed below. For the characterand distributionof the vegetationsee Harshberger ('II, p. 550), and Whitford('05, pp. 99, 194, and 276). Anielanchieralnifolia Nutt ...... Juneberry ArctostaphylosUza-ursi (L.) Spreng...... Kinnickinnick Antennariarose (D. C. Eat.) Greene ...... Everlasting Antennaria.racemtosa Hook ...... Everlasting Berberisrepens Lindl ...... Oregon grape Calypso borealisSalisb. Castilleja sniniataDougl...... Indian paintbrush Ceanothusvelutinus Dougl ...... Mountainbalm Chimlaphilaumbellata (L.) Nutt...... Princesspine Drym1tochallisglandulosa Lindl.. Fragaria sp...... W ild strawberry Linnaea borealis L ...... Twin flower Lonicera UtahensisWats ...... Honeysuckle Pachystimamyrsinites Raf ...... Goat brush Symphoricarposrarcemosus Michx ...... Snowberry Shepherdiaargentia Nutt...... Silver bush Sorbus angustifoliaRydg...... Mountainash Climaticconditions within the Douglas fir-Westernlarch foresttype are taken fromFortine, Haugan, Belton and Upper Yaak. For these stations the mean annual air temperaturevaries between40.6? and 42.4' F. Belton and Fortineare the colderand Haugan the warmer. The growingseason as taken fromthe curvesgives I70 days. These temperaturerecords give con- ditionsnearer the optimumthan the average for the type. It is impossible fromthese records to determinethe average or optimumas well as the minimum temperaturetolerated by this type. Precipitationrecords indicate an average of 27.83 inches for the year. This, however,is far above the minimum,Fortine on the lower borderline showing i8.io inches. The exact requirementwill naturallyvary according to the local soil and its moisture-holdingqualities. The conclusionsare that the lack of moisturelimits the downwardextension of thistype, and thatthe air temperatureswhich prevail here are suitable for the cedar-hemlock-whitefir type but thatprecipitation is generallyinsuffi- cientfor it. SUBALPINE FORESTS Above 5,500 feet elevationin the northernFlathead Valley, and above 6,500 in the BitterrootValley, the forestsare distinctlysubalpine, composed of alpine fir,Engelmann spruce (Fig. 12), and white bark pine. 663 October,I930 FOREST TYPES OF ROCKY MOUNTAINS

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FIG. I2. Engelmannspruce Picea engelfnanniEngelm., Blackfeet National Forest,northwestern Montana 664 J. A. LARSEN Ecology,Vol. XI, No. 4

For this type the same general characteristicspreviously described for the subalpineforests west of the BitterrootDivide hold true. Points of difference are: a muchgreater amount of Engelmannspruce in the northernFlat- head watershed,sometimes forming extensive pure or nearly pure forests; the absence of mountainhemlock; and the greaterabundance of lodgepole pine and Douglas fir,especially on southand west slopes. This typeis chiefly of importancefor watershedprotection. It is moreovertoo inaccessiblefor presentday exploitation. The followingspecies of true subalpineplants are foundabove 6,ooo feet in the Selkirk Mountains (Harshberger,'ii): Botrychiumnsimplex E. Hitchcock Ranunculuspygonaeus Wahl. Phegopterisalpestris Hoppe Viola cania L. v. adunca G. LycopodiumSelago L. Drayas octopetalaL. Lycopodiuxmsitchense Rupr. Rubus arcticusL. v. grandiflorus Festuca brachyphyllaSchultes Sibbaldia procumbensL. Triseturnsubspicatum L. Oxytropis(Aragallus) alpicolaRydg. Carex atratiformisBritton Epilobiumanagallidifolium Lam. Carex pseudo scirpoideaRydg. Bryanthusglanduliflorus G. Carex rupestris Gaultheriahumifusa Graham funcus MertensianusBorg. Androsacesubumbellata A. Nelson JuncusParryi Engelm. Castelleja pallidcaKunth. Luzula spicata,DC. Veronicaalpina L. Tofieldiaborealis Wahlenb. Myosotesalpestris Schmidt Oxyria digynaL. AplopappusLyalli Gray Silene acaulis L. Aplopappus (Pyrrocomea)Brondegei Silene Lya.lliiWats. A. Gray Alsine laeta Rich. Erigeronsimplex Greene (~ E. uni- Arenaria capillarisPoir. v. nordifolia florusHook.) AnemoneDrummondii Wats. Senecio triangularisHook. Aneilmonepatens L. v. Nutt. G. Taraxacumrupestre Greene Ranunculusnivalis L. Taraxacum scopulorumGray Ranunculusalpeophilus A. Nelson For the subalpineforests no trueclimatic records are available. Snowfall measurementsin 1926 on Mt. Silcox (6,870 feet) northof Thompson falls, show the following:

Date Jan. I5 Feb. 24 Mar. 3I May 2 June 2I

Depth,inches ...... 76.5 105.0 156.o 142.0 42.4 Weight,lbs. of cu. ft.of snow . 9...... 195 36.2 27.2 43.7 21.7

The observersin climbingthe mountaintook measurementsat different depthsas follows:

Elevations 3,723 4,924 5,795 6,870

Type Yellow pine Yellow pine Larch-D. f. Subalpine

Feb. 24 I 5.3,, 29.0 52.0 105.0o* t Mar. 31 ...... 21.0 79.0" 156.o0 October, I930 FOREST TYPES OF ROCKY MOUNTAINS 665

Central Montana Throughoutmost of centralMontana the transitionfrom prairies to forest is to the Douglas Fir-LodgepolePine Zonation,thus skippingalmost entirely the zone of westernyellow pine so commonin the two westernsections. A poorlydeveloped forest of yellowpine occurson the Helena National Forest. We occasionallyfind the juniper-limberpine borderassociation on steepslopes overlyingcoarse sandstoneformations. From Douglas fir-lodgepolepine the forestchanges at higherelevations to subalpinein character,with a greater proportionof alpinefir and Engelmannspruce. Eventually,toward the upper limitsof treegrowth true alpine conditionsprevail with limber pine, alpine fir and Engelmannspruce. PRAIRIES The Great Plains sectionin thisterritory changes fromlevel plains at the lower points,to rollingundulating plains and to steeperfoothills. It exhibits wide variationsin floristiccomposition according to differencesin soil and situation. Ecologists and botanistswill recognize Sage-Brush Formation; Grama-GrassFormation; Wheat-GrassFormation, Cercocarpus Formation; Wet Meadow Formation,etc. The studentis referredto the excellentworks of Harshberger,Shantz, Rydberg and Kirkwood. The vegetativecomposition is generally xerophytic, with Stipa, Conmata,Agropyron spicatumI,Astralagus mollissimc, Tradescantia virginica, etc. as leading species. Stationsrepresenting the climateof thesetreeless stretches are located on the lowestand most level parts of the plains withinthe agriculturalsections. They are; Helena, Dillon, Renova, Deerlodge, Phillipsburg,and Virginia City,varying in elevationfrom 4,000 to 6,ooo feet. For these the mean annual temperature is 43.40 F., with extremes of 1030 and - 400; 187 days durationabove 43 degrees,and precipitationunder I5 inchesper year. These values, comparedwith those prevailingin the Westernyellow pine zones to the west, are dangerouslylow. Furthermore,there are on record 3 years, 1917, i9i8 and i9i9, when the average annual precipitation,taking the 3 years together,was below 12 inches. From I902 to I905, inclusive,the annual precipitationrecords for Helena, Havre and Miles Cityaveraged io.47 inches per year per station. Under such criticalmoisture conditions not only the natural restockingof the forest,but its maintenanceand persistencein the face of fireand droughtis seriouslyjeopardized. Temperaturefor the central and easternMontana sectionis given in Table IX, and precipitationin Table X, and the relationof air temperatureto altitudeis shownin Fig. 13. 666 J. A. LARSEN Ecology, Vol. XI, No.4

ON, 00 ,I

~~~ 10 ON a1 - ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~0 10 ~ 10O ~~~ C1 10 10 10 ca -~~~~~~~~~~~~~~~~~~~~~~~~~lCC\ 0t-a1

a1 10 CC0 ~~ 10~~~~~ n C10

Ci~ ~~~~~~~~I C

10 0 t in) 00 ON CCC

o 0 ~t 1 1 0 ~0 Cc; if a) ) to to if

Cc Cc 10 if) ~~~~~~~~~~~~~~~~~~~~~0 ~~0 1000

~4 C 1010

o fm. 0co 0- C 0

ONI Ca 10 C C0 1CC\O ~~~ a1 ON~~~~~~~~~~U _T 10 k-10::k 4 -

0 a 10 1-. 10 10c~~~~~~~~~~~~~~~~~~~~~~0~~~ _ _ _ _ _~~~~~~~~~-. _ _ _ _

Q) 00 U co10 C~a CC ONONU U 1l 0 0I- 10 0' U- t0

C' I CCON Cc ON

U)H Cc) bO ~ ~ - 0 Cc ~ ) 0 C ______a ) O ~~~~~~ Ca- __~~~~~>4- _ _ _ _ _ C 0

I Q C 10- 10 0

CC L 10 C. - C's2 .C I O -. 0 4- Ca ) U~ 0~- >a N C Ca .0a~~~~~~~~~~~~~~~~~0~~~I U a 0 11

10 C/ /

0 0 40 oZ-j 0.0 u 0 - -: :, ca ca b-a 0 a6t ca CaS4 C -I) C-. October, I930 FOREST TYPES OF ROCKY MOUNTAINS 667

Cd 00 ON \600 C-) co 0 C';

00 t- U)

m 00 to (O 4

Ct E Cd b E C0

ON t- Cd

0 Cd -O 00 00 4--) C; E N 00

00 00 Cd U-) 0

let Cn 0 $..

00 00

Cn C) O- 4- 4-j 0 CdV 4t ;a t bk Cd 0

0 0 0 0

blo blo

t-4 4-) 0 4-J U) b.0 b.0 0 > 0 U I- U 0 (n 0 m C'd 1-4 Cd Cd 4-JO 4Z C) a-41-4 '-) V)z 668 J. A. LARSEN Ecology, Vol. XI, No. 4

,Y~fi>6pttwnu>Es

Duck/80 -______

.2ooc Joo. 4ooo C5000 6e.'.. 7... 'E

DOUGLAS FIR-LODGEPOLE PINE FORESTS The outstandingand predominatingforest type throughoutcentral Mon- tana is a mixtureof Douglas firand lodgepolepine. L~eiberg('-) has well describeda typicalarrangement and occurrenceof the species on the Gallatin and AbsarokaNational Forests as follows: from5,ooo to 6,ooo feetelevation limberpine and a small proportionof westernyellow pine may be seen, while scatteredgroups of Douglas fir form thin fringesof forestwhich become fairlywell stockedon northernslopes. From 6,ooo to 8,ooo feet lodgepole pine is abundant,either in closelystocked pure standsor mixed with Douglas fir and Engelmannspruce. The formeris more abundant on dry, rocky slopes,while the latter occurs most commonly along canyonbottoms and places withconsiderable seepage. Above 7,800 feet the lodgepolethins out and is October,1930 FOREST TYPES OF ROCKY MOUNTAINS 669

replaced by limberpine, alpine fir and Engelmann spruce, which together constitutethe subalpineforest. Timberlineis found at 9,300 feet on north and east aspectsand at 9,800 feeton southand west exposures; in some places on theeastern part of BeartoothPlateau it occursat Io,ooo and I IOoo. Near timberlinethe Engelmann spruce is found in greaterabundance and vigor thanany otherspecies, with limber pine second. Both are mereshrubs at the uppermostlimits. These forestsare invariablyeven-aged in character. The Douglas firis seen in greatestabundance on south and west aspects,while pure stands of lodgepolepine are morecommon on northand east slopes. It is herealso that Engelmannspruce occurs especially along thecreeks. Alpine firis also rather prevalentin lodgepolepine stands on east and northaspects. It is a climax species over mostparts now occupiedby the Douglas fir-lodgepolepine type, beingmuch more tolerant of shade thaneither of thosespecies. The greatabundance and wide distributionof lodgepolepine is due to ex- tensivefires, it being well knownthat the fireopens the cones of this species therebymaking the seed available for promptgermination in the ashes. The dense youngstands of lodgepolepine whichhave sprungup on burnedground formwell-nigh impenetrable thickets. Typical and commonplant associates occurring with the Douglas fir-lodgepole pine forest are: Delphinium. cucullatum (tall larkspur) ; Geranium vis- cosissim-um(geranium) ; Sieversia ciliata (sieversia) ; Festuca ovinla (roll leaf festuca) ; Agropyron tenerumt (wheat grass) ; Bromus pasteri (drooping brome grass); Mertensia ciliata (bluebell); Thalictrum occidentale (meadow rue) ; Dasiophora fructicosa (shrubby cinquefoil); Vaccinium scoparium (small blueberry); Cercocarpasledifolius (mt. mahogany). A cross sectionof the climaticconditions of this foresttype is obtainedby records for Red Lodge just below the forestborder at 5,548 feet,and for Hebgen Dam at 6,700,well withinthe upperhalf of the type. At Red Lodge the mean annual air temperature is 39.90 F., average maximum for July of 75.40 F., absolute maximum iio1, absolute minimum -400; and the growing season is 138 days. This would be far too cold for westernyellow pine. Red Lodge receivesan annual precipitationof 19.72 inches,which is probably near the minimumfor Douglas fir,and Hebgen Dam shows 24.16 inches. It is the writer'sconclusion that this region receives insufficient rainfall for such mesophyticspecies as hemlock,white fir, western white pine, larch and cedar, whichare foundwest of the ContinentalDivide. Westernlarch would come nearestto findingsuitable moisture conditions here, but the air temperature appears far too low and the growingseason too short. Larch in western Montana growsunder a mean annual air temperatureof 42.4? and a growing season of I70 days. The recordshere show a mean from34.5 to 40.00 and a growingseason 20 to 30 days shorter. 670 J. A. LARSEN Ecology,Vol. XI, No. 4

SUBALPINE FORESTS In the subalpineforest type Engelmann spruce, alpine fir,and white-bark pine increasein proportionto increasein elevationuntil lodgepole and Douglas firdisappear entirely from the stand. The onlyavailable weatherstation for this zone is Lake Yellowstoneat 7,733 feet. This shows a mean annual air temperatureof 3I.40 F., a growingseason about I50 days, and precipitation of 24.53 inchesper year. This appears muchtoo cold for treessuch as western larch,and bothtoo dryand too cold for westernhemlock, white fir, western red cedar and westernwhite pine. The alpineflora of the YellowstonePark as listedby Tweedy (i886) is as follows: Antennaria,alpine Gaertn. Draba crassifolia Graham Aplopap pus (Pyrrocorna) Lyallii A. Dryas octopetaclaL. Gray Epilobiu-nlatifoliurn L. Arenariaverna L. var. hirtaWats. Eritrichiumaretioides DC. Arenia strictaMichx. Erigeronuniflorus L. Artemisiascopulorum A. Gray Erigeronursinus Eaton Astragalusalpinus L. Erigeronradicatus Hook. Bryanthus (Phyllodoce) enmpetri-Festuca ovinaL. var.brezvifolia Wats. formiisSmith Habenaria obtusataRichards Castilleja pallida Kunth.var. septen- Hulsea nana A. Gray trionalisGray ( C. acuminata Ivesia (Horkelia) Gordonii Hook. Pursh) JuncusDrummondii E. Meyer Carex atrataL. JuncusParryi Engelm. Care alpina Swartz Lloydia,serotina Sweet. Carex concinnaR. Br. Luzula spicata Desv. Carex seirpoideaMichx. Mertensia.alpina Torr. Carex rigida Good. Oxyria digynaCamptd. Douglasia,montana A. Gray PedicularisParryi A. Gray Draba,alpina L. Pedicularis scopulorumnA. Gray Draba area Vahl. Pedicularis groenlandicaRetz.

Summary and Conclusion A studyof the distributionand requirementsof permanentforest types in Montanaand northernIdaho has led to the followingconclusions: i. The occurrenceof climax or permanentforest types is mainlyin alti- tudinalbelts or zonationswhich are determinedchiefly by air temperatureand precipitation. Minor variations or differencesin the forest composition withinany one of thesebelts or zones are due to local variationsin soil, aspect, gradient,etc. 2. Each of theseclimax belts or permanentforest types exhibits a definite requirementin temperatureand precipitationwhich differsmaterially from any othertype. The typesat loweraltitudes require relatively large amounts of heat and littlemoisture, and those of high elevationslittle heat and rela- tivelylarge quantitiesof moisture. October,I930 FOREST TYPES OF ROCKY MOUNTAINS 671

3. In everycase thedownward extension of the typeis limitedby deficient moisture,while the upper limit is determinedby a temperaturetoo low forthe species which occur withinthe given type. The grasslands exhibita wide range of temperature requirement from 390 to 500 F. These grasslands occur whereverthe annual precipitationis insufficientfor tree growth. For most sections 17 inches a year or less resultin grassland,regardless of air temperature. 4. Each permanentforest type exhibits a fairly definiteassociation of vegetationin the formof annuals,shrubs, and otherperennials; and these,as well as densityand compositionof the forestitself, react upon the site and tend to formfor each foresttype a fairlytypical soil and surfacecondition. These conditionsvary considerablyfor the differenttypes of forest. They influenceboth the naturaland artificialestablishment of trees,and therefore the silviculturalmanagement. 5. Since the annual precipitationis much less east of the Continental Divide than in the westernsections, such mesophyticspecies as cedar, hemlock, lowland white firand whitepine are not presentthere. On the other hand, on accountof the very dry summersin the western,compared to the easternsection, Douglas fir,lodgepole pine, and westernyellow pine, require a greaterannual precipitationin Idaho than in centralMontana. 6. The followingare the requirementsfor the varioustypes: The western yellowpine grows with a mean annual air temperatureranging from 440 to 50? F., precipitationfrom I7 to 22 inches per year. The cedar-hemlock- whitefir type in Idaho grows undera mean annual temperaturefrom 400 to 450, and an annual precipitation ranging from 27 to 44 inches. The larch- Douglas firtype in Montana requiresa meanair temperaturealso from400 to 450 F., but requiresless moisturethan the cedar-hemlock-whitefir type. For the larch-Douglasfir the precipitationranges from about 20 to 30 inches annually. The Douglas firand lodgepolepine typethrives with a mean annual air temperature ranging from 350 to 400 F., and an annual precipitation from20 to about 25 inches. Subalpineforests require mean air temperatures from 300 to 350 F. and precipitation varying from 24 to 30 inches in central Montana,and up to 36 inches in Idaho. In subalpineforests the lengthof the growingseason is usually less than I50 days, i.e., the period during which the mean air temperatureaverages above 430 F. In westernyellow pine forestson the otherhand it is from I80 to 24i days. 7. It followsalso that foresttypes of high elevationsshow much greater snowfall,greater depth of snow and longerduration of snow cover than the lower types. Snowfall in the subalpinetype in Idaho is up to 207 inches a year,while that in westernyellow pine is usuallyless than50 inches. Further- more, snow cover in subalpineregions is continuousthroughout the winter, whilein the westernyellow pine forestthe groundis oftenbare duringwarm periodsin winter. 44 672 J. A. LARSEN Ecology,Vol. XI, No. 4

The preparationof this paper was begun on the suggestionof Raphael Zon of the U. S. Forest Service. It could not have progressedwithout the kindcooperation of the membersof the U. S. WeatherBureau and the supervisors of the differentNational Forests. Valuable suggestionsand criticisms have been givenby J. Kittredge,Jr., G. A. Pearson,C. G. Bates, J. F. Preston, J. WV.Toumey, and RobertMarshall.

LITERATURE CITED

Brewster, D. A. 1928. The relation between height growth of larch and weather conditions. Jour. Forestry, 16: 861-870. Gail, F. G. 1921. Factors controlling the distribution of Douglas fir in semi-arid regions of the Northwest. Ecology, 2: 281-291. Gaskill, Alfred. 1906. Why prairies are treeless. Proc. Soc. Am. Foresters, 1: 158- 178. Harvey, Leroy H. 1908. Floral succession in the prairie grass formation of southeastern South Dakota. Bot. Gaz., 46: 81-108. Harshberger, John W. 1911. Phytogeographic survey of North America, pp. 516- 550. Vegetation de; Erde. Vol. 13, New York, G. E. Stechert and Co. Larsen, J. A. 1929. Fires and forest succession in the Bitterroot Mountains. Ecology, 10: 67-76. 1922. Weather records at lookout stations in northern Idaho. U. S. Dept. Agr. Mon. Weath. Rev., Vol. 50, No. 1. . 1923. Physical factors of site. Ms. Report U. S. Forest Service, Missoula, Mont. 1922. Soil shifting and deposits. Science, 55: 457. Leiberg, John B. 1904. Forest conditions in the Absaroka Division of the Yellow- stone Forest Reserve. Prof. Paper U. S. Geol. Surv. No. 29, p. 13. Pearson, G. A. 1928. The relation between spring precipitation and height growth of western yellow pine saplings in Arizona. Jour. Forestry, 16: 677-689. Piper, Charles V. 1906. Flora of eastern Washington. National Herbarium, 11: 38-40. Shattuck, C. H. 1915. Observations within the Clearwater National Forest. Ms. U. S. Forest Service, Missoula, Mont. Tweedy, Frank. 1886. Flora of the Yellowstone National Park. Published by the author. U. S. Dept. of Agriculture, Weather Bureau. 1921-22. Climatological summaries by sections. Weaver, J. E. 1914. A study of the vegetation of southeastern Washington and adjacent Idaho. Lincoln, Neb. Univ. of Neb. Studies, 17: 1-114. Whitford, H. N. 1905. Forests of the Flathead Valley. Bot. Gas., 39: 99, 194 and 276.