hydrometeorologic, pedologic and vegetation patterns along an

• Tropical montane rainforest – Climatic and pedologic altitudinal change – Bolivia

DIE ERDE 139 2008 (1-2) Special Issue: Fog Research pp. 141-168

Hydrometeorologic, Pedologic and Vegetation Patterns along anElevational Transect in the Montane Forest of the Bolivian Yungas

Gerhard Gerold (Göttingen), Marcus Schawe (Hamburg) and Kerstin Bach (Marburg)

The “Mountain Agenda” of the World Summit on Sustainable Development 2002 emphasised theglobal relevance of montane cloud forests for important ecosystem services like water resources andbiodiversity hot spots. Serious concern about the fate of tropical mountain forests has recentlytriggered intensified research on the ecological complexity of these forests. However, in Latin Americaresearch was focused on the Caribbean, Costa Rica and Ecuador (DFG Research Unit 816), whereasstudies in the Andes of Peru and Bolivia were missing. In the framework of an interdisciplinaryproject aiming at understanding the relationships between vegetation and abiotic factors in the mon-tane forest belts of the humid Yungas of Bolivia, hydrometeorologic observations and research on thealtitudinal change of soils along an elevational gradient were carried out. Results suggest that the floristicchange of vegetation belts and the differences in forest stature are influenced by complex interactions ofclimatic and pedologic variables along an elevational transect from 1,700 to 3,400 m a.s.l.

Hypsometrischer Klima-, Boden- und Vegetationswandelim Bergregenwald der bolivianischen Yungas

With 7 Figures, 4 Tables and 5 Photos

1. Introduction

Among the manifold ecoregions of Bolivia and theeastern Andes the Yungas cover the altitudinal zonesof tierra templada and tierra fria including ‘lowermontane forest’ (LMF), ‘upper montane cloud for-est’ (UMCF) and ‘subalpine cloud forest’ (SCF). Thisregion is located north of 18°S and is one of the species-richest regions of the world, i.e. it is a ‘hot spot’ ofbiodiversity (Barthlott et al. 1996, Myers et al. 2000).

An estimated 10,000 vascular plant species can bedistinguished in the Yungas regions of which 7,000are known. This is 50 % of all plant species of Boli-via (Bach 2004, Kessler and Beck 2001). Severalgroups have a maximum of diversity in the cloud for-ests of the Yunga region (e.g., according to Bach2004, Orchidaceae ca. 1,170 species, Melastomata-ceae ca. 100 species). Beck (1988) defines an altitu-dinal range of 1,200 m a.s.l. to 3,400 m a.s.l. as thecore zone of the Yungas forest, including the vege-

142 Gerhard Gerold, Marcus Schawe and Kerstin Bach DIE ERDE

Altitudinal belt LMF UMCF I UMCF II SCF

Altitude (m a.s.l.)

< 2,100 2,100-2,600 2,600-3,150 3,150-3,400

Substrate and texture (B horizon)

Meta-siltstone Lt 2

Meta-siltstone Lt 2

Meta-sandstone Sl 3

Meta-sandstone Sl 2

Mean canopy height (m)

20 18 13 5-10

Typical plant species

Elaphoglossum yungense, Miconia

staphidioides

Hymenophyllum verecundum miconia sp.

Terpsichore semihirsuta,

Ceradenia comosa

Elaphglossum squamipes

Soil type (USDA 1998)

Humic Dystrudept Typic Placaquod Typic Durorthod Typic Placaquod

pH (Ah) 0,01 M CaCl2

4.0 3.4 3.0 2.6

C/N (Ah) 12 12 25 28

ECEC (Ah) (cmol/kg)

6.4 1.5 2.8 2.7

Rainfall (mm/y) 2,310

(altitude: 1,850 m) 3,970

(altitude: 2,600 m) 5,150

(altitude: 3,050 m)

Temperature (yearly average, °C)

16.8 12.8 10.0

T-amplitude (daily average, °C)

9.8 6.6 5.9

Humidity (rF) (yearly average, %)

90.1 96.5 97.5

rF-amplitude (daily average, %)

15.0 9.2 5.1

tation zones of ‘lower montane forest’ (LMF), ‘up-per montane cloud forest’ (UMCF) and ‘subalpinecloud forest’ (SCF) (Bruijnzeel and Hamilton 2000).

Along the altitudinal gradient, the general differ-ences of the mountain forests in contrast to the

evergreen lowland rainforest (tierra caliente)are: declining tree stand heights, emergents arelacking in the SCF, cauliflory is rare, microphyl-lic leaves are found in UMCF, and there is a sig-nificant increase of sklerophylly in SCF (Bach2004, Fig. 3.6) and a high amount of epiphytes

Tab. 1 Climate-vegetation-soil characteristics in the Yungas (Cotapata)Klima-Vegetation-Boden-Charakteristika in den Yungas (Cotapata)

2008/1-2 Hydrometeorologic, Pedologic and Vegetation Patterns in the Bolivian Yungas 143

(maximum in UMCF; Tab. 1). These vegetationchanges are expressed in the occurrence of thedifferent elevational forest types LMF, UMCF,SCF (Kappelle and Brown 2001). The timber lineis reached at approximately 3,400 m a.s.l. in theresearch area of Cotapata National Park but var-ies highly in the Yungas region with exposition,relief (i.e. slope), humidity and anthropogenic in-fluence (maximally up to 3,600 m a.s.l., Beck 1998).The typical diurnal climate of the tropics changeswith altitude as follows: declining barometric pres-sure and air temperature, reduced evapotranspi-ration and increasing intensity of UV-B radiation,higher rates of cloudiness, more fog and cloudprecipitation (Hamilton 1995, Bruijnzeel 2005).The general sequence of the climate, vegetationand land use zones is known for the seasonalhumid tropical eastern Andes (Gerold et al. 2003),but data on detailed hygric altitudinal gradients(gradients of precipitation and complete water bal-ances) accompanied by gradients of soil andplant diversity are missing so far.

The exact factors underlying these altitudinalchanges are still a matter of debate. According tosome scientists, reductions in radiation due to fogand low clouds, coupled with decreases in tem-perature and increases in atmospheric humidity,may limit photosynthetic activity, transpirationand nutrient uptake with elevation (as reviewedby Grubb 1977; Bruijnzeel and Veneklaas 1998).Others have drawn attention to the fact that insome places tree height and leaf area show aninverse relationship with the degree of soil watersaturation (e.g. Hetsch and Hoheisel 1976;Santiago et al. 2000). Thus, unfavourable soilconditions caused by high precipitation excess-es over evaporation may constitute an importantfactor determining forest stature in wet tropicalmountains. However, these and other climatic in-fluences on forest stature have only rarely beendocumented, mostly due to the difficulty in es-tablishing and operating meteorologic and soilwater stations under the adverse topographic andlogistic conditions characterising so many cloud

forest sites (Herrmann 1971; Cavelier and Mejia1990; Bruijnzeel et al. 1993; Pendry and Proctor1996; Hafkenscheid 2000; Holwerda 2005).

Published studies on soil genesis in tropicalmontane forests elucidate stagnic and podzolicproperties as dominant soil forming processes(Schawe et al. 2007). Hetsch and Hoheisel (1976)concluded that hydromorphic processes aredominant for soil genesis and classified thesesoils as Spodic Dystropepts in montane forestsof Venezuela. Working in the Ecuadorian Andes,Schrumpf et al. (2001) report increasingly aquicconditions and placic horizons with increasingelevation. However, at the highest elevations ofaround 3,050 m a.s.l., shallow and less developedsoils prevail. Thus, knowledge about soil gene-sis in tropical montane forests is patchy, infor-mation on soil forming substrate is not alwaysavailable, and no comprehensive soil survey hasbeen conducted (Roman and Scatena 2007).

The objectives in this article are to describe thechanges in climate and soils with elevation in theYungas region of Bolivia and to evaluate their in-fluence on forest stature along the elevationalgradient. Research questions include:

1) How do soil and climate parameters changewith altitude?

2) Which interrelations can be detected be-tween vegetation units and abiotic factors?

The discussion is focused to unravel the key driv-ers for the decrease of tree height and the shiftof vegetation units with elevation.

2. Research Area and Methods

2.1 Research area

The present study was carried out along a transecton a slope facing southeast in the Bolivian National


riods in winter (July, August). The transect extend-ed from 1,700 to 3,400 m a.s.l. and encompassed theentire vertical range of tropical montane forest typesin the ‘Yungas of La Paz’ region (Bach et al. 2003;Tab. 1). The longitudinal slope profile was mostlylinear and had an average inclination of 25° to 30°

Park of Cotapata, in the northeastern part of theBolivian Andes and ca. 80 km north of La Paz(16° 09' S, 68° 55' W; Fig. 1; Photo 1). The easternAndes are part of the seasonal humid tropical zone(Aw in Köppen’s classification), with a maximumof precipitation during summer and short drier pe-

Fig. 1 Study area in the Cotapata National Park (16° 09' S, 68° 55' W). Transects start at 1,750 m a.s.l.,Cerro Hornuni 3,647 m a.s.l. Altitudinal belts: green = 1,100-1,700 m; light brown = > 1,700- 3,100;dark brown = > 3,100 m; shading from W to E / Untersuchungsgebiet im Nationalpark Cotapata(16° 09' S, 68° 55' W). Aufnahmetransekte beginnen in 1.750 m ü.M., Cerro Hornuni 3.647 m ü.M.Höhenschichten: grün = 1.100-1.700 m; hellbraun = > 1.700 -3.100 m; dunkelbraun = > 3.100 m;Schummerung von West nach Ost


although the slope angle in some areas exceeded40°. The geologic substrate consists of Ordovicianmeta-siltstones, slates and meta-sandstones(Mapa geologico de Coroico, Hoja 6045).

Soils along the transect are characterised by highacidity, low cation exchange capacity, highaluminum-saturation in the mineral horizons, anda depth of the organic layer of between 15 and45 cm (in LMF 15-35 cm, in UMCF 25-40 cm, inSCF 20-35 cm, Schawe 2005, Schawe et al. 2007).

Based on vegetation structure and floristic compo-sition of species, three vegetation formations wererecognised (Bach 2004): Lower Montane Forest

(LMF), Upper Montane Cloud Forest (UMCF), andSub-alpine Cloud Forest (SCF). LMF extends up to2,150 m and has a dense, ca. 20 m high canopy andemergent trees of up to 35 m (Photo 2). Myrsine co-riacea (Sw.) R.Br. ex Roem. and Schult. is the domi-nant tree, while the fern Blechnum ensiforme Liebm.and the aroid Philodendron ornatum Schott prevailin the understorey. At around 2,150 m there is a dis-tinct transition to UMCF. This forest type is subjectto persistent cloud incidence and dominated byPodocarpus oleifolius D. Don ex Lamb., Weinman-nia crassiflora Ruíz and Pavón, and Clusia multi-flora Kunth. The canopy of the UMCF is closed andlower than that of the LMF, usually reaching up to15 m (Photo 3). The abundance of epiphytes, most-

Photo 1 Cordillera Oriental withYungas (view from Coroicoto Cotapata National Parkwith forest line at 3,400 ma.s.l.; photo: Kellner 2005)Blick auf die Ostkordilleremit den Yungas (von Coroicozum Cotapata-National-park mit der Waldgrenze in3.400 m ü. M.; Photo: Kell-ner 2005)


ly bryophytes (mosses, liverworts), in the UMCF isnoteworthy; the bryophyte cover of the soil surfacewas estimated at about 45 %. Above 2,800 m UMCFgives way to SCF (Photo 4). Its dominant tree spe-cies Myrsine coriacea and Podocarpus rusbyiJ.Buchholz and N.E.Gray reach heights of around10 m and do not form a closed canopy. Moss coveron the soil surface increases to 75 % and is domi-nated by Sphagnum species. Scleromorphic shrubssuch as Miconia sp., Gaultheria pernettyoides, Gy-noxys ssp., Ilex ssp. and Escallonia myrtilloides L.f.are abundant. Among the Melastomataceae, theproportion of scleromorphic species increases from

11 % in the UMCF (2,300 m) to over 50 % in the SCF(3,050 m). In all of the analysed plant groups (ferns,melastoms, aroids, bromeliads, palms, cacti) á-diver-sity (within habitat diversity: number of plant spe-cies within one plant community) decreases signif-icantly with elevation. The highest overall diversitywas found at 1,900 m.a.s.l. (Bach 2004).

2.2 Methods

Meteorologic measurements were carried out dur-ing the period from October 2001 until April 2004

Photo 2 Lower montane forest (LMF) at1,900 m a.s.l., three tree strata withdense canopy in 20 m, emergenttrees up to 35 m, tree species ofgenera Hedyosmum, Inga, Ficus(photo), highest á -diversity in2000 m (photo: Bach 2002)Montaner Bergregenwald in1.900 m ü. M., drei Baumschich-ten mit geschlossenem Kronendachin 20 m, Überhälter bis 35 m,Baumarten der Gattungen Hedyos-mum, Inga, Ficus (Photo), höchsteá-Diversität in 2000 m (Photo:Bach 2002)


with the end of field work. Because of dataloggerproblems in 2003 the complete meteorologicaldataset from October 2001 to October 2002 wasselected for statistical analysis. Hydrometeoro-logical stations were set up in forest clearings ineach of the three vegetation formations, viz. at1,850 m (LMF), 2,600 m (UMCF) and 3,050 m(SCF). The forest clearings were restricted to2,500 m2 (LMF) and 500 m2 (UMCF, SCF) becauseof national park rules and relief situation. Becauseof forest clearing size in UMCF and SCF the influ-ence of reduced wind speed and restriction ofhorizon at sunrise and sunset occurs. Rainfall (mm)

was measured at 1 m above ground level with atipping-bucket recording gauge (ARG 100, EM;UMS, Munich) with 0.2 mm resolution per tip.Short-wave radiation (W m-2) was measured with apyranometer (8101, Schenk; UMS, Munich), netradiation (W m-2) with a net radiometer (NR-lite,Kipp and Zonen; UMS, Munich), and photosyn-thetic photon flux density (PPFD, wavelength400-700 nm, µmol m-2 s-1) with a quantum sensor(DK-PHAR, Deka; UMS, Munich). All radiationsensors were placed at 2 m height and on supportarms extending 1.5 m from the mast to avoid shad-ing of the instruments. The potential global radia-

Photo 3 Upper montane cloud forest(UMCF) at 2,600 m a.s.l., densecanopy in 15 m, emergent trees upto 25 m, maximum of tree Epiphy-tes with 50-70 % coverage, treespecies Podocarpus oleifolius(photo), Weinmannia crassiflora,Clusia multiflora (photo: Bach 2002)Hochmontaner Bergregenwald in2.600 m ü.M., dichtes Kronendachin 15 m, Überhälter bis 25 m, Baum-epiphytenmaximum mit 50-70 %Deckung, Baumarten Podocarpusoleifolius (photo), Weinmanniacrassiflora, Clusia multiflora,(Photo: Bach 2002)


tion was estimated according to Böhner et al.(1997) using a digital elevation model (exposition,slope angle, relief position with shading) and themean air turbidity of the individual site. Air tem-perature (°C) and relative humidity (%) weremeasured at 2 m height with the sensors placedwithin a Gill-type radiation shield (HP100-A,UMS, Munich). Measured relative humidity val-ues in excess of 100 % were set to 100 %. Windspeed (m s-1) (A100 anemometer, Vector Instru-ments; UMS, Munich) and wind direction (W200Ppotentiometer wind vane, Vector Instruments;UMS, Munich) were also recorded at 2 m height,i.e. well below the level of the surrounding cano-py. It is recognised that the recorded wind speeds

represent underestimates compared to the condi-tions at canopy height where fog droplets are in-tercepted. Data were sampled at 10 min intervalsexcept for wind speed which was sampled everyminute. Data were processed by a Delta T data-log-ger system. Averages and their standard deviationswere calculated over 30 min periods. Reference evap-oration (potential evapotranspiration, PET) was cal-culated from radiation, temperature, humidity andwind speed data using the modified Penman equa-tion advanced by Doorenbos and Pruitt (1988) ac-cording to FAO (refers to well-watered grassland).

Interception loss in the three vegetation belts wasmeasured in 2005 during a three-month period (April-

Photo 4 Subalpine cloud forest (SCF) at 3,200 m a.s.l., increasing canopy openness with tree stratum upto 10 m (Podocarpus rusbyi on the left), forest line at 3,100 m, scleromorphic bushes, e.g. Miconiasp., as opposite slope: forest degraded to secondary bush and grassland (photo: Bach 2002)Subalpiner Bergregenwald in 3.200 m ü. M., Auflichtung der Baumschicht mit Höhen bis 10 m(Podocarpus rusbyi, links), Waldgrenze ca. 3.100 m, scleromorphe Sträucher wie Miconia sp.,Gegenhang mit degradiertem Sekundärbusch und Grasland (Photo: Bach 2002)


June) coinciding with the transition from rainy to dryseason (10 roved gauges, weekly on 500 m2 plots).

Volumetric water content of the mineral soil wasmeasured with two frequency domain reflectometrysensors (FDR, M12-IMCO) placed adjacent to eachof the meteorologic stations. Soil water content wasdetermined every 10 min in the mineral soil layer at10 cm, 20 cm, 30 cm and 50 cm. Water content of theorganic layer was measured daily for a period ofthree weeks, between 24 September and 12 Octo-ber 2002 at 200 m elevation intervals between 1,800 mand 3,000 m. During the measurement period(19 days) rainfall occured on 17 days. At least threedisturbed (non-volumetric) samples were taken at

10 cm depth per sampling site, weighed, dried at 105°Cto constant weight and reweighed. Gravimetric watercontents were expressed in grams of water per gramof oven-dried soil (g g-1).

Three soil transects were selected: a long verti-cal transect from 1,700 m to 3,400 m a.s.l. on anextended ridge and two parallel short transectsfrom 1,800 to 2,600 m, also located on extendedridges (Fig. 1). We augered soils and excavatedsoil pits in 100 m intervals in the long transect andin 200 m intervals in the two parallel shorttransects down to a depth of 1m or lithic contact.This sampling scheme resulted in 26 profiles(Fig. 1). We determined soil colours according

Fig. 2 Soil types (USDA) along the altitudinal transect CotapataBodentypen (USDA) im Höhentransekt Cotapata

Typic Placaquod

Humic Dystrudept

Histic Humaquept

Typic Placaquod

Oi+Oe+OaA horizonB horizonC horizonSkeletonPlacic horizon

Typic Durorthod

Spodic Dystrudept100 cm

UMCF

SCF

A

Bw1

Bw2

Oa

Ah

Oh Ah

Ah Bv

Bv

A

Bw1

Bw2

Oa

Ah

Oh Ah

Ah Bv

Bv

AAg

Cr

Oa

Bv

Oh

AehSw-AeBh/Bs

Sd-Cv

AAg

Cr

Oa

Bv

Oh

AehSw-AeBh/Bs

Sd-Cv

AAg

BCr

Oa

Bg

OhAeBh/Bs

Cv

AAg

BCr

Oa

Bg

OhAeBh/Bs

Cv

AgBg

BCr

Oa OhAeSw-Bs

Sd-Cv

AgBg

BCr

Oa OhAeSw-Bs

Sd-Cv

A

AgBv

Oa

Bg

Oh

Sw Aa

Ae

Sw Bh

Sd Bbms

Sew

A

AgBv

Oa

Bg

Oh

Sw Aa

Ae

Sw Bh

Sd Bbms

Sew

AAg

Cr

Oa OhAhAe

Cv

AAg

Cr

Oa OhAhAe

CvLMF


to the Munsell Soil Colour Chart, sampled the soilpits in each horizon, air-dried the samples in Bo-livia and transported the samples to Goettingen,where they were sieved and analysed.

Following the collection of field and laboratorydata on the augered and analysed profiles, weidentified three characteristic soil zones depend-ing on altitude and selected soil profiles that re-present each soil zone (Fig. 2). The criteria forthe selection of the representative soil profileswere texture, colour and pH. Soils were classi-fied following the Soil Survey Staff (2003). Meth-ods of pedochemical (total nutrient content, nu-trient availability, Ct and Nt) and pedophysicalanalyses (bulk densities) are described in de-tail in Schawe et al. (2007).

Vegetation was recorded along the identicaltransects in homogeneous plots, in regular dis-tances of 50 metres of altitude. For the excep-tionally species-rich zone between 1,900 m a.s.l.and 2,400 m a.s.l. the vegetation was assessedevery 25 metres of altitude and every 100 me-tres of altitude two to three plot replicates werestudied. Transversal transects were installed at1,850 m a.s.l., 2,550 m a.s.l and 3,000 m a.s.l. Al-together 105 plots of 400 m2 each were studiedby Bach (2004). From each plot structural para-meters (tree height, canopy and understoreyheight) and the percentage of coverage for spe-cific plant indicator groups (Araceae, Arecace-ae, Bromeliaceae, Cactaceae, Melastomatace-ae, Pteridophyta) and cover intensity of epi-phytes and soil cover of mosses were determined

Climate variable LMF

1,850 m

UMCF

2,600 m

SCF

3,050 m

Mean daily short wave input Rs (MJ m-2 ) 15.5 ± 5.4 10.4 ± 4.3 9.9 ± 3.8

Rs range (MJ m-2 ) 4.6 – 29.8 3.3 – 28.1 3.1 – 24.8

Average daily photosynthetic photon flux

density PPFD ( mol m-2 s-1) 390 266 252

Average daily maximum PPFD ( mol m-2 s-1) 1,500 910 790

Average daily maximum PPFD for full cloudy

days ( mol m-2 s-1) 505 410 405

Average daily T (oC) 16.8 ± 1.7 12.8 ±1.6 10.0 ± 1.5

Daily amplitude of T (oC) 9.8 6.6 5.9

Average minimum and range of T (oC) 13.3

(7.7-17.2)

10.2

(5.7-14.0)

7.4

(2.1-10.8)

Average maximum and range of T (oC) 22.8

(14.8-29.0)

16.8

(8.8-23.8)

13.3

(6.9-18.5)

Tab. 2 Radiation and temperature at different altitudes (Oct. 2001 to Oct. 2002; ± SD)Strahlung und Temperatur in den verschiedenen Höhenstufen (Okt. 2001 bis Okt. 2002; ± SD)


according to Braun-Blanquet (1964) and vander Hammen et al. (1989). Details on plant iden-tification in collaboration with Herbario Nacionalde Bolivia and botanical and statistical analyses(detrended correspondence analysis, clusteranalysis, parsimony analysis, principal compo-nent analysis) can be found in Bach (2004).

3. Results

3.1 Hydrometeorological characteristics

The montane rain forest, which we examined, hasa generally high rate of incoming radiation givenits latitude, altitude and SE exposition, while thisrate clearly declines from LMF to UMCF and SCF(Tab. 2). The UMCF at 2,600 m.a.s.l. receivesabout 33 % less short-wave radiation (R-s) thanthe LMF at 1,850 m whereas the SCF at 3,050 mreceives ca. 5 % less than the UMCF (Tab. 2,Fig. 3). The degree of reduction in Rs due to the

presence of fog and clouds in the UMCF and SCFwas estimated from a comparison with potentialradiation values as calculated after Boehner et al.(1997). The average inferred reduction in Rs is37 % of the calculated potential value at the LMFsite, 58 % at the UMCF site, and 62 % at the SCFsite (Fig. 4). Thus, radiation reduction is mostpronounced in the SCF where it exceeds 50 % on306 out of the 365 days of observations andreaches as much as 90 % during a single four-dayperiod within one year. Net radiation (Rn) showssimilar patterns to the incoming radiation withmonthly minima from May to August and maximafrom September to November. Mean daily netradiation (Rn, 6-18 h) is 63 % of Rs in LMF anddecreases to 50 % of Rs in SCF due to reducedincoming radiation und increasing outgoingradiation during nights. During clear days, maximaof Rn at noon of between 18 MJ m-2 h (SCF) and41 MJ m-2 h (LMF) are reached, whereas forpersistently cloudy days 2 MJ m-2 h (SCF) and5 MJ m-2 h (LMF) are characteristic (Schawe 2005).

Fig. 3 Averaged daily shortwave radiation (Rs, %) as a percentage of potential global radiation ataltitudes of 1,800 m (square line), 2,600 m (triangle line) and 3,050 m (dotted line) with standarddeviation s / Mittlere tägliche Globalstrahlung (Rs, in %) als relativer Anteil an der potenziellenGlobalstrahlung für die Höhenlagen 1.800 m (Quadrate), 2.600 m (Dreiecke) und 3.050 m(Punkte) mit Standardabweichung s

0

10

20

30

40

50

60

70

80

90

100

Jan Feb Mar April May Jun July Aug Sept Oct Nov Dez

Red

uctio

nof

shor

twav

era

diat

ion

Rs

[%]

Dec


Fig. 4 Average annual pattern of radiation flux density (RFD, W m-2 ) with actual short wave radiation(thin line) and net radiation (bold line), actual photosynthetic photon flux density (PPFD,µmol m-2 s-1); air temperature (°C), and Penman-Monteith evapotranspiration (PET, mm d-1)between 1 Oct. 2001 and 1 Oct. 2002 in LMF, UMCF and SCF. Vertical lines represent thestandard deviation. / Mittlerer Jahresgang der Strahlungskomponenten (RFD, W m-2 ) mitGlobalstrahlung (normal) und Strahlungsbilanz (fette Linie), der photosynthetischen Photonen-flussdichte (PPFD, µmol m-2 s-1), Lufttemperatur ( °C) und Penman-Monteith-Evapotranspiration(PET, mm d-1) zwischen 1. Okt. 2001 und 1. Okt. 2002 im montanen, hochmontanen undsubalpinen Bergregenwald. Vertikale Linien zeigen die Standardabweichung.


The daily amplitude in temperature decreaseswith decreasing net radiation and increasingcloudiness from LMF to UMCF to SCF, where-as the small yearly amplitudes in temperaturesremain (Tab. 1). Photosynthetic photon fluxdensity (PPFD) also decreases with altitude(Tab. 1 and Fig. 5). Average daily PPFD decreas-es by 37 % from the LMF to the UMCF zone, andby a further 7 % in the SCF. In the LMF, PPFDexceeds 1,000 µmol m-2 s-1 on average for4.5 hours per day with a maximum (on average1,470 µmol m-2 s-1) occurring around noon; cor-responding durations in the UMCF and SCFwere 4 and 3.5 hours, with average maximum val-ues (958 and 805 µmol m-2 s-1) occurring ataround 11:00 h and 11:30 h, respectively.

Mean daily maximum, minimum and averagetemperatures at the three sites are given inTable 2. Although the overall decline in tem-perature between the LMF and SCF of 0.59°Cper 100 m is close to the wet-adiabatic lapserate (about 0.56°C per 100 m; Lauer 1975), re-corded lapse rates between the vegetationbelts are quite similar (0.53°C per 100 m be-tween LMF and UMCF and 0.62°C per 100 mwithin the cloud belt). The diurnal range in tem-perature decreases with increasing altitude(Tab. 2). Average relative humidity increasesslightly from 90 % in the LMF to 96 % in theUMCF and 97 % in the SCF whereas the aver-age diurnal range decreases with altitude, from30 % at 1,850 m to 5 % at 3,050 m. Additional

Fig. 5 Average soil water content of the organic layer with increasing altitude and standard deviationMittlerer Bodenwassergehalt der organischen Auflage mit zunehmender Höhe und Standardabweichung

altitude [m a.s.l.]

1600 1800 2000 2200 2400 2600 2800 3000 3200

soil

moi

stur

e [g

g-1

]

2

3

4

5

6

7

8

Altitude [m a.s.l.]

Soil

moi

stur

e [g

g-1]


measurements within the forest at three heights(0.5 m, 2.0 m, 15.0 m, Pareja 2005) and with 200 mdifference of altitude show a distinct increaseof daily average minimum relative humidity from87 % in the LMF to 95 % in the UMCF for the2 m height. The closed forest canopy and thehigh degree of cloudiness cause a maximum ofrelative humidity at altitudes between 2,200 and2,600 m a.s.l. for the entire year.

Yearly rainfall distribution can be divided into adryer season (May-September) and the mainrainy season (October-April). Annual rainfall (P)is high and increases considerably with altitude(Tab. 3). The UMCF site receives 1.7 times morerainfall than the LMF site whereas P at the SCFsite is 2.2 times that recorded at the LMF. The

number of days with < 1 mm/day of rainfall de-creases from 72 % in the LMF to 60 % in the SCFbut rainfall events of 2-10 mm increase from 15 %to 23 % with increasing elevation. The length ofcontinuously dry periods (mostly during the dryseason) decreases from 9 days at 1,850 m to7 days at 2,600 m and to 5 days at 3,050 m.

Daily reference evaporation (PET) values at theUMCF and SCF sites are very similar with 1.1-1.3 mmand almost three times lower than those at theLMF site, although ranges in daily values aresubstantial at all three sites (Tab. 3). Averagehourly PET is 0.14 mm in the LMF but only0.05 mm in the UMCF and the SCF. Rainfall ex-ceeds PET 1.9 times in the LMF, vs. 8.6 times inthe UMCF and 13.2 times in the SCF (Tab. 3).

Fig. 6 Soil moisture in the lower montane forest (1,800 m a.s.l.) and the upper montane cloud forest(2,600 m a.s.l.) in 2002 (field capacity at 48 %; permanent wilking point at 18 %)Bodenwassergehalt im montanen Bergregenwald (1.800 m ü.M.) und hochmontanen Bergregen-wald (2.600 m ü. M.) in 2002 (Feldkapazität bei 48 %; permanenter Welkepunkt bei 18 %)

20.0

25.0

30.0

35.0

40.0

45.0

50.0

55.0

1.10

.200

1

1.11

.200

1

1.12

.200

1

1.1.

2002

1.2.

2002

1.3.

2002

1.4.

2002

1.5.

2002

1.6.

2002

1.7.

2002

1.8.

2002

1.9.

2002

Date

Vol.

%

LMF 10 cm LMF 20 cm LMF 30 cm UMCF 10 cm UMCF 20 cm UMCF 30 cm

Soil

moi

stur

e, (

Vol.

%)


Climate variable LMF

1,850 m UMCF 2,600 m

SCF 3,050 m

Rainfall (mm yr-1) 2,310 3,970 5,150

Interception Ei (mm yr-1) 494 1,040 1,308

Annual evapotranspiration pET (mm)

1,190 462 403

Daily pET (mm) 3.3 1.3 1.1

Range daily pET (mm) 0.3-8.6 0.07-6.4 0.04-5.7

Gravimetric soil moisture content in the organic layerincreases (although somewhat irregularly) with ele-vation, from 2.5 g g-1 at 1,800 m to 6.3 g g-1 at 3,000 m(Fig. 5). The main increase is from 1,800 to 2,000/2,100 m (0.5 g g-1 per 100 m elevation interval), withthe transition zone from the LMF to the UMCF leastaffected by fog and low clouds and from 2,200 m to2,600/2,700 m (0.5 g g-1 per 100 m) with the beginningof the transition zone from UMCF to the SCF at 2,700/2,800 m a.s.l., within the cloud belt (Bach 2004).

The FDR soil water sensors in the mineral soil alsorecorded significant differences between the LMFand the two upper vegetation formations. In theupper soil horizon (Ah, 0-20 cm) soil moisture re-mained at field capacity (48 Vol. %) during the mainrainy season from December 2001 until May 2002and changed in relation to daily rainfall events be-tween 30 and 45 Vol. % (PWP at 18 Vol. %) in theother months in the LMF. The seasonal distributionof moisture content in the mineral soil in both theUMCF and the SCF was relatively constant. The soilin the SCF was saturated throughout the year, where-as in the UMCF the soil was saturated only duringthe rainy season (December 2001 until May 2002)but remained at field capacity at other times (Fig. 6).

3.2 Forest types

Species-richness of all groups declines with altitudeand Pteridophyta always represent the group withthe largest number of species. The maximum ofá-diversity is found for all groups in LMF, below2100 m a.s.l. A significant change in species compo-sition for all groups was rarely found (Fig. 6.1 in Bach2004). The best synchronicity of a change in spe-cies for all groups, i.e. a significant altitudinal bor-derline, was found at 2,000 m a.s.l. (± 150 m). There isa large transitional zone in UMCF with several alti-tudes with significant changes in various plantgroups, from 2,700 m a.s.l. to 3,100 m a.s.l. Due toincreasing canopy openness beginning at 3,100 ma.s.l., a significant increase in epiphytes, Pterido-phyta and Bromeliaceae can be found. Therefore,three altitudinal forest belts may be differentiated,similarly to Grubb (1974): LMF (< ± 2,000 m), UMCF(2,150-2,700 m) and SCF (above 3,000 m).

Apart from the floristic analyses which formedthe basis for the classification into the three al-titudinal forest belts, structural changes alongthe altitudinal gradients were found for the mon-tane rain forest. These comprise 1) stratification,

Tab. 3 Rainfall and evapotranspiration in different altitudes (Oct. 2001 to Oct. 2002)Niederschlag und Evapotranspiration in den verschiedenen Höhenstufen (Okt. 2001 bis Okt. 2002)


O horizon

Elevation [ m a.s.l. ]

1500 2000 2500 3000 3500

C [

t h

a-1

]

0

50

100

150

200

250

300

350

A horizon


1500 2000 2500 3000 3500

C [

t h

a-1

]

0

50

100

150

200

250

300

350

B and C horizon


1500 2000 2500 3000 3500

C [

t ha

-1 ]

0

50

100

150

200

250

300

350

C stocks total


1500 2000 2500 3000 3500

C [

t h

a-1

]

0

100

200

300

400

500

600

2) crown height for trees and understorey vege-tation, 3) cover intensity of epiphytes on the treebranches, and 4) the relationship between leaf lit-ter and moss cover at the soil surface (Bach 2004).

3.3 Soil types

In the lower montane rain forest, (1,700 to2,200 m a.s.l.), silty, brown soils with shallow ector-

ganic horizons (10 to 30 cm) dominate. The repre-sentative soil type for this zone is a Humic Dys-trudept (Fig. 2), located at 1,850 m a.s.l. The weaklydeveloped Inceptisols give way to more strongly de-veloped soils in the UMCF (2,200 to 2,700 m a.s.l.).The dominant soils in this zone differ from the soilsin the lower montane rain forest (LMF) by a thickerectorganic horizon (30 to 40 cm) and the presence ofa grey E horizon and a Bhs horizon (placic horizon).The soil profile that best represents this zone is a

Fig. 7 Carbon stocks (t/ha) in different soil horizons along the altitudinal transectC-Vorräte (t/ha) in den verschiedenen Bodenhorizonten im Höhenverlauf


Typic Placorthod at 2,200 m a.s.l. (Fig. 2). Above2,500 m a.s.l., hydromorphic properties (mottles, Feconcretions) increase with altitude. In the SCF(3,000 to 3,400 m a.s.l.), hydromorphic soils domi-nate. The representative soil type is a TypicPlacaquod, located at 3,200 m a.s.l. (Fig. 2, Photo 5).Beneath the thick (35 cm) ectorganic horizon thereis a dark grey silty to loamy Ag horizon. A placichorizon limits rooting depth. Below the placic ho-rizon, aeration is improved.

The soils in the research area are all acidic. ThepH of the ectorganic layers ranges from 2.4 to 3.5.In the mineral horizons, it increases gradually withdepth (Table 1 in Schawe et al. 2007). Between1,850 and 2,000 m a.s.l., the pH in the A horizon isaround 4, and above 2,000 m a.s.l., the pH isaround 3. Above 2,100 m a.s.l., the pH in theA horizon never exceeds 4.0, and above 3,100 ma.s.l., it decreases again to 3.5. The elevationalchange of the pH of the mineral subsoil differs lit-tle from that of the organic horizons; the pH ofthe B horizons shows a similar pattern: Up to2,300 m a.s.l. it remains at 4.5 and above 2,300 m itdecreases to 3.5. A low pH is common in tropicalmontane rain forests (Grieve et al. 1990, Tanneret al. 1998). The effective cation exchange capac-ity (ECEC) in the research area is very low. In theA horizons, it is < 15 cmolc kg-1, and in the ector-ganic horizon, it amounts to < 25 cmolc kg-1. Be-tween 1,800 and 2,100 m a.s.l., average ECEC is9.3 cmolc kg-1, above 2,100 m, it decreases to7.5 cmolc kg-1, and in the SCF it is just 6.3 cmolc kg-1.The ECEC in the A horizon of the LMF and UMCFis dominated by Al-ions that occupy more then 80 %of the exchange capacity (Schawe et al. 2007). Highconcentrations of Al or low base saturation havebeen noted in tropical montane rain forests before,for instance by Hafkenscheid (2000), Foelster andFassbender (1978), Grieve et al. (1990).

In the lower montane rain forest, < 30% of thesoil organic carbon (SOC) are accumulated inthe ectorganic horizon. With increasing alti-tudes, the ectorganic horizon increases

Photo 5 Placaquod in SCF (Schawe 2002) / Anmoor-Bänderstaupodsol im SCF (Schawe 2002)

(30-40 cm) and more than half of SOC is accu-mulated in the O-horizon (Fig. 7). According to


Altitude (m) 1,800 2,000 2,200 2,400 2,600 2,800 3,000 3,200

Emergent tree height (m)

28 31 25 24 - - - -

Mean canopy height (m)

20 20 17 18 15 11 12 6

Mean understory height (m)

12 10 8 10 9 8 - -

Cover intensity of epiphytes (%)

15 ± 10 30 ± 19 45 ± 20 55 ± 22 65 ± 20 45 ± 35 40 ± 15 35 ± 13

Soil cover of mosses (%)

18 50 65 80

Skleromorph plant species (% of Melastomataceae)

15 30 30 35 33 60 75 100

the Wilcoxon test (p < 0.05) a significant differ-ence exists between the altitudes 1,800-2,100and 2,200-2,600 m a.s.l. (O-horizon). But thereis no continuous increase of total C-stocks withaltitude: In the O-horizon, SOC stocks increaseup to 2,400 m. In the mineral soil, SOC stocksincrease up to 2,000 m. No altitudinal patterncan be detected at higher altitudes. In the re-search area, total SOC stocks range from 220 to530 t ha-1 with average values of 363 t ha-1, withthe maximum of above 350 t ha-1 at heights be-tween 2,000 and 2,400 m a.s.l., due to a thick(35-40 cm) ectorganic layer (Schawe 2005).

4. Discussion

4.1 Forest types

Forest structure parameters measured by Bach(2004) are summarised in Table 4. Measure-

ments of above-ground biomass and fine rootbiomass were not part of the research project.From tropical mountain forests of EcuadorLeuschner et al. (2007) report a decrease from13,200 (Eq. 1: lny = -3.375 + 0.948ln (D2H) ) or19,900 (Eq. 2: lny = 21.297-6.953 D + 0.740 D2)g dry matter m-2 at 1,890 m (LMF) to 7,380 (Eq.1) or 15,640 (Eq. 2) at 3,060 m (SCF), corre-sponding to a decrease in mean tree heightfrom 9.9 m (maximum 18.7 m) to 5.2 m (maximum9.0 m)! In contrast to southern Ecuador, whereonly three plots were investigated between1,890 m and 3,060 m, there is no continuous de-crease in tree height and probably also inabove-ground biomass in our transects. Meancanopy tree height decreases within the tran-sition zone from UMCF to SCF (Tab. 4). Themaximum of cover intensity of epiphytes andthe strong increase of soil cover by mossesat around 2,500 m a.s.l. indicate the importanceof fog and cloud water input.

Tab. 4 Elevational gradient of forest structure (average estimation, ± = standard deviation)Höhengradient der Waldstruktur (mittlerer Schätzwert, ± = Standardabweichung)


4.2 Hydrometeorological variables

4.2.1 Radiation

Daily inputs of short-wave radiation (Rs) in thestudy area decrease by roughly one third from theLMF to the SCF, due to the pronounced increasein fog and low clouds above 2,200 m. Within thecloud belt, however, cloudiness increases onlymarginally with elevation (Tab. 2). The averageRs of 15.5 ± 5.4 MJ m-2 day-1 in the LMF at 1,850 mis comparable to 13.2 MJ m-2 day-1 reported foran LMF at 1,975 m in southern Ecuador (4° S;Motzer 2003), and 13.8 ± 4.6 MJ m-2 day-1 meas-ured at 1,850 m in the Jamaican Blue Mountains(18° N) under conditions of light cloud incidence(Hafkenscheid 2000). Under the more cloudy con-ditions prevailing at 900-1,050 m.a.s.l. in easternPuerto Rico (18° N), Holwerda (2005) obtainedaverage Rs values of 9.7 ± 4.0 MJ m-2 day-1

(stunted ridge-top elfin cloud forest) and10.5 ± 4.8 MJ m-2 day-1 (upper montane palm for-est). The latter values are very similar to the dailyRs totals determined for our Bolivian UMCF andSCF (Tab. 2). Holwerda (2005) estimated Rs in thePuerto Rican cloud belt to be reduced by rough-ly 50 % compared to potential solar radiation in-puts, which is again similar to our estimated58-62 %, which confirms the extreme cloudinessof the Bolivian UMCF and SCF sites.

During an only five-day micrometeorologicalexperiment in the Chamaecyparis cloud forestin Taiwan Klemm et al. (2006) measured astrongly reduced short-wave radiation of 90Wm-2 for foggy conditions. Figure 4 indicatesan interannual variance in the SCF from 30 to60 Wm-2 and in the LMF from 65-150 Wm-2.

Changes in photosynthetic photon flux density(PPFD) along the elevational gradient reflect themeasured changes in Rs

(Tab. 2). Average PPFDtotals in New Guinea varied from 38 mol m-2 day-1 at1,100 m to 22 mol m-2 day-1 at 3,480 m (Koerner et al.1983) and were similar to values observed in the

present study area (33.6, 22.0 and 20.5 mol m-2 day-1 at1,850, 2,600 and 3,050 m, respectively).

Taking the 500 µmol m-2 s-1 threshold of light sat-uration proposed by Aylett (1985) for treecrowns (Cyrilla racemiflora, Clethra occiden-talis) in a Jamaica UMCF, PPFD values in theLMF exceed this value for 64 % of the time vs.52 % in the UMCF and SCF. Applying the lowerlimit of 200 µmol m-2 s-1 (Garcia-Nunez et al.1995) increases these percentages to 76 % in theLMF, 72 % in the UMCF, and 76 % in the SCF.Even under fully overcast conditions the PPFDremains above 200 µmol m-2 s-1 for 8 h in the LMF,for 6.5 h in the UMCF and for 7 h in the SCF.

According to Urban et al. (2007) and Beider-wieden et al. (2006) diffuse photosynthetic photonflux density (PPFD) has a much higher canopyphotosynthesis efficiency during cloudy periodsthan during direct radiation due to the higher solarequivalent leaf area. Our observed average dailymaximum PPFD values, also during fully overcastsky (Tab. 2), are all well above the threshold valuesdiscussed above. Thus, the PPFD data suggestthat despite the marked difference in radiationlevels between the LMF and the two higher vege-tation units, the reduction in PPFD is unlikely tobe a prime cause for the reduction in forest struc-ture with increasing elevation.

4.2.2 Evaporation

As observed in other tropical montane forest belts,reference evaporation PET decreases with altitudedue to reduced radiation, temperature (Fig. 4) andincreased relative humidity (Keig et al. 1979; Ki-tayama 1995; Bruijnzeel and Veneklaas 1998).Annual PET at the UMCF site is 61 % lower thanthat of the LMF, and decreases by an additional13 % in the SCF. It should be noted that PET is astandard climatic measure of the water use of well-watered grass (Doorenbos and Pruitt 1988) andtherefore not directly comparable to actual evapo-


ration (ET) from tall vegetation types like mon-tane rain forests or cloud forests where rainfalland fog water interception evaporation comeinto play next to soil water uptake (transpiration).However, expressed as an annual total, the PETvalue for the LMF site (1,190 mm) is within therange of ET values determined for a number ofequatorial LMF sites with negligible fog incidenceusing water budget techniques (1,155-1,380 mmper year; Bruijnzeel and Proctor 1995).

In 2005, additional measurements were taken fora three-month period (April-June) with the transi-tion from rainy season to drier season for the es-timation of interception losses in all three vegeta-tion belts (10 rovine gauges, weekly sampling on500 m2 plots, Kellner 2006). Increase of rainfallwith elevation is similar to 2001/2002 with the samefactor of 1.7 in UMCF and 2.1 in SCF comparedwith LCF. Based on measurements with record-ing troughs (Hawaii SCF, Juvik and Nullet 1995)or roving gauges (Jamaica UMCF, Hafkenscheid2000) Ei was calculated with 25 % respectively22 % of incident rainfall. For the LMCF in Vene-zuela (2,300 m a.s.l.) Ataroff (1998) reported a highvalue of 45 %. Bruijnzeel (Table 18.2, 2005) summaris-es recent studies on apparent rainfall interception(Ei) for lower and upper montane cloud forests andgives a range from 8 to 29 %. In other Andean stud-ies, Ei has high values: 39 % (Ecuador) and 45 %(Venezuela). Our medians – 21 % (LMF), 26 %(UMCF) and 25 % (SCF) – match well with the aver-age by Bruijnzeel (2005) of 25 to 29 % (Tab. 3).

Conversely, the annual PET values for the UMCF(462 mm year-1) and SCF (403 mm year-1) cannotbe compared with water-budget based ET valuesfor similar forests elsewhere (310-390 mm year-1)because only apparent ET values can be derivedwith the latter approach due to the confoundingeffect of unmeasured cloud water inputs (Bruijn-zeel and Proctor 1995). Evapotranspiration (ET)estimated from the annual difference ofthroughfall + stemflow and run-off in the LMF inEcuador range between 315 and 369 mm (Yasin

2001). The estimation of Et (transpiration, 980 mmyear-1) of a tall LMF in Venezuela by energy budgetcalculations leads to 2,310 mm year-1 because ofhigh interception evaporation, which seems rath-er high compared with the total evapotranspira-tion values by Bruijnzeel (2005, Table 18.3,1,050-1,260 mm). Based on direct measurementsof water budget components Holwerda (2005) es-timated annual ET for the cloud-affected uppermontane palm and stunted ridge-top elfin cloudforests in Puerto Rico cited earlier at 674 mm andca. 480 mm, respectively. Et evaluated from soilwater budget of the hydrometeorological stations(grass and herb soil cover) for the main soil rootlayer above soil water divide indicates Et valuesof 406 mm (LMF) and 38 mm (UMCF). This unre-alistically strong decrease shows the still existingproblem of experimental measurement uncertain-ty in the cloud forest with the great influence ofthe ectorganic layer in water storage and watersupply to the forest stand. Further, the realisticmeasurement of fog and cloud water input is a se-rious problem (Bruijnzeel 2005). From ourthroughfall measurements with roving gauges in2005 we can get an idea of the lower limit of cloudwater input (throughfall > incident rainfall, net pre-cipitation method) with 7 % for LMF, 13 % UMCFand 15 % SCF, equivalent to 0.17 mm/d, 0.54 mm/dand 0.77 mm/d. Using the same method Hafken-scheid (2000) reports 0.53 mm/d for UMCF in Ja-maica and Holder (2003) gives a range of0.5-1.0 mm/d for the cloud forest of Guatemala.

Transpiration rates for the two Puerto Rican forestswere 1.33 ± 0.95 mm day-1 and 0.81 ± 0.97 mm day-1,respectively (Holwerda 2005) and comparable tothe daily PET values derived for the presentcloud forest sites (Tab. 3, UMCF, SCF). Al-though these rates are low, even lower rates(< 0.5 mm day-1) of xylem water movement havebeen demonstrated in a Hawaiian cloud forest(Santiago et al. 2000). Despite the fact that re-ductions in montane forest stature and produc-tivity have been attributed to strongly reducedtranspiration rates by some researchers (Odum


1970; Kitayama and Mueller-Dombois 1994),this idea was refuted on theoretical grounds byGrubb (1977) and Tanner and Beevers (1990).Moreover, recent evidence suggests that thedecline in above-ground forest stature with ele-vation reflects increased investment in below-ground productivity, possibly in response togradually more adverse edaphic conditions(Leuschner et al. 2007, see also below).

4.2.3 Precipitation and soil moisture

The presently observed increase in P above2,000 m a.s.l. is atypical for tropical regions. Ac-cording to Lauscher (1976) P in tropical regionsis often highest between 1,000 m and 2,000 m. Vis(1986) and Veneklaas and van Ek (1990) report-ed P in the central Cordillera of the Colombian An-des to decrease from 3,150 mm year–1 at 1,700 mto 1,700 mm year-1 at 3,050 m. On Mount Kinaba-lu (East Malaysia) changes in P with elevationseemed less pronounced (Kitayama 1992). How-ever, the presently observed increase in P withaltitude is matched by various other observationsin the eastern Andes. Weischet (1969) reportedincreased P between 2,600 and 3,300 m in the east-ern Cordillera of the Colombian Andes due to theoccurrence of a secondary maximum of precipita-tion caused by heating of the high mountain pla-teau and forced convective precipitation on the up-per slopes. In addition, a marked increase in P, fromca. 2,000 mm at 1,900 m to ca. 5,000 mm at 2,380 mand 4,500 mm at 3,060 m, has been recorded in theAndes of southern Ecuador (Leuschner et al. 2007).

The pronounced changes in the climatic water bal-ance (i.e. P minus PET) result in major changes insoil moisture content along the altitudinal gradi-ent. In the LMF water-logging of soils is absent inthe Ah-horizon (10-20 cm depth), but reaches fieldcapacity during the main rainy season at a depthof 30 cm (Fig. 6). In the UMCF, field capacity isexceeded all the year and SCF soils are saturatedthroughout the year. Similar patterns of precipita-

tion excess and soil wetness with elevation havebeen reported for northern Colombia (Herrmann1971), Mount Kinabalu (Kitayama 1992) and Puer-to Rico (Silver et al. 1999; Holwerda 2005).

Persistently wet conditions have been reported forsome, but certainly not all, tropical cloud forestsoils (Bruijnzeel and Proctor 1995; Roman andScatena 2007), providing further illustration thatnot all cloud forests show an equal hydrologicbehaviour (Bruijnzeel 2005). However, increasesin the degree and duration of soil-water loggingare often paralleled by a decrease in montane for-est stature (Hermann 1971; Hetsch and Hoheisel1976; Bruijnzeel et al. 1993; Silver et al. 1999; San-tiago et al. 2000). Interestingly, such parallels havebeen observed both at high and low elevations,suggesting that precipitation excess overridesany temperature effects. Where water logging isparticularly persistent, as in the current SCF, aerialroots tend to be particularly common (Bach 2004)which has been observed in other very wet cloudforests (e.g. Gill 1969; Lyford 1969).

With the FDR soil moisture measurements a cal-culation of water uptake from the A horizon withthe soil water budget method was possible (Et).Without measurements of water uptake from ect-organic horizon (principal rooting zone in UMCFand SCF), in 2002 Et was 406 mm for the LMF. In-cluding the interception loss (Ei), total ET addsup to 900 mm/year, comparable with the results ofthe post-1993 water budget studies in Bruijnzeel(2005, Table 18.3) for Puerto Rico and Jamaica.

4.3 Soil development

Acidic brown soils with a loamy texture (Lt2) dom-inate the LMF. The transition to the UMCF is strik-ingly exhibiting grey eluvial, humic and iron/alu-minium illuvial horizons (grey E- and Bhs horizon).Dominating soil forming processes change fromthe LMF to the UMCF, with increasing podzolisa-tion and an increasing importance of hydromor-


phic properties with increasing altitude (SCF).Podsolic soils are reported from cloud forests inMalaysia (main tree species Podocarpus sp.,Burnham 1974). Schrumpf et al. (2001) reported in-creased water saturation as well as the occurrenceof placic horizons of soils in the Ecuadorian An-des. In the LMF (1,700 to 2,100 m a.s.l.), Dys-trudepts with shallow ectorganic horizons areprominent. Nutrient concentration and pH are rel-atively high. Precipitation is lower and soil satu-ration is restricted to short periods of the year. Thepronounced change in the climatic water balancebetween the LMF and the UMCF controls soil for-mation with more acidic conditions, deep ect-organic horizons and an increasing translocationof sesquioxides. In the UMCF (2,200 to 2,700 ma.s.l.), Placorthods are dominant. In the SCF (3,000to 3,400 m a.s.l.) hydromorphic processes exceedpodsolisation. The Aa/Ae-horizon exhibits extremeroot density. A Sw/Bhs-horizon follows which ham-pered deeper rooting. Placic horizons originate fromvertical and lateral Fe transport after reductivemobilisation. Whitmore and Burnham (1984) referto peaty Gleysols with thin Fe bands in Malaysia.Podsols with strong hydromorphic properties wereexamined by Sevink (1984) and Grieve et al. (1990)in Columbia and Costa Rica. Hetsch and Hoheisel(1976) also describe hydromorphic soils in the up-per cloud forest of Venezuela.

Mineralisation rates in the UMCF and SCF are low.Measurements on litter production (April 2003-April 2004) and total organic stock in the litter ho-rizon (Oi-horizon) allow the calculation of the year-ly mineralisation rate (koi) (Schawe 2005). From theLMF with koi = 2,1 (litter production/Oi-stock) themineralisation rate decreases to 0.9 (UMCF) and0.7 (SCF) (Schawe 2005). This correlates with wideC/N ratios and very low nutrient concentrationsin the mineral horizons. Correspondingly, the partof exchangeable cations in the ECEC of the wholesoil profile increases in the O-horizon from 42 %(LMF) to 52 % (UMCF) to 70 % (SCF) (see Schaweet al. 2007 for details). Dominant soil types in theSCF are nutrient-poor Placaquods.

5. Conclusions

Similar to soils of other tropical montane cloudforests, soils of the Yungas of Bolivia are char-acterised by high acidity, low cation exchangecapacity, high Al-toxicity and thick organic lay-ers. Soil types and soil genesis are distributedalong the altitudinal transect in close relation tothe vegetation belts of Lower Montane Forest(LMF), Upper Montane Cloud Forest (UMCF)and Subalpine Cloud Forest (SCF). This corre-lates with a decrease in temperature to 7-8°C(average daily T) in 3,400 m a.s.l., decrease in PETto 1/3 and an increase in annual rainfall up to 5,000mm. Pedologic indicators of the climatic gradientare decreasing pH (upper soil horizon), increas-ing O-horizons and increasing translocations ofsesquioxides (spodic Dystrudepts and typicDurorthods in the UMCF). The degree of soil mois-ture and water logging in the Ah- and O-horizonsincreases from the LMF to UMCF and stagnantconditions are present all around the year in theSCF and highly promote hydromorphy. Thereforein these altitudinal belts podsolisation and hydro-morphy are the dominant soil forming processesand conditions. The decrease of temperature withaltitude and the increase of upper soil water log-ging lead to a significant decrease of mineralisa-tion rates in the UMCF and SCF. Owing to soilacidity and very small nutrient availability, root de-velopment and root mass are concentrated moreand more in the ectorganic layer of the soil.

Despite the marked differences in short-waveradiation, PPFD and PET between the lower mon-tane forest and the upper montane and subalpinecloud forest formations, these hydrometeorologicfactors do not seem to exhibit a direct influenceon forest stature or composition and probably alsoon forest productivity. Similar to other cloud for-ests altitudinal gradients exist for vegetation pa-rameters (e.g. canopy height, emergent trees, soilcover of mosses, and decrease in á-diversity). Thecorrelation between á-diversity for the studiedplant families (Bach 2004) and the complex factor


of podsolisation indicates the importance of pre-cipitation excess with elevation, nutrient leach-ing, hydromorphic and stagnant soil conditionsin addition to plant-specific factors (e.g. plantevolution, plant competition). Conditions of C-allocation, high Al-toxicity resistance of the fineroot complex, N- and S-deficit in the soil (UMCFand SCF) must be investigated in more detail.

Acknowledgments

This study was carried out in close collaborationwith the Instituto de Ecología of the UniversidadMayor de San Andrés, La Paz and the NationalPark Administration (SERNAP) in Bolivia. Thestudy was funded by Deutsche Forschungs-gemeinschaft (DFG) (Grants GE 431/12-3 toG. Gerold and GR 1588/4 to S.R. Gradstein).

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Summary: Hydrometeorologic, Pedologic andVegetation Patterns along an Elevational Transectin the Montane Forest of the Bolivian Yungas

Floristic composition, structure and functioningof tropical montane rainforests depend on variousabiotic and biotic factors although the precisenature of the interaction is still a matter of debate.As part of an interdisciplinary project aiming tounderstand the relationship between vegetation

and abiotic factors in the montane forest belt inthe Yungas of Bolivia (Eastern Andes), hydrom-eteorologic observations and research on the alti-tudinal change of soils along an elevational gradi-ent were carried out. Earlier studies have revealeda decrease in biodiversity and forest stature withaltitude. Between October 2001 and October 2002three weather stations were in operation at 1,850m (lower montane forest, LMF), 2,600 m (uppermontane cloud forest, UMCF) and 3,050 m.a.s.l.(sub-alpine cloud forest, SCF). Precipitation in-creases strongly with elevation from 2,310 mmyear-1 at 1,850 m to 5,150 mm year-1 at 3,050 m.Compared to clear-sky conditions, reductions inshort-wave radiation inputs by fog and clouds areestimated at 37% at 1,850 m vs. 58-62% at 2,600m and 3,050 m. However, intensities of photo-synthetically active radiation (PAR) remain wellabove the light saturation point for local vegeta-tion, and changes in PAR with elevation are there-fore unlikely to control vegetation zonation. Pen-man-Monteith reference evaporation rates de-crease from 3.3 mm day-1 in the LMF zone to 1.4and 1.3 mm day-1 in the UMCF and SCF zones,respectively. Three zones of different dominantsoil forming processes can be found: In the LMF(1700- 2200 m a.s.l.), Dystrudepts with highnutrient concentration and acidity are common.The pronounced change to the UMCF (2200-2700 m a.s.l.) coincides with the appearance ofPlacorthods with strong acidic conditions, deepectororganic horizons and podzolization. In theSCF (2700-3400 m a.s.l.) hydromorphic process-es dominate, resulting in Placaquods with lowmineralisation rate and nutrient availability. Be-tween 1890 m and 3060 m there is no continuousdecrease in tree height, however mean canopy treeheight decreases in the transition zone from UMCFto SCF. The maximum of cover intensity of epi-phytes and the high increase of soil cover bymosses at around 2500 m a.s.l. indicates theimportance of the factor of fog and cloud waterinput. The present results suggest that the differencein forest stature of LMF and UMCF is primarilydue to the different radiation climate, while thedifference between UMCF and SCF seems to bedominated by the strong increase in precipitation,leading in turn to persistently saturated condi-tions, high acidity and leaching in the SCF.


Zusammenfassung: Hypsometrischer Klima-,Boden- und Vegetationswandel im Bergregenwaldder bolivianischen Yungas

Vegetationszusammensetzung, -struktur undökologische Funktionen tropischer Bergregenwäldersind im Zusammenhang mit zahlreichen abiotischenund biotischen Einflussfaktoren zu sehen; vieleökofunktionale Zusammenhänge sind kaum bekanntund werden weltweit diskutiert. Im Rahmen einesinterdisziplinären Projektes in den montanen Berg-regenwäldern der Yungas Boliviens wurden dieZusammenhänge von Klima, Boden und Vegetationentlang eines Höhengradienten untersucht. FrühereStudien in tropischen Bergregenwäldern zeigteneine Abnahme der Biodiversität und der Waldhöhemit zunehmender Meereshöhe. Drei installierteKlimastationen in 1.850 m (montaner Bergregen-wald LMF), in 2.600 m (hochmontaner Bergregen-wald UMCF) und in 3.050 m Höhe (subalpinerBergregenwald SCF) lieferten zwischen Oktober2001 und Oktober 2002 kontinuierliche Daten. Mitder Höhe nehmen die Niederschläge sehr stark zu,von 2.310 mm/Jahr (1.850 m) bis auf 5.150 mm/Jahrin 3.050 m Höhe. Verglichen mit der potentiellenGlobalstrahlung (unbewölkt) nimmt die Verminde-rung der Globalstrahlung durch den Wolken- undNebeleinfluß von 37% in 1.850 m auf 58-62% in2.600 und 3.050 m Höhe zu. Die photosynthetischaktive Strahlung (PAR) unterschreitet nie die Licht-sättigung der montanen Vegetation und stellt für denHöhenwandel der Vegetation keinen Kontrollpara-meter dar. Die nach Penman-Monteith (FAO) be-stimmte potentielle Verdunstung nimmt im Mittelvon 3,3 mm/Tag (LMF) auf 1,4 mm/Tag (UMCF)und 1,3 mm/Tag (SCF) ab. Drei Stufen dominanterBodenbildungsprozesse wurden identifiziert: ImLMF (1.700-2.200 m ü.M.) dominieren Braunerden(Dystrudepts) mit relativ hoher Nährstoffversor-gung und Versauerung. Ein markanter Bodenwandeltritt mit Übergang zum UMCF (2.200-2.700 m ü.M.)auf, gekennzeichnet durch Podsole mit starker Ver-sauerung, mächtigen organischen Auflagehorizontenund Podsolierungsprozessen. Im SCF (2.700-3.400 m ü.M.) sind Vergleyungsprozesse domi-nant. Anmoor-Staupodsole (Placaquods) sind durcheine sehr geringe Mineralisationsrate und Nähr-stoffverfügbarkeit charakterisiert. Untersuchungenzur Vegetationsabfolge (Waldstruktur) zeigen

zwischen 1.890 m und 3.060 m keine kontinuierlicheAbnahme der Baumhöhe. In der Übergangszone deshochmontanen Bergregenwaldes (UMCF) zur sub-alpinen Waldstufe (SCF, Nebel- bzw. Wolkenwald)nimmt die mittlere Baumhöhe deutlich ab. Aus demMaximum des Epiphytenvorkommens und dem star-ken Anstieg der Moosbedeckung des Bodens ab2.500 m ü.M. kann indirekt der große Einfluß desNebel- und Wolkenwassereintrags in das Waldöko-system abgeleitet werden. Der Wandel der Wald-struktur vom montanen zum hochmontanen undsubalpinen Bergregenwald korreliert vor allem mitder Änderung des Strahlungsklimas und einem zu-nehmendem Niederschlagsüberschuss mit ganzjäh-riger Bodenwassersättigung im UMCF und SCF,was zu starker Bodenversauerung mit Nährstoff-auswaschung und Prozessen der Podsolierung undVergleyung (SCF) führt.

Résumé: Le changement hypsométrique du climat,du sol et de la végétation dans les forêts tropicalesmontagnardes des Yungas en Bolivie

La composition de la végétation des forêts tropicalesmontagnardes ainsi que sa structure et ses fonctionsécologiques sont à voir par rapport à de nombreusesinfluences abiotiques et biotiques. Beaucoup derelations éco-fonctionelles ne sont à peine connueset on en discute dans le monde entier. Au cours d’unprojet interdisciplinaire dans les forêts tropicalesmontagnardes des Yungas en Bolivie, des recherchesont été faites sur les relations entre le climat, le solet la végétation le long d’un gradient d’altitude. Lesétudes précédentes faites dans les forêts tropicalesmontagnardes avaient montré une diminution de ladiversité biologique et de la hauteur des arbrescorrespondante à l’augmentation d’altitude. Troisstations climatologiques, installées à 1.850m (LMF),à 2.600m (UMCF) et à 3.050m (SCF), ont rendu desdonnées continues entre le mois d’octobre 2001 et lemois d’octobre 2002. Plus l’altitude augmente plusles précipitations sont puissantes : de 2,310mm paran (1.850m) jusqu’à 5,150mm par an à 3.050m.Comparé à la radiation globale potentielle (sansnuages), la diminution de la radiation globale causéepar l’influence des nuages et du broullard augmentepar rapport à l’altitude du terrain de 37% à 1.850m


jusqu’à 58-62% à 2.600 et 3.050m. La radiationphotosynthétique active (PAR) reste toujoursinfèrieure à la saturation de lumière de la végétationmontagnarde et ne présente donc pas de paramètrede contrôle pour le changement de la végétation avecl’altitude. L’évaporation potentielle définie parPenman-Monteith (FAO) diminue en moyenne de3,3mm par jour (LMF) jusqu’à 1,4mm par jour(UMCF) et 1,3mm par jour (SCF). Trois stages dansle procès dominant de formation du sol ont étéidentifiés : Dans le LMF (1.700-2.200m d’altitude)les terres brunes (Dystrudepts) dominent avec unefourniture copieuse de substances nutritives et uneforte acidification. Un changement marqué de sol semanifeste au niveau de la transition dans l’UMCF(2.700-3.400m d’altitude) charactèrisé par despodsols avec une acidification prononcée,d’importantes couches organiques et procès depodsolification. Dans le SCF (2.700-3.400md’altitude) les procès de gleyification sont domi-nants. Placaquods sont charactèrisés par un taux deminéralisation très bas et une fourniture de substancesnutritives peu élevée. Les recherches sur le change-ment de la végétation (structure de la forêt) nemontrent pas de diminution continue de la hauteurdes arbres entre 1.890m et 3.060m. Dans la zone detransition entre la forêt tropicale montagnard éle-

vée (UMCF) et la forêt tropicale subalpine (SCF,la forêt dans les nuages et le brouillard) la hauteurmoyenne des arbres diminue nettement. En sebasant sur le maximum de gisement épiphyte etl’augmentation prononcée de la couverture demousse sur le sol à partir de 2.500m, on peutconstater indirectement la grande influenced´importation de brouillard et d’eau des nuages surle système écologique des forêts (Yungas).

Prof. Dr. rer. nat. Gerhard Gerold, GeographischesInstitut der Universität Göttingen, Goldschmidtstr. 5,37077 Göttingen, Germany, [email protected]

Dr. rer. nat. Marcus Schawe, Hartwig-Hesse-Str. 11,20257 Hamburg, Germany, [email protected]

Dr. Kerstin Bach, Philipps-Universität Marburg,Fachbereich Geographie, Deutschhausstr. 10, 35032Marburg, [email protected]

Manuskripteingang: 18.6.2007Annahme zum Druck: 07.01.2008

hydrometeorologic, pedologic and vegetation patterns along an

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