Tropical Rain Forests: Are Nutrients Really Critical?
Author(s): Carl F. Jordan and Rafael Herrera
Reviewed work(s):
Source: The American Naturalist, Vol. 117, No. 2 (Feb., 1981), pp. 167-180
Published by: The University of Chicago Press for The American Society of Naturalists
Stable URL: http://www.jstor.org/stable/2460498 .
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Vol. 117, No. 2
TROPICAL
The AmericanNaturalist
RAIN FORESTS:
ARE NUTRIENTS
February1981
REALLY
CRITICAL?
CARL F. JORDAN AND RAFAEL HERRERA
Instituteof Ecology, Universityof Georgia, Athens, Georgia 30602; Centro de Ecologia,
InstituteoVenezolano de InvestigacionesCientificas,Apartado 1827, Caracas, Venezuela
SubmittedFebruary27, 1979; Accepted October 4, 1979
Part of the lore of tropicalecology is thattropicalrain forestsare particularly
susceptibleto leaching loss of nutrientsas the resultof rapid decompositionof
litterand heavy, frequentrains. Clearingthe tropicalforestforagriculturalpurand decline
loss of ecosystemfertility,
poses resultsin rapidleachingof nutrients,
in productive potential. Ecologists are citing these ideas as reasons why the
Amazon Basin should be leftin an undisturbedcondition(Goodland and Irwin
1975).
Harcombe (1977a, 1977b)has recentlychallengedtheidea thatnutrient
leaching
is severe in the tropics. On the basis of several experimentalplots in Costa Rica,
he concluded thatnutrientleaching,even afterclearing,was not greatenoughto
affectsubsequentvegetativegrowthin the tropics.Is Harcombe correct?If so, it
would have importantimplicationsnot only for understandingtropical biogeochemical cycles, but also for futureplanningfor agricultureand forestryin
the tropics. The problem lies in expectingthat all tropical forestshave similar
structureand function.Certainlyno one expects similarstructureand functionin
all temperateforests,whichrangefrompine barrenson the Eastern Coastal Plain
oftheUnitedStatesthroughrichmesophyticcove forestsoftheSmokyMountains.
Why then should all tropicalforestscycle nutrientsin similarways?
Here we propose thatthereare basically two types of nutrientcyclingstrategies, or perhapsbetter,a gradientwiththetwo typesrepresenting
extremesof the
gradient.One strategy,an oligotrophicstrategy,occurs on nutrient
poor soils, and
the other,a eutrophicstrategy,occurs on nutrientrichsoils. This distinctionhas
notgenerallybeen used, because in thetemperateregionswheremostoftheforest
research has been done forestsare generallyof the eutrophictype. In contrast,
oligotrophicecosystems constitutea much greaterproportionof the tropics,at
least in South America and especially in the easternand centralAmazon Basin,
but oligotrophicecosystemsalso occur in Africa(McKey et al. 1978) and Southeast Asia (Brunig 1974; Janzen 1974).
The soils ofthecentraland easternAmazon ecosystemsare extremelylow in all
nutrientelements(Aubert and Tavernier 1972). The reason theyare low is that
there has been no geologic activity such as mountainbuilding in the recent
geologic past, and while therehas been depositionof sediments,theyare princiAm. Nat. 1981. Vol. 117, pp. 167-180.
1981 by The Universityof Chicago. 0003-0147/81/1702-0001$02.00
?
167
168
THE AMERICAN NATURALIST
pally low nutrientcontent sands derived from the nearby sandstone shields
(Fittkau et al. 1975b). This topographicstabilitycombined withthe continuous
hot, wet climate results in a very deep weatheringand leaching of underlying
parentmaterial.Lack of erosion over the generallyflatto rollingAmazon Basin
has had the result of eliminatingbedrock as a freshsource of nutrients.
There are manyeutrophicregionsin tropicalAmerica,especiallyin the Andes,
CentralAmerica, and some of the Caribbean islands, where the nutrientsin the
underlying
rocks of the mountainsbecome available as the exhaustedtop soils are
eroded. The differencein nutrientcycling strategiesbetween the oligotrophic
ecosystems of the Amazon Basin and the eutrophic ecosystems of Central
America has caused the differencein opinion regardingthe question of nutrient
leaching.
To begin the discussion of differencesin nutrientcyclingbetween oligotrophic
and eutrophicsystems,we compare structureand functionof bothtypesof forest
ecosystems. While it is difficultto findcomparable data on a large numberof
data fromone oligotrophic
ecosystemsofeach type,we were able to findsufficient
tropicalecosystem,one eutrophictropicalecosystem,one oligotrophictemperate
ecosystem, and one eutrophictemperateecosystem. If commonalitiesor differences appear between oligotrophicand eutrophicecosystems when the ecosystems being compared occur in different
latitudinalbelts, our assumptionis that
these characteristicsare importantenough to definedifferencesin most oligotrophicand eutrophicforests,regardlessof latitude.
ECOSYSTEM
COMPARISONS
The basic factorwhichseparateseutrophicfromoligotrophicecosystemsis soil
Soils withhighfertility
fertility.
supporteutrophicecosystems,and soils withlow
to findan adequate indifertility
supportoligotrophicecosystems. It was difficult
such as percentagesaturationwithexchangeablebases or ion
catorof soil fertility
exchangecapacitywhichhad been reportedforthefourtypesofecosystems.However, we were able to eitherfindor calculate calcium concentrationsin the soil.
we used this
Since calcium concentrationis highlycorrelatedwith soil fertility,
as a basis forseparatingeutrophicfromoligotrophicecosystems(table 1, row 1).
Our temperateeutrophicecosystem is the Liriodendrontulipferaforestof the
Oak Ridge National Laboratory (Reichle et al. 1973). The forest is a second
growth,mesophyticdeciduous forest,located on Karst topography.Oldest trees
were about 48 yr in 1973, and tallesttrees had a maximumheightof about 30 m.
The soil is a deep alluvial Emory silt loam (Reichle et al. 1973). Exchangeable
calcium in the soil averages 3.3 meq/100g, and total exchangeable calcium to a
depth of 75 cm is 3784 kg/ha(Shugartet al. 1976).
Our eutrophictropical ecosystem is the lower montane rain forest near El
Verde, Puerto Rico (Odum 1970). The forestis of a type described as Tabanuco
(Dacryodes excelsa) by Wadsworth(1951). The foresthas never been clear cut,
but occasional selective loggingtook place in the area in the late 1930's (F.
Wadsworth, personal communication).The average height of the top of the
canopy is about 20 m, althoughthere are a few emergentswhich reach 25 m
NUTRIENTS IN TROPICAL RAIN FORESTS
169
(Jordan 1971). Soil type is Los Guineos clay, but the clay is aggregatedinto
particleswiththeresultthatthebulkdensityat the surfaceis onlyaround.66 g/cm3
(Edmisten1970). Calcium concentrationwas between 1 and 5 meq/100g, and total
exchangeable calcium per hectare is 1,900 kg, to a depth of 40 cm (table 1;
Edmisten 1970).
Our oligotrophictemperateecosystem is the oak-pine forestat Brookhaven
National Laboratory, Long Island. The forestis "small and unimpressiveto a
forester"(Whittakerand Woodwell 1968, 1969). The canopy is open, withlight
to supporta shrubstratumof Ericaceae. The
intensitybelow the trees sufficient
forestis growingon level outwash sands and gravels of the Wisconsinglaciation
(Whittakerand Woodwell 1969). The soils are droughty,acidic, and poor in
nutrients,and are classified as Duke sand, with a sand contentof 90%-100%
(Reiners 1965). Because the descriptionof Duke sand did not give exchangeable
calcium, we used instead data fromLakewood sand (Lutz and Chandler 1946).
Lakewood sand is the upland soil along the New Jerseyouter coastal plain in a
regionlocally called the "Pine Barrens" (Tedrow 1979). Lakewood sand is very
similarto Duke sand in that they both are podsols, support scrubbypine-oak
vegetation,are almostpure sand and supportthe same vegetationtype(Woodwell
Lakewood sand
1979). However, thegeologic originofthesands maybe different.
has an exchangeablecalcium concentrationof .22 meq/100g, and totalexchangeable calcium is about 990 kg/ha.
Our oligotrophictropical ecosystem is the type of tropical rain forestof the
centralAmazon Basin known locally as "caatinga" or "campina" (Klinge et al.
1977, but it should not be confused withthe thorn-scrubforestof Brazil, which
also is called "caatinga." The predominanttrees are Micrandra and Eperua and
theirheightrangesbetween20 and 30 m. Duringperiods of heavy rain,the water
table reaches the forestfloorsurface. The soils are classifiedas tropicalhumus
podsols (Klinge et al. 1977). Under the litterlayeroccurs a deposit of humus,rich
in fineroots. Beneath this is an eluviated layer of sand, very poor in nutrients.
Calcium concentrationis 0.38 meq/100g, and totalcalcium per ha is 306 kg to a
depth of 40 cm. The B horizon is an "ortstein" in which iron, aluminum,and
organic matterare precipitated.The agriculturalpotential of the soil in this
ecosystem is very low. Afterclearingan area of forestfor shiftingcultivation,
local farmersusuallyget one good crop and one or two poor crops. Afterthat,the
site is abandoned. At our experimentalsite at San Carlos de Rio Negro,
Venezuela, we found a good correlationbetween declining productivityand
nutrientleaching. Our preliminarydata on rates of tree growthand rates of
dead treefallin the Amazonian caatingaecosystemnear San Carlos de Rio Negro
indicatethattheforestmaybe in a steadystate. There is no recordof the siteever
having been cut withinthe historyof the village of San Carlos de Rio Negro.
Table 1 summarizesthe comparisons of the structureand functionof these
temperateand tropicaloligotrophicand eutrophicecosystems.
Since calcium in the soil differsby a factorof 2.5 to 25 betweenthe eutrophic
and oligotrophicecosystems,it mightbe expected thatthe biomass of the forests
would also differgreatly.However, the differencesare not striking.In fact, the
oligotrophicforestat San Carlos has the highestlivingbiomass and the highest
170
THE AMERICAN NATURALIST
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THE AMERICAN NATURALIST
totalorganicmatterof all fourforests(table 1, rows 6 and 9). This maybe because
the San Carlos forestis probablyin steady state, or close to it, while the other
forests are in various stages of late succession. However, if the net primary
productivity
of the livingtrees is compared, successional age of theforestshould
not matter.The growthrate of individualimmaturetreesin a "climax" forestwill
continue to be high as long as there are gaps caused by fallen trees or canopy
irregularities.In a "climax" ecosystem, net biomass incrementis zero only
because thenet growthof youngtreesis balanced by the deathof old trees. When
net wood primaryproductivity
of the fourforestsis compared,thereis verylittle
differenceamong all of the forests(table 1, row 10).
Rates oflitterfall
are higherin thetropicsthanin thetemperatezone, regardless
of ecosystemtype (table 1, row 11). However, litterfall
rates in temperateoligotrophicand temperateeutrophicecosystemsdo not appear verydifferent,
nor do
litterfallrates in tropical oligotrophicand tropical eutrophicecosystems. One
strikingdifferencebetween oligotrophicand eutrophicforestsoccurs in the biomass of the humus layer (table 1, row 8) which is not distinguishablein the
eutrophicforestsbut constitutesabout 30% of the total organic materialin the
oligotrophicforest.Comparisons of productivityand biomass in temperateand
tropicalforestsreveal no strikingdifferencesbetween eutrophicand oligotrophic
forests except for the presence of a humus layer in the oligotrophicforests.
Perhaps calcium in the oligotrophicecosystemsis lower. However, a comparison
(table 1, rows 12-21) shows that while concentrationsand total amountsin the
eutrophicforestsare usually higher,differencesare nowherenear as greatas soil
calcium differences.It mustbe concludedthatbiomass, productivity,
and nutrient
cyclingdo not varygreatlybetween oligotrophicand eutrophicforests.There are
butnowherenear as largeas would be expectedconsideringthe
slightdifferences,
differencesin soil fertility.An importantpoint here, however, is that no great
differencesappear as long as the naturalforestsare undisturbed.What happens
when the naturalforestsare cut and agricultureis attempted?Do the ecosystems
maintaintheirproductivityequally?
In attemptingto answer this question, it was difficult
to findcomparable data,
especially on the same crop, forthe fourecosystemtypes. Further,we specified
thatthe data should be fromunfertilized
plots, since additionsof fertilizerwould
mask naturallyoccurringecosystemphenomena. However, by choosing yield of
corn grain, we were able to get enough data to see what happens when these
ecosystems are convertedto agriculture.
The tropical eutrophicsystem is a series of experimentalplots near Ibadan,
Nigeria (Kang et al. 1977). The firstyear afterforestcutting,in the no-fertilizerbut-weedingplots, yieldwas 6 tons per ha, the second year 4.5, the third3.5, the
fourth2.9, and the fifth2.7. MidwesternUnited States would qualifyas a eutrophictemperateecosystem, but most of the recent corn yield data are from
fertilizedfields. However, since fertilizerapplication was not importantin the
U.S. beforethe 1940's, if we take average yield data forbefore 1940, it can be
assumed thisrepresentsthe more or less continualoutputcapabilityof an unfertilized temperateeutrophicagriculturalsystem(about 2.5 tons/ha/yr,
Thompson
1975). The experimentalfieldstationof North Carolina State Universityin the
NUTRIENTS IN TROPICAL RAIN FORESTS
173
Amazon Basin of Peru (Sanchez et al. 1975) representsan oligotrophictropical
forest converted to agriculture.There firstyear grain yield in an unfertilized
corn-soybean-rice
croppingsystemwas 2-7 tons/ha.Second yearyieldswere 1-2
tons/haand thirdyear yieldswere less than 1 ton/ha.We could findno grainyield
data fromunfertilizedoligotrophictemperateareas, simplybecause farmersdo
in
not tryto raise corn there,at least withoutfertilizer.The necessityforfertilizer
nutrientpoor areas such as theoutercoastal plainof theeasternUnitedStates has
been known for at least 100 yr (Cook 1868; Coman 1892).
How can the relativelylarge differencesin potential sustained agricultural
productivitybetween oligotrophicand eutrophic systems be explained when
undernaturalconditionsdifferencesin biomass productionare not so large?Table
1, rows 22-24 shows the amountof nutrientsenteringthe ecosystem via rainfall
compared to the amount lost throughsubsurfacesoil water runoff.In the two
(table 1, row 24).
eutrophicecosystems,the differencesare 16.9 and 21.3 kg/ha/yr
In the Brookhaven ecosystem the differenceis only 6.4. In the San Carlos
ecosystem, there is a net gain of calcium.
There are some veryeffectivenutrientcyclingmechanismsin the oligotrophic
forestswhich keep the nutrientcycles tightand preventnutrientloss. What are
some of these nutrientconservingmechanismsthat oftenoccur in oligotrophic
forests?In the followingparagraphswe discuss mechanismsof nutrientcycling
and nutrientconservationin oligotrophicecosystems. This discussion is based
principallyupon the resultsof our ecosystemprojectin San Carlos de Rio Negro,
in the northcentralportionof the Amazon Basin (Medina et al. 1977; Jordanand
Medina 1977).
NUTRIENT
CONSERVING
MECHANISMS
Root Mat
The mostimportantmechanismsfordirectnutrientcyclingand nutrientconservation in the Amazonian Rain Forest are located in the mat of roots and humus
which occurs on the soil surface. At the field station near San Carlos de Rio
Negro, Venezuela, thematvaries between 15 cm and 40 cm in thickness,and 58%
of the small feederroot biomass occurs in this mat (Stark and Spratt 1977). We
observed root mats of similarthicknessin the forestsnear Manaus, and Belem,
in theAmazon regionof Peru
Brazil, and similarmatsexist on soils of low fertility
(P.A. Sanchez, personal communication).However, a surveyof root mat thickness taken along the Rio Negro, Casiquiare, and Orinoco Rivers showed thatthe
root mat decreased in thicknessas one approached the Roraima Shield and the
highlandsof the upper Orinoco. This gradientof decreasing thicknessparallels
the gradientof increasingsoil fertility
predictedby Fittkauet al. (1975a) and the
gradientof increasingsedimentload in streamsobserved by Rice (1921).
One of the most importantnutrientconservingmechanismsin the root mat is
directphysicaladsorptionof the nutrientswhichenterit. Starkand Jordan(1978)
foundthat99.9% of all 45Ca and 32P sprinkledon these root matswas immediately
adsorbed and only 0.1% leached throughthe root mat. When the radioisotopes
174
THE AMERICAN NATURALIST
were applied to the plot in the formof labeled leaves, no detectable activity
leached through(Jordanand Stark 1978). Althoughphysicaladsorptionwas very
rapid,biologicaluptakeand translocationwere considerablyslower. Experiments
using32P as a tracershowed thatmycorrhizalfungiiare one of themechanismsby
which nutrientsare transferredfromdecomposing leaf to root (Herrera et al.
1978b). Phosphorus32 incorporatedinto a leaf which was isolated witha living
root inside a petridish on the forestfloorwas laterfoundin boththe root and the
mycorrhizalconnectionbetweenthe leaf and root. In Herreraet al. (1978a) a set
of photographsshows how a freshlyfallenleaf is initiallycovered withroots and
thenbeginsto decompose so thatafterseveral months,onlya "skeleton" of roots
in the shape of the originalleaf remains. Rapid growthof small roots is another
nutrientconservingmechanism(Jordanand Escalante 1980).
There are undoubtedlymany other mechanismsof nutrientconservationand
transferwithintherootmat. For example,algae in thematcould take up dissolved
nutrientsfromthe throughfall,
and store it untilthe death and decompositionof
the algae. Microorganismsin the root mat of the Amazonian rain forest fix
nitrogen(Herreraand Jordan,in press) whichsooneror laterbecomes available to
the rest of the ecosystem. Conditions in the root mat inhibitpopulations of
bacteria which would cause losses of nitrogen.Gamble et al. (1977) foundthat
bacteria could not be detected in soil samples fromthe San Carlos
denitrifying
forest.Jordanet al. (1979b) showed thatcuttingthe surfacerootmatand applying
calcium did not induce growthof nitrifying
bacteria and subsequent nitrateloss
withthe drainagewater.They attributedthe lack of growthto the maintenanceof
low pH and high concentrationof tanninsby the humus.
Leaves, Bark, and Soil
Evergreen scleromorph leaf types have been interpretedas a successful
ecological strategyin dealingwithextendeddryperiodsduringthe summer.They
also could be an adaptation to low nutrientavailabilitybecause scleromorphic
leaves withtheirrelativelythickcuticle and wax cover and theirrelativelylow
water contentare longer lived and may be more resistantto nutrientloss by
parasitesand herbivores(Grubb 1977). Medina et al. (1978) foundthatscleromorphic leaves were commonin the San Carlos area, and J. Yantko (in prep.) working
in the same forestfoundthathighlyscleromorphicleaves have less insectdamage
than thinner,more mesophyticleaves. Evergreen sclerophyllleaves are very
efficientin the utilizationof nutrients,because nutrientsinvested in leaves are
used for photosynthesisduringprolonged periods of time (Monk 1966; Small
1972). In addition,these leaves are moreresistantto nutrientleachingby rainfall,
and a large proportionof some nutrients,notablyP, N, and K are retranslocated
to the twig before leaf shedding(Small 1972).
The ability of canopy leaves to scavenge nutrientsfromthe rainfallin the
Amazonian forestsresultsin throughfall
witha lower nutrientconcentrationthan
rainfall(Jordanet al. 1979a). The mechanismhere appears to be adsorptionof
nutrientson the surfaceof algae and lichens whichcover the surfacesof manyof
the leaves. While it is not clear thatthe nutrientsscavenged by the microfloraare
immediatelyavailable foruptake by the leaves, certainlywhenthe leaves die and
NUTRIENTS IN TROPICAL RAIN FORESTS
175
decompose, the nutrientsassociated with them then become available to the
roots. Microfloraon the leaves also fixnitrogenwhicheventuallybecomes available to the roots (Herrera and Jordan,in press).
Nortcliffand Thornes (1977) suggestedthat much of the runoffin Amazonian
soils tends to be channelledthroughlarge pores, therebydecreasingthe area of
contact between drainage water and soil surfaces,and lesseningthe leaching.
Dean and Smith(1978) considered whetheror not drip tips on tropicalleaves
cause rainwaterto drainoffthe leaves more rapidly.Their experimentsshowed
that water remainedfor a shortertime on leaves with drip tips. For leaves on
which the water runs off more quickly, there is less opportunityfor leaching
nutrientsout of the leaf by the water. Thus the driptips on the leaves of the San
Carlos forestmay be anothernutrientconservingmechanism.
Because roots mustexpend more energysearchingfornutrientsin less fertile
soils, along a gradientfromrichto poor soils it mustbecome more energetically
economical for a plant to produce herbivore-deterrent
toxic secondary compounds thanto replace leaves consumed by herbivores(Janzen 1974). McKey et
al. (1978) testedthistheoryin two tropicalrainforestsin Africaand foundthat,in
fact,theforestgrowingon the soil lower in nutrientcontenthad thehighercontent
of phenolics. In the San Carlos forest,preliminarydeterminationsalso have
indicatedthatthe phenolic contentof manyof the species is high(Sprick 1979).
A stemflowstudyin the San Carlos forestshowed thattotalamountofnutrients
leached per year was considerablylower than the amountleached per year in a
richerforestin Puerto Rico and approximatelythe same amountleached during
the summergrowingseason of forestsin the temperatezone (Jordan1979). Low
leachingby stemflowin the San Carlos forestmay be due to the insulatingeffect
of therelativelythickbark,whichconstitutesalmost 10% of thetotalweightof the
trunk(Jordanand Uhl 1978). The thickbarkinhibitsdiffusion
of nutrients
out from
the phloem and subsequent loss by stem flow.
Recovery Mechanisms
A nutrientconserving mechanism observed in northeasternU.S. forests is
secondarysuccessional vegetationwhichquicklyinvades a disturbedarea before
much leaching takes place and incorporatesthe nutrientsinto the secondary
successional biomass (Marks and Bormann 1972). A similarphenomenonoccurs
in a cleared area of the Amazon region,ifthe secondarysuccessional vegetation
becomes establishedbeforethe root matdecomposes. Because it takes about 2 yr
forthe root matto decompose, cuttingand burningof the forestdoes not destroy
it. If the area is not continuallycropped, thick stands of Cecropia will be
establishedbeforethe root mat disappears, and, in fact,a new mat begins to be
established over the old decaying one (C. Jordan,in press).
NutrientConservingMechanisms in TemperateRegions
Marbut and Manifold(1925) pointed out that the nutrientpoor soils of some
regionsof the Amazon Basin were quite similarto the soils of the easterncoastal
plain of the United States. Monk (1966) workingin this coastal plain region,
176
THE AMERICAN NATURALIST
suggestedthat"evergreenness" (sclerophylly)of thepines typicalin thisregionis
a nutrient
conservingmechanism.Also, itis well knownthatmycorrhizalfungiare
importantin the nutrientcycling of the coniferous species growingon these
nutrientpoor soils (Wilde 1968). A studyon sandyoutwash soils in northernNew
York State indicates that nutrientconserving mechanisms may also be very
effectivein temperateoligotrophicecosystems. After9 yr,60% of the potassium
applied to a Pinus resinosa standwas stillcyclingwithinthe ecosystem,and after
23 yr40% was stillcycling(Stone and Kszystyniak1977). The site of thisstudyis
similarto a nearbyundisturbedsite in whichhumusand littercoveringthe podsol
sands is about 25 cm deep (McFee and Stone 1965).
AlthoughWoodwell (1979) called the Brookhavenforest"leaky" withregardto
nutrients,it reallyis relativelytightcompared to eutrophicecosystems(table 1,
row 24), especially since the periodic fireswhichburnthe area probablydestroy
the nutrientconservinghumus layer.
I MPLICATIONS
It has been knownfora long timethatbecause of theirlow nutrientcontentthe
upland soils of the Amazon Basin are unsuitedto continuousintensiveagriculture
(Sioli 1973). What has been less clear is why relativelylargeforestscan maintain
themselvesmoreor less indefinitely
in theAmazon Basin, despitethelow fertility
of the soil. We now believe the reason is that oligotrophicforests conserve
nutrientsby the mechanismsdescribed above.
These nutrientconservingmechanismsare the reason thatthe productivity
and
nutrientcyclingvalues fortheundisturbedoligotrophicforestsshownin table 1 do
not differgreatlyfromthe values for the eutrophicforests. The mechanisms
diminishtheproblemof nutrientscarcityin the oligotrophicforests.An important
point, however, about the nutrientconservingmechanismsin the oligotrophic
forestis thattheyare partof the livingorganicstructureof theundisturbedforest
which is destroyedwhen the forestsare cleared foragriculture.That is why the
productivityof these systemsquicklydrops offonce the forestsare destroyed.
For eutrophicecosystems,in contrast,the nutrientconservingmechanismsdo
not play nearlyso large a role. If the eutrophicforestis destroyedby cuttingand
burning,the mineral soil still has the capacity to adsorb most of the released
nutrients,and the systemretainsmuch of its productivecapacity. This is why
Harcombe(1977a, 1977b)concluded thatnutrients
were notlimitingto production
afterclearing in a Costa Rican ecosystem. Many Costa Rican ecosystems, includingHarcombe's, are eutrophicbecause theirsoils are derivedfromvolcanic
rock rich in nutrients.
Do these same principlesregardingeutrophicand oligotrophicforestsapply to
temperateregionsas well as tropicalregions?Monk (1966) postulatedthatoligotrophictemperateforestsconservenutrients.However, he did notemphasize that
the nutrientconservationoccurs principallyin the thick humus layer of these
forests.We believe thatour results,gainedfromexperimentsand observationson
the root and humuslayerof the oligotrophicecosystemof the Amazon Basin, are
equally applicable to theoligotrophicforestsofthetemperatezone. As longas this
NUTRIENTS IN TROPICAL RAIN FORESTS
177
humus and root layer is maintainedin the temperateoligotrophicforests,tree
growthcan continue. But when the forestis cut for agriculture,nutrientsare
quicklylost because the humus is destroyed,and the mineralsoil does not have
the capacity to retainthe leached nutrientsor to supply new nutrients.
CONCLUSION
Are nutrientsreallycriticalin the tropicalrain forest?Yes, theyare in ecosystems on ancient highlyleached soils derived fromprecambrianrock or nutrient
poor sand deposits. However, in eutrophicecosystems, such as those covering
much of the Andes, where mountainbuildinghas resultedin unweatheredparent
rock materialswhich are relativelyshallow and rich in essential elements,nutrients are not critical.Oligotrophicforestsoccupy most of the easternand central
portions of the Amazon Basin (Fittkau et al. 1975b), parts of the Far East
(Richards 1952; Whitmore1975; Janzen 1974), and evern some areas in Africa
(McKey et al. 1978). In these areas nutrientsare likely to be limitingto plant
growthonce the naturalforestsare destroyedand agriculturalor forestryplantations are attempted.
SUMMARY
Nutrientpoor forestsare more common in the tropicsthan in the temperate
regions,but most of the work on nutrientcyclingin the tropicshas been carried
out on nutrientrichsites such as those in PuertoRico and Costa Rica ratherthan
nutrientpoor sites such as those in centraland easternAmazonia. Consequently,
therehas been confusionas to whethernutrientsare criticalin tropicalforests.
Productivityand nutrientcyclingin nutrientrichand nutrientpoor forestecosystems do not differgreatlyas long as the ecosystems are undisturbed.However,
when the forestsare cleared foragriculturalpurposes, the nutrientpoor systems
quicklylose theirproductivepotential,whereas the nutrientrich systemsdo not.
The nutrient
poor ecosystemis able to maintainitsproductivity
underundisturbed
conditionsthrougha varietyof nutrientconservingmechanisms,the mostimportantof whichare associated withthe humusand root layer on top of the mineral
soil. These mechanismsare destroyedwhenthe nutrientpoor forestis cleared for
agriculturalpurposes.
ACKNOWLEDGMENTS
This paper was preparedwhilethe authorswere receivinggrantsfromthe U.S.
National Science Foundation, UNESCO, CONICIT de Venezuela, and the Organizationof American States.
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