Structural-floristic differences Atlantic-Pacific montane Quercus forests Costa Rica - Kappelle 1992

/

aram.o An Andean Ecosystem. under Hun1an Influence Edited by

H. BALSLEY Department of Systematic Botany, Aarhus University, Denmark

AND

J.L. LUTEYN Institute of Systematic Botany, The New York Botanical Garden, USA

ACADEMIC PRESS Harcourt Brace Jovanovich, Publishers London San Diego New York Boston Sydney Tokyo Toronto

Contents

Foreword by Heinz Ellenberg ................................................................... .

Contributors ................................................................................................. .

Contents ........................................................................................................ .

Introduction ................................................................................................. .

J. L. Luteyn- Paramos: Why study them?

•••••••••••••••••••••••••••••••••••••••••••••

0. Hedberg- Afroalpine vegetation compared to paramo: Convergent adaptations and divergent differentiation

v • •

Vll •

lX •

Xl

1

•••

15

..... ..........................................................................

31

A. M. Cleef and A. Chaverri P. - Phytogeography of the paramo flora of Cordillera de Talamanca, Costa Rica .. .. .. .. .. .. ... ..... ..

45

J. Fjeldsa- Biogeography of the birds of the Polylepis woodlands of the Andes

M. Kappelle - Structural and floristic differences between wet

Atlantic and moist Pacific montane Myrsine-Quercus forests in Costa Rica ..................................................................

61

J. L. Luteyn, A.M. Cleef and 0. Rangel Ch.- Plant diversity in paramo: Towards a checklist of paramo plants and a generic flora ••••••••••••••••••••••

85

.. .... .. .... .. .... .... .. .. .. .. .. .. ........ .. ...... .. .. .. .. .... .. .

95

K. Romoleroux - Rosaceae in the paramos of Ecuador H.

J. M. Sipman- The origin of the lichen flora of the Colombian paramos

T. Ahti - Biogeographic aspects of Cladoniaceae in ~he pararnos A. F. Bosman - Ecology of a tropical high Andean cushion mire G.

71

•••••

111

.... 119

J. Tol and A. M. Cleef- Nutrient status of a Chusquea tessellata bamboo paramo

.. .. ... ..... .. .. .. ..... .. .. ... .. .. ... .. .. ... .. .. ... .... .. ... .. .. ... .. .. .. 123

Stru ctur al and floristic diff eren ces betw een wet Atla ntic and moi st Pacific mon tane Myr sine -Qu ercu s fore sts in Cos ta Rica M. Kappe lle

Hugo de Vries Laboratory, University of Amsterdam, The Netherlands, and Escuela de Ciencias Ambientales, Universidad Nacional, Heredia, Costa Rica Abstrac t. Two new subcom muniti es in the montan e Myrsin e pittieri-Quercus costaricensis forest commu nity are provisi onally describ ed from Costa Rica. First, the Ilex discolor- Quercus costaricensis subcom munity on the wet Atlanti c slope of the Cordill era de Talama nca, and second ly the Comarostaphylis arbutoidesQuercus costaricensis subcom munity on the moist Pacific slope. At 3100 m alti-¡ tude many structu ral and floristic dissimi larities betwee n these subcom muniti es were found. Atlanti c forests are almost twice as tall as Pacific forests, which in turn show more dwarfis h feature s (e.g., gnarled trees). Tree density is lower and average stem diamet ers are higher in Atlanti c stands, which also house a great variety of hygrop hilic fern species, while dense Pacific forests are rich in smallstemm ed tree and shrub species with coriace ous leaves. Floristic similar ity between both slopes is great; about one-thi rd of all species is found on both the Atlantic and the Pacific slope, and the remain ing 65°/o are equally propor tioned between the two slopes. Structu ral and compo sitiona l dissimi larities are mainly caused by differe nt climatic conditi ons prevail ing on the two slopes. Especially, the more stahl~ mesoclimatic conditi ons caused by the trade winds on the Atlanti c slope and the extrem e diurnal temper ature fluctua tions resultin g from cloud absence during the mornin g and mid-da y hours on the Pacific slope determ ine the differe ntiatio n of Talama nca montan e Myrsine pittieri- Quercus costaricensis forests just below timber line. .

Resum en. Se describ ieron provisi onalme nte para Costa ~ica dos subcom unidad es nuevas pertene cientes a la comun idad boscos a montan a de Myrsine pittieri Quercus costaricensis. En primer Iugar la subcom unidad de Ilex discolor- Quercus costaricensis, la cual se presen ta en la vertien te muy humed a atlantic a de la Cordillera de Talama nca yen segund o Iugar la subcom unidad de Comarostaphylis arbutoides- Quercus costaricensis, la cual se presen ta en la vertien te humed a pacifica de la mism.a. A 3100 m.s.n.m. se observ an varias diferen cias en estruct ura PARAMO ISBN 0-12-460442-0

Copyrigh t Š 1992 by Academi c Press Limited All rights of reproduc tion in any form reserved

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y composici6n florfstica entre ambas subcomunidades. Los bosques de la vertiente atlantica son aproximadamente dos veces mas altos que los de la vertiente pacifica. En esta ultima, las formas arborescentes son mas enanas y presentan tallos torcidos. Las clases de distribuci6n de dap y la diversidad de helechos son mas altas en Ia vertiente atlantica, en cambio la densidad de arboles y la diversidad de especies leftosas son mas altas en la vertiente pacifica. La similaridad florfstica entre ambas subcomunidades es grande: alrededor de una tercera parte de todas las especies se han encontrado en ambas vertientes. Asi mismo, una tercera parte de las especies esta restringida a cada una de las dos vertientes. Las diferencias en estructura y composici6n floristica entre los dos tipos de bosque se deben prindpalmente a las condiciones climaticas que prevalecen en cada vertiente: en el Atlantica las condiciones mesoclimaticas son mas estables debido a los vientos alisios, mientras que en el Pacifico ocurren diariamente extremas fluctuaciones de temperatura por la ausencia de nubes en las horas de la mafi.ana.

Introduction Different studies on the classification of montane Chusquea-Quercus forests present in the Costa Rican Cordillera de Talamanca have been carried out before or are under way (Kappelle, 1987, in press; Kappelle et al., 1989, 1991). In general, these studies detail the structure and composition of altitudinally-zoned oak forest communities. However, thus far no study has focussed on the structural and floristic differences between montane Chusquea-Quercus forests at similar altitudes but on different slopes.

Therefore, a comparison is made between an Atlantic and a Pacific montane ericaceous forest at ca. 3100 m elevation in the Chirrip6 National Park, Costa Rica. Both forests belong to the upper montane Myrsine pittieriQuercus costaricensis community, which is described elsewhere (Kappelle et al., 1989).

Materials and methods Study area. The study area (Fig. 1A) is situated in the central part of the Costa Rican Cordillera de Talamanca and forms part of the Chirrip6 N ationa! Park (50,150 ha; Boza, 1988) which belongs to the Amistad Biosphere Reserve. Here, the highest peak of southern Central America, Cerro Chirrip6 (3819 m), is located (9°30'N; 83°30'W). Cordillera de Talamanca is formed of intrusive rocks, Tertiary volcanic rocks, and marine sediments (Weyl, 1980; Castillo, 1984). Pleistocene glaciations have left traces, such as U-formed valleys and glacial lakes, mainly above 3000 m elevation (Weyl, 1980; Barquero and Ellenberg, 1986). Slopes are moderately to very steep.

Soils are developed from volcanic ashes from the Cordillera Central to the north; at higher altitudes they are dark-colored, rich in organic matter, medium-textured, moderately fertile, very acid, and excessively drained (Vasquez, 1983). Average annual temperature at the town of Villa Mills, 10 km west of the study area at 3000 m, is 10.9°C, while average rainfall is 2812

Myrsine-Quercus forests in Costa Rica

Atlantic Ocean

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10.9 (2812) Precipi tation (mm)

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Figure 1. A. Map of Costa Rica showing the study area. B. Walter climate diagram from Villa Mills at 3000 m alt., based on records of the Instituto Meteorol6gico Nacional (1988).

C. Cloud distribution during late morning hours in the Cordillera de Talamanca. Arrows on the Atlantic slope indicate trade winds coming from the Caribbean. The open circle represents the location of the Pacific, sun-exposed, Comarostaphylis- Quercus forests, and the black dot the wet and cloudy, Atlantic flex- Quercus forest stands (based on Coen, 1983).

mm (Fig. 1B). Diurnal mist during the afternoon is considerable, especially d uring the rainy season from May to November. In general, the Atlantic slope is much wetter than the Pacific due to the heavy rainfall produced by the northeastern trade winds coming in from the Caribbean (Fig. 1C) . .

Methods. In order to understand the basic patterns of altitudinal vegetation zonation in the Costa Rican Cordillera de Talamanca, undisturbed mature p rimary plant communities have been studied at 2000-3300 m elevation along an altitudinal transect through the Chirrip6 National Park. The m ountain transect methodology proposed by van der Hammen et al. {1989) w as employed. The vegetation characteristics on the Atlantic and Pacific slopes were analyzed separately in order to reveal differences in vegetation

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and community patterns. Along the altitudinal transect, seven 0.05 ha plots were established at 3000-3200 m elevation. These support evergreen, upper montane Chusquea-Quercus forests just below the tree line. Three of the seven plots are located on the Atlantic slope and four on the Pacific. These plots are all situated about 1 km northwest of the 3333 m high Cerro Uran near Paso de los Indios (Kohkemper, 1968). Structure and composition was studied in each plot. Data on canopy height, tree density, stem diameter, and species richness were recorded and compared. Plant specimens were collected, identified, and deposited at ASD, B, CR, F, NY, and U. Profile diagrams covering 40 square meters were drawn for one Atlantic and one Pacific plot. Finally, Atlantic and Pacific data subsets were compared in order to discover similarities and differences in vegetation patterns.

Results and discussion The Talamanca upper montane Chusquea-Quercus forests at 2900-3400 m elevation, belong to the Myrsine pittieri-Quercus costaricensis community, which has been described earlier for the area between Cerro Las Vueltas and Cerro de la Muerte in the western part of the Costa Rican Cordillera de Talamanca (Kappelle et al., 1989). In the Chirrip6 National Park, this upper montane forest community extends to the upper tree line, where Diplostephium costaricense shrub and Chusquea subtessellata bamboo paramo replaces the forest stands. Here, the Myrs.ine pittieri-Quercus costaricensis community is characterized by Quercus costaricensis canopy trees (15-30 m tall) and a mixed subcanopy layer (8-15 m tall) of Myrsine pittieri, Vaccinium consanguineum, Oreopanax capitatus, Rhamnus oreodendron, Schefflera pittieri, Drimys granadensis, Miconia schnellii, Weinmannia trianae var. sulcata, Symplocos austinsmithii, Clethra gelida, and Clusia spp. Important understorey species (4-7 m tall) are the dominant bamboo Chusquea talamancensis and the shrubs Macleania rupestris, Viburnum venustum, Cavendishia bracteata, Gaiadendron punctatum, Hesperomeles heterophylla, and Fuchsia microphylla. The herb Maianthemum paniculatum, the climbing vine Bomarea acutifolia, and the ferns Arachniodes denticulata, Elaphoglossum spp., Eriosorus glaberrimus, Grammitis spp., Plagiogyria spp., Polypodium spp., and Vittaria graminifolia are freque11t. All these species are abundant on both the Atlantic and Pacific slopes, whereas many others are only or mainly present on one of the two slopes.

The Atlantic slope. In the subcommunity !lex discolor var. lamprophyllaQuercus costaricensis, the canopy layer reaches to about 25 m height and is composed of Quercus costaricensis trees sometimes with stem diameters at breast h.eight over 200 em (Fig. 2C). The 12-15 m tall subcanopy layer consists of such species as Weinmannia pinnata, !lex discolor var. lamprophyl

65

Myrsine-Quercus forests in Costa Rica

fLEX DISCOLOR VAR. LAMPROPHYLLA •

COMAROSTAPHYLIS ARBUTOIDES •

QUERCUS COSTARICENSIS SUBCOMMUNITY

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Figure 2. A, l3. Profile diagrams and aerial stem projection of :fwo 0.015 ha forest plots at 3100 m alt., Cordillera de Talarnanca., Costa Rica. A. Ilex discolor- Quercus costaricensis forest on the Atlantic slope (plot 71). B. Comarostaphylis arbutoides-Quercus costaricensis forest on the Pacific slope (plot 62). C, D. Distribution of stem diameter classes for Atlantic and Pacific slopes. Black bars represent dead stems and shaded bars living stems.

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M. Kappelle

Drimys granadensis, and Miconia schnellii. A profile diagram

rep~

resenting this subcommunity is given in Figure 2A. This forest type is characterized by the diagnostic presence of two species of holly in the subcanopy: !lex discolor var .. lamprophylla and the less abundant Ilex pallida.

Other important woody species restricted to this forest are the tree Symplocos serrulata and the shrubs Disterigma humboldtii which is often also epiphytic on Quercus, Palicourea adusta, and a species of Senecio: The 5 m tall species of Chusquea, probably a new species (Y. Widmer, pers. com.), sometimes codominates the understorey together with the more common Chusquea talamancensis and the tree fern Culcita coniifolia. Many herbs are found on the ground or as low epiphytes, among others the diagnostic

Anthurium concinnatum, Peperomia saligna, Maxillaria biolleyi, Rhynchospora vulcani, and Centropogon costaricensis. The herbaceous Mikania cordifolia and Dioscorea standleyi make use of Chusquea bamboo stems, shrubs, and treelets as support for climbing towards the better light conditions of the subcanopy stratum. In the herb layer and as epiphytes one finds a series of characteristic ferns, such as Arachniodes denticulata, Elaphoglossum latifolium, Blechnum aff. viviparum, Elaphoglossum furfuraceum,

Elaphoglossum squamipes, Plagiogyria semicordata, Grammitis ctenopteris, Grammitis myriophylla, and Polypodium ptilorrhizon/rhodopleurum. At perhumid places one may encounter Hymenophyllum consanguineum or H. elegantulum.

The Pacific slope. In the subcommunity Comarostaphylis arbutoides ssp. arbutoides-Quercus costaricensis, the canopy and subcanopy tree layers are not present as different strata. Instead, there is only one, dense and diverse tree stratum, 5-15 m tall, consisting of species such as Comarostaphylis arbutoides ssp. arbutoides which is heavily gnarled and inclined, Quercus

costaricensis, Schefflera pittieri, Drimys granadensis, Weinmannia trianae var. sulcata, Vaccinium consanguineum, and Zanthoxylum scheryi (Fig. 2B). In general, tree stem diameters do not exceed 70 em at breast height

(Fig. 2D). This subcommunity is characterized by Persea vesticula, Wein-

mannia pinnata, Viburnum costaricanum, Cleyera theaeoides, Crossopetalum tonduzii, I lex tristis, Zanthoxylum scheryi, and C l usia rose a.

Important characteristic shrubs are Pernettya prostrata, Gaultheria erecta, Diplostephium costaricense, Escallonia myrtillioides, Ugni myricoides, and Hypericum irazuense. In the herb layer, one observes the diagnostic Vriesea williamsii and the locally important ferns Elaphoglossum tectum, Grammitis moniliformis, Polypodium myriolepis, and Eriosorus glaberrimus. The caulescent Blechnum buchtieni is a conspicuous element on humid places near the upper forest line. Dendrophthora costaricensis and D. squamigera occur frequently as shrubby parasites.

Structural differences. Comparing structural aspects of the Atlantic and Pacific Myrsine-Quercus subcommunities, one notes immediately the differ-

Myrsine-Quercus forests in Costa Rica

67

ence in canopy height and stem diameter spectra (Fig. 2). Atlantic forest patches are almost twice as high as Pacific forest startds at the same altitude (Fig. 2A,B). Canopy dominating Quercus costaricensis trees with dense crowns and numerous .Chusquea bamboo clumps in the Atlantic forest do not favor a diverse subcanopy with lots of young trees (Fig. 2C). On the other hand, Pacific forests are much richer in tree and shrub species (Table 1). Here the small woody stems (Fig. 2D) share a rather low tree layer and Quercus becomes less important. Instead, gnarled trees like Comarostaphylis arbutoides ssp. arbutoides and shrubs such as Crossopetalum tonduzii, Escallonia myrtillioides, Ugni myricoides, and Hypericum irazuense codominate. These species are much better adapted to the harsh environment on the Pacific slope. Also on this side of the Cordillera de Talamanca, Schefflera pittieri becomes abundant probably as a result of a high gap frequency close to the timberline. Mesic conditions on the Atlantic slope generate wet microclimatic niches, which are occupied by various ferns in Elaphoglossum, Grammitis, Hymenophyllum, and other genera.

Looking at species richness of both slopes separately (Fig. 3) and floristic similarity between both slopes, one notes some interesting trends.

In total, 91 terrestrial vascular plant species have been recorded (Table 1).

About 70°/o of these species were found on the Atlantic slope (64 spp.) and a similar number (66°/o) appeared on the Pacific slope (61 spp.). Considering the 64 species recorded in the three Atlantic forest plots, about 53°/o occurred also in the four Pacific plots (34 spp. ), whereas 47°/o ·did not (30 spp. ).

Looking at the total number of species found in the plots on the Pacific slope (61 spp.), 44°/o appear to be restricted to this slope at this altitude (27 spp.), while another 56°/o is shared with the Atlantic slope (34 spp.).

In other words, about one-third of all species are found on both slopes, while a second third is restricted to the Atlantic slope; another third is only recorded in Pacific plots. It is remarkable that many ferns (28 spp.)

20 16

12 8 4 0

T

S

H

F

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Figure 3. Species numbers of trees (T), shrubs (S), herbs (H), ferns (F), and bamboos (B) for a Pacific (bold line, plot 62) and an Atlantic (fine line, plot 71) 0.05 ha forest stand at 3100 m alt., Cordillera de Talamanca, Costa Rica.

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M. Kappelle

Table 1. Checklist of species of the Myrsine pittieri-Quercus costaricensis forests, Costa Rica. A= Atlantic slope, P= Pacific slope.

PTERIDOPHYT A . Aspleniaceae: Asplenium serra Langsd. & Fisch. (A). Blechnaceae: Blechnum buchtienii Resent. (P), Blechnum aff. viviparum (Broadh.) C. Chr. (A) . Cyatheaceae: Culcita coniifolia (Hook.) Maxon (A). Dennstaedtiaceae: Dennstaedtia aff. cicutaria (Sw.) Moore (A).

Dryopteridaceae: Arachniodes denticulata (Sw .) Proctor (AP), Elaphoglossum alfredii Rosenstock (A), Elaphoglossum fournieranum Gomez (AP), Elaphoglossum furfuraceum (Mett.) Christ. (A), Elaphoglossum Iatifolium (Sw.) J. Smith (A), Elaphoglossum mathewsii (Fee) Moore (P), Elaphoglossum squampipes s.l. (Hook.) Moore (A), Elaphoglossum tectum (Willd.) Moore (P) . Hymenophyllaceae: Hymenophyllum consanguineum Morton (A), Hymenophyllum elegantulum Van den Bosch (A). Plagiogyriaceae: Plagiogyria costaricensis Mett. ex Kuhn (P), Plagiogyria semicordata (Presl) Christ (A). Polypodiaceae: Grammitis (Ctenopteris) sp. (A), Grammitis cultrata (Willd. in L.) Proctor (A), Grammitis flabelliformis (Poir. in Lam.) Morton (A), Grammitis meridensis (Klotzsch) Seymour (A), Grammitis moniliformis (Se.) Proctor (P), Grammitis myriophylla (Mett. ex Baker) Morton (A), Polypodium myriolepis Christ. (P), Polypodium ptilorhizon/thodopleurum (AP).

Pteridaceae: Eriosorus glaberrimus (Maxon) Scamman (AP). Thelypteridaceae, Thelypteris gomeziana Smith and Lellinger (A). Vi ttariaceae: Vittaria graminifolia Kaulf. (AP).

MONOCOTYLEDONAE. Alstroemeriaceae: Bomarea acutifolia (Link & Otto) Herb. (AP).

Bromeliaceae: sp. Cyperaceae: Rhynchospora vulcani Boeckeler (A). Liliaceae : Maian themum paniculatum (Mart. & Gal.) LaFrankie (AP). Orchidaceae: Maxillaria biolleyi (Schltr.) L. 0 . Williams (AP). Poaceae: Chusquea sp. nov. (A), Chusquea talamancensis Widmer & L. G. Clark (AP).

DICOTYLEDONAE. Aquifoliaceae: !lex discolor var. lamprophylla (Standley) Edwin (AP), Ilex pallida Standley (AP), !lex tristis Standley (P). Araceae: Anthurium concinnatum Schott (AP) .

Araliaceae: Oreopanax capitatus (Jacq.) Decne. & Planchon (AP), Schefflera pittieri (Marchal) Fredin (AP). Asteraceae: Diplostephium costaricense Blake (AP), Mikania cordifolia (L.f.) Willd. (A), Neomirandea araliaefolia (Less.) R. King & H. Robins. (P), Senecio heterogamus (Benth .) Hemsley (A), Senecio sp. (A) . Caprifoliaceae: Viburnum costaricanum (Oersted) Hemsley (P), Viburnum venustum Morton (AP). Celastraceae: Crossopetalum ¡tonduzii (Loes.) Lundell (P). Clethraceae: Clethra gelida Standley (AP). Clusiaceae: Clusia rosea Jacq. (P), Clusia cf. stenophylla Standley (A).

Cunoniaceae: Weinmannia pinnata L. (P), Weinmannia trianae Wedd. var sulcata (Engl.) Cuatrec.

(AP). Dioscoreaceae: Dioscorea standleyi Morton (A). Ericaceae: Cavendishia bracteata (R. & P. ex J. St. Hil.) Hoer. (AP), Comarostaphylis arbutoides Lindl. ssp. arbutoides (P), Disterigma humboldtii (Klotzsch) Niedenzu (AP), Gaultheria erecta Vent. (P), Macleania rupestris (Kunth) A. C. Smith (AP), Pernettya prostrata (Cav.) DC. (P), Sphyrospermum cordifolium Benth. (A), Vaccinium consanguineum Klotzsch (AP). Fagaceae: Quercus costaricensis Liebmann (AP) . Hypericaceae: Hypericum irazuense Kuntze ex N. Robson (P). Lauraceae: Persea vesticula Standley & Steyerm. (P).

Lobeliaceae: Centropogon costaricae (Vatke) McVaugh (A). Loranthaceae: Dendrophthora costaricensis Urb . (P), Dendrophthora squamigera (Benth.) Kuntze (P), Gaiadendron punctatum (R. & P.) G. Don (AP). Melastomataceae: Miconia schnellii Wurdack (AP). My):'sinaceae: Grammadenia minor Lundell (A), Grammadenia pellucido-punctata (Oersted) Mez (P), Myrsine pittieri (Mez) Lundell (AP). Myrtaceae: Ugni myricoides (Kunth) Berg (P) . Onagraceae: Fuchsia microphylla Kunth (AP). Piperaceae: Peperomia alpina (Swartz) A. Dietrich (A), Peperomia galioides Kunth (P), Peperomia saligna Kunth (AP) . Rhamnac-ea e: Thamnus oreodendron L. 0 . Williams (AP).

Rosaceae: Hesperomeles heterophylla (R. & P.) Hook. (AP), Rubus eriocarpus Liebmann (AP).

Rubiaceae: Palicourea adusta Standley (A). Rutaceae: Zanthoxylum scherye Lundell (AP) .

Saxifragac eae: Escallonia myrtillioides L.f. var. patens (R. & P.) Sleumer (P). Scrophulariaceae : Castilleja talamancensis N. Holmgren (P). Solanaceae: Cestrum irazuense Kuntze (P).

Symplocaceae: Symplocos austin-smithii Standley (AP), Symplocos serrulata Kunth (A). Theaceae: Cleyera theaeoides (Swartz) Choisy (P). Winteraceae: Drimys granadensis L.f. (AP).

Myrsine-Quercus forests in Costa Rica

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are restricted to o.n e slope; very few ferns (18°/o) are present on both slopes.

Over half of all fern species (61 °/o) are only found in Atlantic plots (Fig. 3, fine line) indicating very humid conditions in the understorey. On the other hand, many coriaceous shrub and tree species are observed on the Pacific slope (Fig. 3, bold line). These are Comarostaphylis arbutoides ssp. arbutoides, Crossopetalum tonduzii, Dendrophthora costaricensis, D.

squamigera, Escallonia myrtillioides, Gaultheria erecta, Grammadenia pellucidopunctata, Hypericum irazuense, Ilex tristis, i prostrata, and Ugn i myrcioides. In general, these species have relatively small and thick leaves clustered at the top of gnarled branches. This xeromorphic adaptation helps to overcome the extreme temperature fluctuations characteristic for the high-altitude zone of the Pacific slope (Fig. lC). In addition, several species found on only one slope at these altitudes may occur on the other slope at lower or even higher altitudes. Conclusions drawn in the present paper refer merely to the Myrsine pittieri-Quercus costaricensis forests at 30003200 m altitude in the western part of the Chirrip6 National Park. Data from a larger altitudinal range or other areas in the Cordillera de Talamanca may shift tl1e picture (Kappelle, in press). Also, we have considered an unequal number of plots on both slopes; three on the Atlantic slope and four on the Pacific. Thus only preliminary conclusions may be drawn from the results.

Overall, the structural and floristic dissimilarities observed suggest strong climatic differences between the two slopes. The wet and foggy Atlantic slope offers a good climate for forest growth producing tall stands of robust trees and pteridophyte diversity, whereas the drier Pacific slope with extreme diurnal temperature fluctuations due to changing cloudiness and high variation in insolation gives rise to leaf xeromorphistn and dwarf habits in woody species, Ericaceae among others.

Acknowledgments Financial and logistic support was received from the Programa ECOMA (Proyecto 762090) of Universidad Nacional (Heredia, Costa Rica), University of Amsterdam, the Treub-Maatschappij, and Hugo de Vries Foundation. Most species have been identified with the help of N. Zamora (angiosperms) and L. D. Gomez (pteridophytes). Scientific support came from A. M. Cleef and A. Chaverri. An earlier draft of the manuscript was commented on by Wm. Burger and an anonymous reviewer. The Spanish summary has been prepared with the friendly help of 0. Rangel. Local re1 search assistance was provided by M. Monge.

Literature cited Barquero, J. and Ellenberg, L. (1986). "Geomorphologie der alpinen Stufe des Chirrip6 in Costa Rica." Eiszeitalter und Gegenwart 36, 1-9.

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Boza, M. A. (1988). "Parques Nacionales de Costa Rica." Fundaci6n

Neotr6pica Heliconia, San Jose.

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