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Climate and Vegetation in the Villavicencio Region of Eastern Colombia

Author(s): Marston Bates

Source: Geographical Review, Vol. 38, No. 4, (Oct., 1948), pp. 555-574

Published by: American Geographical Society

Stable URL: http://www.jstor.org/stable/211443

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CLIMATE AND VEGETATION IN THE VILLAVICENCIOREGION OF EASTERN COLOMBIA*

MARSTON BATEST HE easternAndesof Colombia erveasa boundingwall to thevastlevelareadrainedby thetributariesf theOrinocoandAmazon iversystems.Theadjective vast"s not misapplied:rom Arauca n thenorthern rontierof Colombia to Leticiaon the Amazon the air-linedistanceis about I200 kilometers; from Villavicencio at the base of the Andes toPuerto Carreno on the Orinoco the air-line distance is 700 kilometers. Theregion is perhaps not quite as level as some maps would lead one to believe,since there are large areas of highly dissected savannas, and several ranges oflow mountains rise out of the endless Amazonian forest to the south. Mostof it, however, is flat, and one can stand on the mountain flanks and look outto where the plains merge into the horizon, as though one were looking outto sea.

The eastern Andes rise abruptly out of the plain. The town of Villavi-cencio is built against the base of the mountains, and the road to Bogotareaches the upper tropical life zone (I200 meters) in the first Io kilometersof twisting highway. Villavicencio is 500 meters above sea level, and althoughthe country to the east seems perfectly level, the altitude drops Ioo metersin the first 12 kilometers, and another Ioo meters in the next 70 kilometers.The streams in the immediate vicinity of Villavicencio and to the northflow into the Orinoco system, but 200 kilometers to the south the drainageis Amazonian. The Orinoco drainage basin is characterized by great grassyplains with forest along the streams, the extent of forest decreasing steadilynorthward. The southernmost savannas are small areason the south bank ofthe Guaviare River, and south of these is the continuous Amazonian forest.

Villavicencio is thus in a transition area: between mountains and plains,between Orinoco and Amazon drainage, between savanna and forest. Thearea shows a great variety of landscape and habitat types and has a richnessof fauna and flora that it would perhaps be difficult to equal elsewhere.* The observationson which this paper s basedwere carriedout at the VillavicencioField Labo-ratory,which is supportedby the InternationalHealth Division of The RockefellerFoundation n co-operationwith the Ministryof Hygiene of the Republicof Colombia. The author is indebted to theTropicalOil Companyfor rainfalland river-leveldata for PuertoL6pezandto the Aerovias Nacionalesde Colombia for meteorologicaldatafrom severalof theirstations, ncludingthatat Apiay.

>DR. BATESs an entomologist on the field staff of the International Health Divisionof The Rockefeller Foundation,stationedin Colombia since I940, before that in Albaniaand Egypt.


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THE GEOGRAPHICALREVIEWA biological laboratory was established at Villavicencio in 1938 under the

joint auspices of the Colombian government and The Rockefeller Foundationfor the purpose of studying the epidemiology ofsylvatic yellow fever, knownto be endemic in the area. The program of yellow-fever research was sus-pended at the end of I945, but the laboratory has been continued with the

FIG.I-The VillavicencioField Laboratorywas founded in 1938 by the Colombian governmentand the RockefellerFoundation or the study of yellow fever; at present t is dedicatedto the study ofthe biology of anophelinemosquitoes.primary objective of studying the natural history of anopheline mosquitoes.Meteorological records have been kept at the laboratory and at various otherpoints in the region since 1941, and the object of the present article is tosummarize some of the observations on climate and landscape that haveaccumulated incidentally in the course of the laboratory work.

LANDSCAPE TYPES: THE MOUNTA1N FORESTSOriginally, the eastern slopes of the Colombian Andes were undoubtedly

continuously and heavily forested. The forest is still continuous except forareas accessible by roads or trails, where there has been considerable replace-ment with pasture grass. In places the pastures have a permanent and naturallook, but I have no doubt that they are all man-made. We have watchedthe process of forest destruction during the past seven years, and by projec-

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EASTERN COLOMBIAtion backwardn time it is easyto account or all themountain rassland.Verysteepslopesaresometimesleareddeliberately,ut moreoftenforeston suchslopes s accidentally estroyedby fires started t lower levelsinunusually ry periods.Undernormalconditions ainforestis practicallynoninflammable,uta firestartedow on a steepslopemaysweepupwardforconsiderableistances,ausingdisproportionateestruction.

es?.'..7~;,.~r.ep

0 I10 MILES Footh Pedo Ga//eo b KILOMETERS Road, E: forest frest vana 1 eFIG. 2-Distribution of the chief vegetation types of the lower tropicalzone in the Villavicencioregion. The savannaareas nclude varioustypes of associated orest.We have ound tconvenientocall hebandof forestonthe owerslopesof the mountains"foothill orest"andhaveplaced ts upper imit ratherarbitrarilyt I200 meters,sinceits characterhanges omewhatat aboutthisaltitude ecause f thegreatlyncreasedmountof fog.The lower imitwouldbethelinewhere heabruptmountainlopegiveswayto thehardlyperceptibleradient f the talus.Thechangeof slope s sharply efined;ntheVillavicenciorea t occurs tabout500meters.Thechangenthecharac-ter of the forest s alsodistinct, houghwe have attemptedno botanicalanalysis f the change n species ompositionwith the change n gradient.The forestat thebaseof themountainswe have called"piedmont."nthe souththisforestwouldhardlybe definable,ince t is continuouswiththe Amazonforest;but in the Orinocodrainageareait forms a well-definedbeltextending utforI5 to 20 kilometers, here t giveswayto thesavannasndassociatedorests f other ypes.Thispiedmontorest s classi-cal rainforest,with little or no underbrushecause f the heavycanopy,which, nitself, scomplexwithseveralayers f treeheights.Thepiedmontforesthasbeenalmost ompletelylearedntheimmediateicinityof Villa-vicencio,but thestumps nd tru/nkstill itterthepastures,nd theoriginalpatternof forestcan be seena few kilometersrom the roadsand trails.The piedmont orest s supported y a rainfall f four to five metersannually,airlyevenlydistributedhroughheyear.On themountainlopestherainfallncreases,o a totalof nearly evenmetersat theupper imitof

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THE GEOGRAPHICALREVIEWthe lower tropical zone (I200 meters). Here rainfall and fog result in aprofuse development of epiphytesof all kinds-bromeliads, mosses, ferns,peppers,orchids.As regardsthe mosquito fauna,the outstandingdifferencebetween thetwo forests s in type of aquatichabitat.The foothill forest has a rich mos-quito fauna,but it is exclusivelyof the sort that breeds n water containers-tree holes, the leaf basesof bromeliads,aroids,and other plants,the fallenleaves of palm-flowerbracts.There areno accumulations f waterexcept insuchcontainers.The mountain streamsaretoo precipitousand too subject ofloodingto providemosquitobreedingplacesexceptforwaterheldin depres-sions in the great boilders, again a containerhabitat.The piedmontforest,on the otherhand,providesmany sorts of ground-water accumulationsn additionto the containerhabitats.Shallow depres-sionsmay hold rain waterfor daysor weeks;seepageareasgive riseto smallswampsand pools; the meanderingstreams orm quiet pools and leave oc-casionalsmall oxbows; and the streamswith gravel beds have pools andseepage areas that are occupied by mosquitoes during the drier months.

THE SAVANNA LANDSCAPESThe savannasbegin I5 to 20 kilometers out from the mountain front.From an airplane, he nearersavannasook like streamsystems n the forestmatrix, narrowbelts that graduallywiden and converge; fartherout, it isthe forests that seem like streamsystems,and in fact they do outline thesavanna treams.The plantcompositionof the savanna orestsdiffersnotablyfrom thatof the piedmontandfoothill forests, hough againwe have carriedout no botanicalsurveys.The flood plainsof the majorrivers,such as the

Meta, the Guaviare,and the Casanare,are coveredwith anotherdistinctivetype of vegetation, for which the term "gallery forest" is perhapsmostappropriate.The developmentof savannas s apparentlyrelatedto rainfall.In a linefromVillavicencio o PuertoL6pez,the firstnatural avannaoccursat Apiay,about 13 kilometersfrom the mountainfront, where the annual rainfall ssome 500ooillimeters less than at Villavicencio.Perhapseven more im-portantthan the total rainfall s the severityof the December to April dryseason. Rainsin Januaryand Februaryare usuallyinsignificant hroughoutthe savannaarea,and I have been told by reliablewitnessesthat sometimesthere is no rainat all for threemonths,though we have no recordof morethan a month without rain. The relation between total annualrainfallanddistancefrom the mountain front is shown on the accompanyinggraph

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FIG. 3-One can stand on the flanks of the Eastern Andes and look over the plains of the llanos ason a coastal range looking out to sea.FIG. 4-The Eastern Andes rise abruptly out of the plain, with no nonsense about intermediatezones of foothills.


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FIG. 5-The piedmont forest has mostly been clearedaroundVillavicencio,but small patcheshave beenleft, like this on the Ocoa River.

FIG. 6-The rain forest has little undergrowth because the canopy is too dense forthe penetration of much light.

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FIG. 7-The forest along the river banks is constantly changed by erosion and deposition.

FIG. 8-In the course of yellow-fever work, detailed studies were made of thestructure of selected areas of forest.

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THE GEOGRAPHICALREVIEW

(Fig. I7) of the recordsfor I946 at Buenavista,Villavicencio, Apiay,and Puerto L6pez.Scattered records frompoints farther east seemto indicate that there islittle further changewith increased distancefrom the mountains.

The present vege-tation of the savannasis determined by fireresistance, since thegrass is constantlyburned by cattlemento provide fresh pas-ture. The grass burnsreadily after only threeor four rainless days,and the burning maythus be done at anytime of the year; butthe most extensive firesFIG.9-For studies of upper zones in the forest, ladderswerebuilt on trees;the tagson this tree markholes in which mosquitoes are those of the drybreed. season. The smoke at

this time forms a haze that overcasts the whole sky. The custom of burningthe savannas is probably pre-Colombian: various early observers report thatsuch burning was habitual with the Indians. It is unlikely, however, that itcan be prehuman, since lightning would seem an improbable agent forstarting grass fires with any regularity.

Whether or not grass was the prehuman vegetation of the savannas, thisvegetation must have been different from that of the present forest areas:the one was inflammable, the other is not. Human clearing of forest can beeliminated as an agent. Even today clearing is a minuscule factor in theColombian llanos because of the scattered population, engaged entirely incattle raising. It seems unlikely, however, that the savannas represent ageologically ancient landscape type, inasmuch as no highly peculiar animalsor plants have developed. The fauna is characterized by its poverty and the

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EASTERN COLOMBIA

apparently recent arrivalof its chief components.The deer, rabbits, andquail are all only slightlydifferentiated representa-tives of northern types.The rich grazing faunaof the African savannashas no counterpart, yetsurely these grasslandscould support a densegrazing population. Thepoverty of the grasslandfauna and the absence ofhighly characteristic ocaltypes contrast stronglywith the condition ofthe fauna and flora ofthe forest. In the forestthe profusion of localanimal and plant types,with obvious forestadap-tations, gives evidenceof the antiquity and sta- FIG. Io-Water courses in the savannasare typically markedbilityof the South Amer- by rows of Mauritia moriche) palms.ican forest habitat.

The present distribution of savanna and forest reflects drainage condi-tions. The ilanos east of Villavicencio show a dense network of streams,which rise in springs and seepages that have a fairly even flow throughout theyear. Some springs that we have watched show the least flow in May, whenlocal rains are heavy, an indication that the ground water has its origin inthe mountains one or two hundred kilometers away. The forest is always inlow areas, the grassland in higher, and typically there is a flowing streamsomewhere in the forest. In a few places there are strange forest islands,in depressions that center in ponds without surface drainage, but this forma-tion is rare. The seepage that forms the head of a stream sometimes occursin grassland, where it is invariably marked by a clump of Mauritiapalms, aplant whose nearest approach to the mountains corresponds exactly with thenearest approach of the natural savannas. The stream may flow for several

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FIG. II

FIG. 12FIG. II-Flood plain of the Meta, with gallery forest, as seen from the serrania of the south bank.FIG. I2-The Meta, like other meandering rivers, leaves numerous oxbows; the gallery forest herehas been cleared for pasture.

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FIG. 13

FIG. 14FIG. 13-This is typical "serrania"; highly dissected savanna, with forest in the well-drained valleys.FIG. 14-In many places in the serrania there are outcrops of ferrugineous sandstone.

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THE GEOGRAPHICALREVIEWhundredmetersthrough grass, ined by Mauritiapalmsin a widening bandthatdevelopsgradually nto the usualcomplex forestvegetation.

TYPES OF GRASSLANDThe savannasare far from uniform in type, and severalvarietieshavereceiveddistinctive ocal names.Perhaps he most fascinatingof these typesis the serrania.This is highly dissectedcountry, usuallycharacteristic f thesouth banksof the largerrivers.These rivers seem to be graduallyworkingsouthward, heirgalleryforestson the north giving way to level plains,andon the southboundedby high bluffs anddissectedserrania.The hills of the

serraniaoften have fantasticshapesand very steep slopes, protectedfromobvious erosion by the grass.The soil surfaceis a gravel layer over claywith occasionaloutcropsof a reddishrock.The rockis apparently ne of theminerals hathardenon exposure,giving erosionpatternsike those of lime-stone, except that the base is iron rather hancalcium.More commonly, perhaps, he savannas regently rolling,with extensiveareas,locally called bajos, hat are marshya good partof the year. Some-times the bajosform sheets of shallow open water during the wet season,in which casethey arecalledesteros. he low savannas refrequentlymarkedwith termitemounds,which may be built up as much as a meter above thegeneralgroundlevel.There are various transitionstagesbetween this type of landscapeandthe type calledsural,which represents he extreme of difficultmarshyter-rain. The suralespresenta reticulatepatternof deep ditches surroundingmounds ameteror two in diameter; hetop of the moundisameter or moreabove the bottom of the surroundingditch. There are large areasof suchcountry, particularlyon the north side of the Meta, and its origin has longpuzzledme. The reticulateditchingis like the pattern ormedby the dryingof a giganticmud flat.There is usuallya barelyperceptible low of water inthe ditches,but it seems difficult o account for any suchpatternon a basisof erosion.Each moundtypicallycontainsaninhabitedor abandoned ermitenest,but I do not know whetherthisdistributions a cause or a resultof thesuralpattern.In some areaswhere the suralespresumably re eithervery oldor very young, they show gentleslopes ike those of the "Mima mounds"ofthe westernUnited States.'

The origin of the Mima mounds is underactive discussion.V. B. Scheffer(The Mysteryof theMima Mounds, ScientificMonthly, ol. 65, 1947, pp. 283-294) believesthey are causedby the pocketgopher. His argumentshave been criticizedby ChapmanGrant (Journ.of Geol., Vol. 56, 1948, pp.229-231), and he has replied (ibid.,pp. 231-234). From AlaskananalogiesT. L. Pewe (Origin of theMima Mounds,Scientific.Monthly,ol. 66, 1948,pp. 293-296) ascribes periglacial riginto the mounds.

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FIG. I5-A flooded savanna with open water, like this, would be called an estero; the "marsh buggy" isbeing tested as a possible method of transport in this terrain.

FIG. I6-Termite nests may be numerous in low savannas, often occurring onsmall hummocks, making an uneven terrain called sural.

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THE GEOGRAPHICALREVIEW

Well-developedsuralcountry is difficultto traverse. f you are on foot,you have to decide whether to follow the endlesstwistings of the boggyditches or to jump from mound to mound, both awkwardexpedients.Ifyou are mounted, the animal has to make the same decisionand generallyendsup in completefrustration: have heardstoriesof manandmule firmlystuck in a narrow,deep ditch between two suralmounds. One of the oilcompaniestried a "marshbuggy" in this terrain,but though some progresscould be made by winching, the frameof the vehicle soon started o breakup.The surales, he esteros,the marshes,and the savannaseepagesprovideexcellentbreedingplacesfor a varietyof mosquitoes.They belong to specieswhichare ittle known to science,but which arepresentn incrediblenumbersin the llanos. The human population, however, is too sparseto supportmalariaexcept in the rivervillages.

THE LLANOS FORESTSThe forests of the savannacountry show as much varietyas the grass-lands,or more. In serrania, he forestsoccupy the valleysand may be well

drained,covering steep slopes. In the rolling savannas, he forests are indepressions, ut the groundmay be solid anddry, or boggy, or coveredwithstandingwater (swamps).The physicalstructureof the savanna orests s notas complex as that of the piedmontrainforest;the canopy, in particular,susuallynot well differentiatednto zones of various tree heights, though itmay be denseand theremay be occasionalgianttrees.The forests,especiallythe swamp forests,sometimesalmostachieve theaspectascribedby Hollywood and thepopularwriters o "jungle."The foot-hill and piedmont forests are majestic,with a cathedral-likebeauty in themassive architectureof buttressedtrunks and the intricatedetail of leafstructure.But the savannaswampsarecovered with 30 centimetersof algae-filledsludge,with in placesa denseundergrowthof variousperverted ousinsof the banana,with spinyferns,andwith a sufficiententanglementof slendervines and cobwebs for annoyance,if not really for impediment.In theseswampsI am alwaysverymuch awareof the giantanacondas ndthejaguarswhosehabitattheyare;when crossinga sluggishstream thinkof the electriceels, the sting rays, and the many storiesof caribeish (the piranhaof theBrazilians).These last are more typically inhabitantsof the clear oxbowlagoons,but I think of them anyway.We have a cattleranchon the llanos,and there we once cut opena hugeanaconda, o find a smallmissingcalf inside;a big boar came back to the

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EASTERN COLOMBIAhouseacoupleof monthsagoterriblyaceratedromsomeencounter ithajaguar;and smallerpigs frequentlyail to come backat all. I haveseenfriendsufferingromtheincrediblepainof a sting-raywound,so that hereis realreason o be carefulof the streams.The mostprobable xplanationof several rownings f goodswimmerssthetemporary aralysisesultingfrom an encounterwithanelectric el. Thesecreaturesreallpresentn theforest,andthe localpeopleareaptto be even more timidthanI amaboutwanderinglonethere.

TABLE I-CLIMATE DATA FROM LABORATORY STATION AT VILLAVICENCIO, I94I THROUGH 1946

M. A. M. J. J. A. S. O. N. D. J. F. YearTotal or MeanRainfall mm. 189 599 695 463 492 480 426 519 435 214 93 65 4670Days with rain I2 24 27 24 27 24 19 22 20 13 9 7 228Mean temp. ?C. 27.7 26.6 26. I 25.5 25.3 25.8 26.5 26.7 26.7 26.4 26.9 27.7 26.5Mean noon R.H. 66 72 75 77 75 74 72 72 72 68 63 6i 70.6Evap. mm. I40 78 58 56 60 72 86 87 89 IOI I23 134 o084

Comparethe rainfallgraphfor Villavicenciofor 1925 and 1926 in F. O. Martin:Explorations nColombia, Geogr. Rev., Vol. 19, 1929, pp. 621-637 (Fig. 5, on p. 627). The yearly average for thesetwo yearswas almost the same as that for the six-year period above. The driestmonth was Februarythe wettest,June.

But theseforestsare not unfriendly;heyaremerelydisinterested.orme theirgreat ascinationomes rom theirremoteness,rom the fact thattheyareuntouchedby humanactivity, hatannoyingactor orwhichal-lowancemustconstantlybe made in studiesof vegetation ormationsnmostparts f theworld.Hereyoudo notfindstumpseftbythewoodcutter,or haveto worryaboutsuccessionfterclearing, r about hedepletion fgameby hunting. t shouldbeanecologist's aradise-ifanecologist ouldmanageo gettherebeforeagriculture,attle,highways, ndoilcampshavebrandedhelandscape iththeirdominatingatterns.

CLIMATE AT VILLAVICENCIOBasicdataon theVillavicencio limateare summarizedn TableI. The

controllingactorntheannualcycle sthedryseason,xtendingromsome-time n December o theendof March.Thevarious lantandanimalcyclesshowrhythmsdependent n thisdry season, ndin consequence e havefound t mostconveniento analyze llseasonalatawitha yearbeginningon the firstof March,hemonth nwhich ifeprocessesrerenewedby theonsetof therains.The rainfalls high, with an average or the six-yearperiodof 4670

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THE GEOGRAPHICAL REVIEWmillimeters (184 inches); the lowest total recorded is 3964 millimeters (1941),the highest 4987 millimeters (in both 1942 and I943). Distribution throughthe year is fairly even: for eight consecutive months, April through No-vember, the fall is more than 400 millimeters a month, with a maximum inMay (695 mm.); the minimum occurs in February (65 mm.). There may beconsiderable rain, by temperate-zone standards, even during the dry season,but the evaporation rate is so high, and the runoff so rapid, that this pre-cipitation has little effect on the plants and animals.Air temperature is remarkably stable throughout the year. The highesttemperatures generally occur in February, the lowest in June orJuly; how-ever, the highest monthly mean of which we have record, 28.2 C. (Febru-ary, I942), does not differ greatly from the lowest monthly mean, 23.9 C.(August, 1946). The annual mean is 26.5? C. (80? F.). There is considerabledaily fluctuation in temperature, night temperatures being between 20? and22? C. (about 700 F.) and maximum day temperatures between 30? and 34?C. (86? to 93? F.). The average daily fluctuation is, in fact, much greaterthan the annual fluctuation in monthly means. In 1946, for example, thedifference in mean temperature between the hottest and coldest months was3.7? C.; the average daily fluctuation throughout the year was 7.5? C. Thisgreat daily temperature range makes for pleasantness; the hot afternoonsare invariably followed by cool nights.We measure evaporation daily from a free water surface in a beaker keptin the shade. The method is crude but convenient and seems to give consistentresults, and comparisons between different seasons and different localitiesare possible. We also measure the relative humidity with a sling psychrom-eter three times a day, but the noon figure seems to be the best to usebecause it shows the greatest seasonal variation. Both these records combineto prove that we have a rather dry climate, in spite of the high rainfall. Thedryness shows up more strikingly in little events of daily life. I am unendinglyastonished to find the asphalt pavement dry when I walk to the laboratoryin the morning after having listened to a heavy rain pounding on the roof allnight. Or to find dust on the road within 24 hours of a rainfall of a hundredmillimeters. Our shoes do not mold, nor do envelope flaps stick, unless theenvelopes are carelessly stored. The annoyances of a "tropical climate" seemmuch greater in Florida than in Villavicencio, in the interior of the SouthAmerican continent.

I am not sure how much the characteristicsof our climate result from ourmid-continental position, and how much from our nearnessto the mountains.Both are probably influential, the position perhaps more important in the

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case of temperaturend humidity,since conditions eem to be similarhundredsf kilometersut fromthe mountains.We areaptto thinkof amaritimelimatewhen we thinkof thetropics-of Panama, rTrinidad,rtheWestIndies-and heir ventemperaturesndhighhumiditiesreproba-blymoredirectly elated o themaritime osition han o thetropical.Thegreat orestedontinental reasperhapsavea similar ufferingffectontheclimate,but thisis nullifiedat Villavicencio y the extensive learingnearthetown andby the savannaso theeast.

meIers/2zo-9 Buenavista

1000

800-s3 \\ Nmi/limeters -7000

00 Villavicencio -6000^ Apiay ^ -5000_Pto. L6pez ZZ

N200Y1, 2000 t1

0 6 18 ki/ometers 81FIG. I7-To show the relation between altitude, distance from mountain front, andtotal annual rainfall (close-ruled bars).

CLIMATIC GRADIENTSTheclimatemeasuredt a standardmeteorologicaltation as,of course,only a limitedsignificance.t is usefulfor comparison etweendifferentgeographicalegions,andit perhaps as some smallbiologicalmportanceso farasman sconcerned,incemannormallyives n openshelterswhereconditions resomewhat imilar o those n a Stevensoncreen.Formost

biologicalpurposes, owever, he significantlimates thatwhichprevailsin theactualhabitat f theanimalor plantbeingstudied, climatewhichmaydiffergreatlyromthatmeasuredbystandard ethods.nVillavicenciowe havehadconsiderableopportunityo studyboththedifferencesamonghabitatlimates nd hedifferencesnclimatebetween imilar ituationsonlya few milesapart.The moststriking egionalgradients thatof rainfalln relationo themountainfront.The figures or fourstationsn I946 arecharted n Figure17.Buenavista s at the crestof the first mountainridge, at I200 meters;Villa-vicencio, about six kilometersaway in a straight ine, is at the base of themountains;Apiay is 12 kilometersfarthereast;PuertoLopez,75 kilometerseastof Villavicencio, s the headof navigationon the Meta River. The drop

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THE GEOGRAPHICALREVIEWin rainfallbetween Buenavista and Apiay is enormous (from 6400 milli-metersto 3200 millimeters)andregular:stationsa kilometerapartalong thisline show a uniformdecrease.Widely separated tations hatwe have estab-lishedat equaldistances rom the mountain front show closely correspond-ing precipitation.The dropbetweenApiayandPuertoL6pez,63 kilometers

900- mniI/meters800 -

00 A500IG. Average monthly level of Meta Riverpez (line) and rainfall

at Villavicencio (bars)in I946.

~400 --4 1300- ii -i

FtG. 8-Averag level of Meta River at Puerto Lpez (line) andrainfallinlat Villavicencio (bars) in 1946.

away, is, however, slight, and fragmentary records from several points stillfarther east show a rainfall closely similar to that of Puerto L6pez, so that thefigures for this town are probably characteristic of the llanos east of Villa-vicencio away from the influence of the mountain front. The appearance ofthe vegetation makes us think that there is a rainfall gradient in the llanos,from a higher precipitation n the Amazonianarea to a lower one towardArauca,but no figuresareavailable.The changein vegetationmight also beaccounted or by a more even distributionof rainfall o the south.

Figuresare availablefor the fluctuations n the level of the Meta RiveratPuertoL6pezin I946.These,with the Villavicenciorainfallsuperimposed,are charted n FigureI8. The river level at thispoint represents summationof the rainfallalong some 30 kilometersof the mountainfront, and the riseandfall, thoughconsiderable, reusuallynot abrupt.The total annualrangeof fluctuations about four meters(13feet). The fluctuationn level is, how-ever, a poor index of the actualvariation n volume of water, since at highlevelstheentirefloodplainof the river(aboutfourkilometerswide atPuertoL6pez) is inundated.This periodic inundation is undoubtedly the controlling factor deter-mining the character f the galleryforestof the riverflood plain.This foresthas been extensivelycut, sincethe soil is fertile,but afterone or two cropsthe clearedareasare abandonedto second growth. The Meta and other

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majorrivers akea meanderingourseabout30 kilometers ut fromthemountainrontand eave requentargeoxbowlagoons.Theres alsoacon-stantundercuttingf thebanks, nddepositionf sandbars.The erosion nddepositionmean hat hegalleryorest s in anunstabletate, venaside romhumanactivities,nd hevegetationhangesormaninterestingxample f

Temperature30? - C. - Laboratory station

..........---o Piedmont forest28?- / *- Foothill forest

o ------- -..... ...26?o--.- / .- ---220-

Hour ' 4

. 24? I tHour 2 4 6 8 10 12 14 16 6 20 22 24

FIG. 9--Comparative temperaturerecords (averagesat two-hour intervals for 7days)at the laboratorystation and in two near-byforeststations.

ecological"succession"hatwelldeserves etailedstudy.Thevegetation fthe older sandbars s similar o the secondgrowththatfollowshumanclearing, ndthesesandbarsareprobablyhenatural abitat f thehost ofweeds and shrubs hat are now closelyassociatedwith humanactivitiesthroughoutheregion.Fromthebiologicalpointof view it is useful o distinguishhreecate-goriesof climate: hegeographicallimate,measuredby standardmeteoro-logical tations;hehabitat limate f different indsof environmentalitua-tions;andthemicroclimate,hephysical onditionshatactually urroundthe individualorganism.Habitat limatesn an areasuchas thataroundVillavicenciorelikelyto be particularlyiversified,ndwe were at onetime especiallynterestedn studying orestclimates.The most strikingdifferencesn forestenvironmentsrefoundn temperature,elative umid-ity, andlight.Theforesthasa considerablebufferingffecton temperature,so that hedaily luctuationsmuch ess han n openstations.Temperaturesalsodiffer n differenttypesof forestandin differentituationsn thesameforest.Theamountof differencehatmayexistbetweenground tationssshown n Figure 9, basedon simultaneousthermographecordsora weekmade n anareaof foothill orest,one of piedmontorest,andat the labo-ratory tation.

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THE GEOGRAPHICALREVIEWThe reative humidity of the air in the forest remainshigh all throughthe day, and light, of course,is greatlydiminishedby the tree canopy. All

the elements of forest climate-the low temperatures,he high humidity,the reducedwind movement-tend to result n a very low evaporationrate.One of the outstanding haracteristicsf forest climate s its stratification.Conditionsin the floor zone and in the canopyarevery different,and thereis a fairly regular gradientof environmentalchange from the forest floorto the air above the trees. At midday, temperature s lowest, humidityhighest, light least,at the forestfloor, and conditionsgraduallychangewithascent.The changesare reflected n a stratification f the animal and plantlife, eachkind of organismhavinga preferredevel in the forest at which itis found in greatestabundance.High in the canopyconditionsarerelativelywarm and dry, and organisms ound theremay show specialadaptationsothe aridity-metallic colors in insects,tubers and fleshyleaves in epiphyticplants.The list of the specialconditions of various habitatclimates could bealmost endless.The study of these specialclimateshas been relativelyneg-lected even in the temperatezones, and in the tropicsthey have receivedalmost no attention.It is in the tropics,however,that thesehabitatdifferencesforce themselveson the attention of the observer:walking three or fourmetersfrom a Colombian savannanto a forestmargin,you changeclimateas completely as though you had moved from Floridato Maine in July.And you can,of course,changeconditionsfrom Florida o Alaskaby climb-ing a few kilometersup the slopesof the Andes.

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