The phytogeography and vegetation of Chiapas (Mexico) - Breedlove 1973 by JPZ.

VEGETATION-AND VEG-ETATIONAL HISTORY OF NORTH ERN LATIN AMERICA PAPERS PRESENTED AS PART OF A SYM POSIUM, " VEGETATION AND VEGETAT IONAL HISTORY IN NORTH ERN LAT IN AMERICA", AT TH E AMERI CAN INSTIT UTE OF BIOLOGICAL SCIENCES MEETJN G,S. BLOOMINGTON, IN D. (U.S.A.), 1970

edited by

ALAN GRAHAM Departmelll of Biological Sciences, Ken/ State University, Kent, Oflio (U.S .A.)

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ELSEVIER SCIENTIFIC PUBLISHING COMPANY Amsterdam - London- New York

. 1973

Contents Preface . . . . . . . . . . . . . . . . . .

Chapter 1. The Vegetation of the Antilles RICHARD A. HOWARD Summary . . . . . . . . l ntroduct iOJl . . . . . . . Vegetation types in the Antilles Coastal formations . Lowland formations Montane forma tions Qrigins of the existing vegetation of the Antilles Relationships and distribution of the Antillean vegetation Disjunct distribution patterns in the Antilles . . . . . Distribut ion patterns wit hin the islands . . Long-distance 1elationships and disjunct distribution patterns Conclusions References . .

5 8 12 16 17

29 31 34 37 37

Chapter 2. A-Paleoclimatic Interpretat ion of the Eocene Floras of Southeastern

North America DAVID

D ILCHER

Summary . . . . . Introduction . . . Individual components of the fl ora Identification to modern taxa Evolution of the fossil taxa . Relation of range and ecological tolerance. Community structure . . . . . . . . . . . . Stability through time . . . . . . . . . Separation into equivalent modern communities Foliar physiognomy . . . . Leaf-margin analysis . . . . . . . . Leaf-size analysis . . . . . . . . . Interpretation of foliar physiognomy . Application to this nora Additional evidence Wood . . . . . . . Pollen . . . . . . . Paleoclimatic interpretations References . .

40 41 4t 42

43 44 44 45

46 48

50 53 54 55 55 55 56 57

Chapter 3. G eographical Relationships ofthe Flora of Mexican Dry Regions J. RZEDOWSKI Suillmary . Introduction

61 61

f X

CONTENTS

CONT ENTS

Relationships between diOcrent arid zones of Mexico Relationships with other arid zones of the world Relationships with zones of non-a rid climate . Conclusions References . . . . . . . . . . . : . â&#x20AC;˘ .

63 1;6'

70 71

Clrapter 4. Ecology of the Vegeta tion of Veracruz ARTURO G6MEZ-POMI'A

Summary . . . . Introduction . . . Physical condi lions Topography Climate Soils . Vegetation . . Forests Selvas . High sclvas . Low selvas . Grasslands . Arid or semi-arid vegetation Secondary vegetation References . . . . . . . . . . .

73 73 73 73 75 85

89 94 105 106 118

128 135

145 147

Chapter 5. The Phytogeography and Vegetation ofChiapas (Mexico)

149

149 150 154

155 155 158 161 163 164

Chapter 6. The Vegetation of Panama: a Review D UNCAN M. PORTER

Summary . Introduction Climate Soils Man Vegetation .

Vegetation maps . . . Koppen macroclima tes Holdridge Life Zones Human innucnccs . . . Darien forest types . _ . A summary of plant comnumiti<.,-s Incipient Forest . . . _ Early Secondary Forest Late Secondary Forest Liltoral Forest . . . . Thorn Forest _ . . . . Deciduous Seasonal Forest . Savanna .. . . . . _ . S<tltwatcr Swamp Forest . Saltwater Riparian Forest. Brackish Swamp Forest _ llrackish Riparian Forest Freshwater Marsh . . . Freshwater Swamp Forest Frcshw<tter Riparian Forest . Monsoon Forest . . . . Evergreen Seasonal Forest Premontane Rain Forest Cloud Forest Elfin Forest Acknowledgements ~ References . . . .

17 1

171 172 173 174 175

175 177 179

182 183 184 185 187 187 187

189 189 190 192

193 197

197 198 199 199 200

Chapter 7. Phytogeographic History of the Isthmus of Panama during the Past

DENNIS E. B REEDLOVE

Summary . . . . . . . Introduction . . . . . . Phytogeography of Chiapas . Vegetational formations ofChiapas Description of the formations . Optimum forma tions. . . . Seasonal format ions . . . . Wet-land forest formations . Treeless formations References . . . . . . . . . . .

167 167 169 170

170 171

12,000 Years (A History of Vegetation, Clima te, and Sea-level Change) ALEXANDRA S. B ARTLETT AN D ELSO S. BARGHOORN Summary . . . . . . . . . . Introduction . . . . . . . . . Northern South America . Southern Central America Panama . . . . . . . Geological history of Panama .. Pre-Quaternary history . . Late Quaternary and Recent history Physiography' and climate of Panama . Climate of the Canal Zone The present vegetation of Panama Preparation ofsamples . . . . Preparation techniques . Preservation of samples The pollen diagrams . . . . . Theoretical considerations The Gatun Basin diagrams The modern pollen rain . Climatic change in Panama . . . Identification and nomenclature of the fossil pollen .

203 204 .-

204

206 207 208

208 209

211 211

212 216 219

219 2 19 2 19

222 225 228

229

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A. GOM EZ-POMPA

Gomcz-Pom ra, A., Hcrmindcz. P., L. and Sousa, S. M., l%4b. Es!udio fi!occoltigico de Ia cuenca in1crmedi:1 del Rio Papaloapan. Pub/. Esp. Ins/. Nac. !111·. For. Mh·., 3:37- 90. Gomez-Po m pa, A. and Ncvling, L. 1., 1970. La llora de Veracruz. Anales Ins/. Bioi. Uni••. Nac. lv/i!x., Ser. 13oL, 4 1( 1) : 1-2. Howard, R. A., 1968. The ecology of an elfin fo rest in l'tiCrlo Rico, I. ln!mduction and composition studies. J. Amold Arboretum, 49(4):38 1-41 &. Ja nzen, D. H., 1970. Jacquiuia fi/1/I.!feus, a heliophilc from the understory of tropical d eciduous forest. Biotropica, 2(2): 11 2- 119. Johnson, E. L., 1970. Mmfogcucsis y Clasifrcacivu de Algunos perfiles de Suelos deril•tulos de Ceuizas volcanicas del Pica de Orizaba, Puebla y Veracruz. Thesis Fac. Ciencias, UN AM, Mexico, 78 pp. leon, C. J . and Gomez-Pom pa. A., 1970 . La vcgelacion del surcstc de Veracruz. Pub/. Esp. lust. Nac. btv. For. Mex., 5:15-48. Lol, A., 1971. Estudios sobre fancr6gamas marinas en las ccrcanias de Veracruz, Ver. Anales Ins/. Bioi. Unil•. Nac. Mi:x., Ser. Bot., 4 2 (1): 1-48. Lundell, C. L., 1937. The vegetation of Pclen. Camegie lust. Wash. , Pub/.,478: 1- 244. Miranda, F., 1955. Fonnas de vida vcgelales y el problema de Ia delimitaci6n de las zonas aridas de Mexico. In: M esas lledonda.\· sabre Problema.,· de las Zonas tiridas de Mexico. Edic. lnst. Mex. R ec. Nat. Renov., Mexico City, pp.85-1 19. Miranda, F ., 1959. Posible significaci6n del porcentajc de gcneros bicontincnla les en America tropical. Anales lmt. Bioi. Unh•. Nac. Mex., 30: 117- 150. Miranda, F. and H ernandez X., E., 1963. Los lipos de vegelacion de Mexico y su clasificaci6n . Bot. Soc. B ot. Mh:., 28:29-179. Miranda, F. and Sharp, A. J., 1950. Characlerislics of !he vegetal ion in certain temperate regio ns of eastern Mexico. Ecology, 31 :313-333. Miranda, F., G6mez-Pom pa, A. and Hernandez, X., E., 1967. Un metoda para Ia invesligaci6n ecol6gica de las rcgioncs lropicales. Ana/es lnst. Bioi. Univ. Nac. Mex. , Ser. Bot., 38 ( 1):101-110. Mohr, E. C. J. and Van Baren, F. A., 1954. Tropical Soils. Interscience, Londqn, 498 pp. Mosii\o, A. P. and Garcm, E., 1966. Evaluaci6n de Ia sequia in!racstival en Ia Republica Mexicana. Uni6 n Geognifica lnlcrnaciona l. Confc rencia Regional Lal inoamerican a . Edic. Soc. ·Mex. Geogr. Estat!is., 3:500-516. Pennington, R. D. and Sarukh{m, J., 1968. Arboles tropica/es de M exico. lost. Nac. Invest. Forestales, Mexico, Mexico City, 413 pp. Ramos, C., 1971. Vegetacion de Ia Zona arida Veracmzana. Thesi; Fac. Ciencias, UNAM, Mexico, Mexico City, 30 pp. Rzedowski, J ., 1963. El exlremo boreal del bosque tropical siempre verde en Norteameric a Conlinenlal. Vegetatio, 11(4):173-198. Rzedowski, J. and McVaugh, R., 1966. La vege1aci6n de Nueva Galicia. Coutrib. Uni••. Mich. Herb., 9(1): 1- 123. Saru khan, J., 1964. Estudio succsional de un area talada en Tuxlepcc, Oax. Pub/. Esp. l ust. Nac. !nv. For. Mex., 3:107-172. Sauer, J., 1967. Geographic reconnaissan ce of seashore vegetation along the Mexican Gulf coast. Coastal Stud. Inst., La. State Univ., Tech. Rep., 56:59 pp. Solo, M., 1969. Consideraciones ecoclimliticas del Estado de Veracmz. Thesis Fac. Ciencias, UNAM, Mexico, Mexico City; 43 pp. Sousa, S. M., 1964. Estudio de Ia vegetaci6n secundaria en Ia region de Tuxtepec, Oax. Pub/. Esp. Just. Nac. !nv. For. Mex., 3:91-105. Sousa, S. M ., 1968. Ecologia de las leguminosas de Los Tuxtlas, Veracruz. Aua!es Just. Bio i. Uuiv. Nac. M ex., Ser. Bot., 39(1):121-160. T o ledo, V., 1969. Diversit!ad de £species en/as Se/vas altas de Ia Platticie Costera del Golfo de Mexico. Thesis F ac. Ciencias, UNAM, Mexico, Mexico City, 55 pp. Vazquez, C., 1971. La vegetaci6n de Ia laguna de Mandinga, Veracruz. Anates !nsf. Bioi. Univ. Nac. Mex., Ser. Bot., 42:49-94. Whiteside, E. P., 1960. Observations 011 Great Soil Groups in Humid Tropical Portions of Mexico. Rockefeller Found., Mexico (unpublished ). W illia ms, L. L., 1939. Arboles y arbustos del Istmo de Tehuantepec , Mexico, Lil/oa 4:137-171.

Chapter 5

The Phytogeography and Vegetation of Chiapas (Mexico) DENNIS E. DREEDLOV E Department of /Jot any, Cali}imtia Academ y of Sciences, San Francisco, Calif. (U.S.A.)

Summary Chiapas has one of the most diverse and la rgest flo ras of any of the Mexican states. The study of this vegetatio n is facilitated by reference to the seven physiographic regions outlined by Mulleried (1957). These regions have consistently sim ilar fl oristic association s. Th1 ee of these regions, the Central H ighlands, the Central Depression and the Northern Highlands , are restricted to Chiapas. T he Sierra Madre a nd the Eastern H ighlands extend southward into Guatemala while the P acific and Gulf Coastal Plains are southern extensions of large central Mexican regions. T he result is that many of the floristic associations i11 Chiapas are unique without wiclespread relationships. The classification of the vegetationa l formations of tropical America establi shed by Beard (1944) is remarkably adequate for t he vegetation ofChiapas . Some adjustment is necessary to account for the temperate forests of the Central Highlands and the Sierra Madre. T here are nineteen fo rmations described in this work and they are arranged into five formation series. The four optimum formations are characterized by a climate in which significant rain falls in every month of the year and a well drained substrate. The six seasonal formations also occur on well drained soils. .The climate, however, is marked by a distinct dry season of 2-6 months. The wet-land forest a nd treeless fo rmations are much less natural groups. T hey are in general narrowly restricted in distributio n and dependent on some special feature of the environme nt. The dominant and most common species of trees and shrubs are listed for each formation. Second growth formations are common and very d iverse, and one example of t hese is described.

Introduction Chiapas is the southernm ost state of Mexico, situated on the Pacific Coast between the Guatemala border and the Isthmus of Tehuantep ec, and occupying more than 74,000 sq. km. The present population of Chiapas is near 1,500,000, with almost one half of this total being Mayan agriculturalists living in dispersed hamlets. The climate is diverse, ranging from semi-desert to rainforest and fr om sea-level with stands of

150

D. E. DREEDLOVE

PIIYTOG EOGRAP HY AND VEGETA TION O F C HTAPAS (M EXICO)

mangroves to peaks as high as 4,000 111 with sHb-alpinc vegetat ion. There arc over 8,000 species of "f'ascular plants recorded from the state. The annual rainfall varies from under 800 mm in some localities to over 3,500 mm in others. ll is impossib le to estimate the percent of endemism for the state as a whole but there arc several areas in which local endemism is q uite high.

Phytogeography of Chiapas Following Mulleried (1957), Chiapas can be divided into seven physiogr aphic regions (see map, Fig. !). Florist ic associations within one of these regions are more similar to each other than the same floristic associations from two different physiographic regions. That is to say, two pine-oak forest communities from the same region are more alike than two pine-oak forest communities from two different regions. For this reason, the vegetational associations are best studied region by region, even though

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Fig.I. Map of Chiapas with the physiogra phic regions outlined.

151

there will be an overlap of plant associations. Many local endemics a1e restricted to a particular plant association with in a particular physiograph ic region. The regions and the plant associations fo und with in them arc: (/)The Pacific Coastal Plain- a narrow strip of metamo rph ic strata and intrusive rocks of Precambrian and Paleozoic age with several ranges of low hills. This region is flat and relatively dry in the north and hilly and wet in the south. Mangrov e swamps (a) with patches or coastal strand (b), fo rm a continuo us band along the Pacific Ocean. Tropical deciduous ro,:esl (c), palm fo rest (d), thorn woodland (e) , and shorttree savanna (/), are all quite common in the remaining inland areas. Evergree n and semi-evergreen seasonal fo rest (g) is fo und most abundan tly in the south. The plant associations found in th is region arc widespread types sca!lered along the Pacific Coast from Si na loa to Panama (Stand ley, J940). T here appear to be very few e ndemic species. (2) The Sierra Madre de Chiapas, which is parallel to the coast from the southern end of the Isth mus of Teh uantepec extending through Chiapas into Guatema la, is a narrow steep range of volcanic mountains ranging in elevation from I,500 m in the north to 4,000 m on the Guatem ala bord er. (a) Tropical deciduous fores t is common at lower elevations on both escarpments especially in the north, whereas (b) evergreen and semi-evergreen seasonal forest is restricted to riparian situation s in the north and middle elevations of both escarpments. (c) Lower montane rain forest is found only on the lower slopes of Volcan Tacana in the Soconusco district. Most of this Pacific rainforest has been destroyed or altered for cultivation of coffee. (d) Pi ne-oak forest is widespread at middle and higher elevation s in the north. It is replaced by (e) pine-oakLiquidambar forest, (f) montane rain forest, and (g) evergreen cloud· forest in the south. T here are many local endemics in t his region, especiall y in t he higher-el evation associations about Yolcim Tacana. Arclisia oJ•andensis Lundell, Buddteia o1·andensis Lundell, Clethra matudai Lundell, Deherainia matudai Lundell, Eugenia siltepecana Lundell, Quercus ovandensis Matuda, Quercus paxtalensis C. H. Muller, Symplocos tacanensis Lundell, and Z inowiewia matudai Lundell are a few of the conspicuous endem ic species. (3) The Central Depression (Valle Central: Helbig, 1964a, b; Depresi6 n Central: Miranda, 1952; Rzedowski, 1965). Beginning with the fast m oving streams flowing out of the Cuchumatan Massif, the Rio Grijalva descends slowly through a broad terraced valley from I,200 m at the Guatemala border to 500 m at the mouth of El Sumidero. This giant· basin is characterized by a dry climate (in some places, less than 800 mm annual rainfall). The strata are most ly marine limestones and slates. The depression terminates abruptly as the Grijalva flows into El Sumidero, a sheer faced canyon with walls up to 1000 m high. This dry valley(Contreras Arias,l955, refers to this area as semi-arid) is more than 200 km long and up to 70 km br oad . It is complete ly surrounded by moist, densely forested mountain areas, affording it complete isolation from other pockets of xeric vegetation. Quite understandably there are a large number

t52

D. E. BR EEDLOVE

PIWTOG EOGRA PH Y AND VEGETATION OF CHIAPAS (MEXICO)

of endemic plants, especially a mong the sh rubs and small trees occurring on dry, exposed slopes. Catoplieria chiapensis Gray, Erytllrina goldmanii Standley, Lop11zia /angmaniae Miranda, and Wimmeria acuminata L. Williams arc particularly conspicuou s endemics. · The flora of the Central Depression, although dry, and containing a number of Central Mexica n elements, has a st riking number of species from the d ry flora of Oaxaca completely missing, even though some of these species occur in the dry regions of eastern Guatemala (Ampllipterygium adstringe11s (Schlecht.) Schiede, Apoplanesia panicu/ata Pres!, Megastigma, Myrtillocactus, a nd Plocosperma). Another interesting facet of the flora of this regio n is the occurrence of many species known elsewhere only from t he dry regions of the Yucatan peninsula (Miranda, 1952; Clusia flm•a Jacq., Coccoloba cozumelensis Hems!., Dioon spinulosum Dyer, Guaiacum sanctum L., Guettarda combsii Urban, GJ'mnopodium antigonoides ( Rob.) Blake). Most of the valley was probably originally covered with (a) tropical decid u ~us forest; however, extensive cultivation and grazing has Jed to large tracts of (b) thorn wood l~nd , and (c) savanna. In certai n protected riparian areas, (d) everg reen and semi-evergreen seasonal fores t occurs, a nd (e) palm forest is CO!nmon o n alluvial flats in t he southe1 n end of the depression. (4) T he Chiapas Plateau (also commonly referred to as the Central H ighlands (Vogt, 1969), La Meseta Central de Ch iapas (Miranda, 1952) and Chiapas H ighlands (Goldman and Moore, 1945). A highland mass, about 220 km along its principal axis and from 50 to 100 km in width lies directly east of the Central Depression. It is composed mostly of marine limestones with extrusions of volcanic rocks on the higher peaks. The broad summit is from 2, 100 to 2,500 m in elevation with a few peaks reaching as high as 2,900 m. There is a mat ked d iffere nce in the rainfall between the east and the west escarpments and the vegetation reflects this difference. Tropical deciduous fo rest (a) and pine-oak forest (b) occu r on the drier west side, bordering the Central Depression. Pine-oak-Liquidambar (c) and montane rain forest (d) covel the eastern escarpment. The summits and eastern slopes of the higher peaks and ridges have a diverse evergreen cloud forest (e) which contains a large number of endemic species : Carex chiapensis F. 1. Hermann, Eugenia ravenii Lundell, Eugenia tonii Lundell, Hida/goa breed!ol•ei Sherif, Magnolia sharpii Miranda, Miconia ravenii Wurdack, Parathesis breedlo1•ei Lundell, Sah>ia querceto-pinorum Epl. and Jat. , and Symp!ocos exce/sa L. Wms. are a few of the more conspicuous ones. Another large group of species is restricted to t his region and the limestone areas of Huehuetenango, Alta Verapazand Baja Verapaz, Guatemala. This area supports a large population of Mayan-swidden agriculturalists. The 1960 census estimates the populatio n size conservatively between 250,000 and 300,000. T hese people live in a dispersed "vacant town" manner and cultivate most of the arable land available in t his region. As a result, very little primary forest is left. (5) Eastern Highlands. Eastward from the Central Plateau, thete are several ranges of steep mountains gradually sloping off into the drainage of the Rio Usuma-

cinta. The area is mostly limestone with some sandstone and volcanic extrusions. It ranges in elevation from 400 to 1500 m. The vegetatio n is for the most part uniform with lower mont ane rain fores t (a) being most common. T here are, however, patches of short-tree savanna (b) and pafm forest (c) scattered throughout the area. Everg reen and semi-evergreen seasonal fo rest (d) is com mon in d ryer situations in the northern portion of the area. Tropical rain fores t (e) is present in the flat areas surround ing the u pper drainage of the Rio Usumacinta . Monta ne rain forest (f) is present only on the crests of the highest ridges. In contrast to the Central Plateau, this area is very sparsely populated and large tracts of virgin forest remain. The florist ic associations present here are continuous with associations in the Peten 1egion of Guatemala, and the two areas taken together contain many endemic taxa. (6) T he Northern Highlands. To the north of the Central Plateau and the Central D epression, lies a series of steep ranges of volcanic origin but geologically quite complex. The chasm of the Rio Grijalva traverses this area with steep faced cliffs and high ridges on both ·sides. This area is bounded to the north and northwest by an extension of the Sierra Mad re which lies across the border in Oaxaca. Lower montane rain forest (a) occu rs along the bases of the ranges while montane rain forest {b) is common o n the higher ridges. Pine-oak-Liquidambar and evergreen and semievergreen seasonal forest (c) are comm on in drier sit uations in temperate and lowland areas, respectively. This region has no major~popul a tion centers and few roads; much of it can only be reached by fo otpat hs. It is quite understandably the most poorly k nown of the physiographic regions. (7) The Gulf Coastal Plain. Two small segments of this large physiographic region a re found in the northeast corner of Chiapas. The vegetation is mostly secon d growth f rom what once was t ropical rain forest (a) or evergreen and semi-evergreen seaso na l forest (b), but now is short-tree savanna (c) and second-growth tropical forest with scattered palm fo rests (d). Lundell (1942) and Williams (1938, 1939) discuss the p la nts of part of this area. Perhaps the most striking aspect of the Chiapas flora is the fact that most of the area of the state is contained within nart:owly restricted physiographic areas. This alone accounts for the large n umber o f taxa endemic to the state. Only two smal! portions of the state contain floristic associations which are closely related to similar associations in tile rest of Mexico. Both the Gulf Coastal Plain and the Pacific Coastal Plain have their vegetational associations continuously linked far to the north. Two other physiographic areas (the Sierra Madre and the Eastern Highlands) along with their vegetational associations extend to the south into Guatemala and El Salvador and are strictly Central American in aspect. The remaining three areas (the Northern H ighlands, t he Central Depression and the Central Plateau) are strictly C hiapas phenomena. The fl ora, therefore, although having elements of both Central Mexico and Central America, has an aspect quite its own.

!53

154

D. E. llREEDLOVE

PH YTOGEOGR APHY AND VEGETATION OF CHIAPAS (MEXI CO)

Vegetational formations of Chiapas The classification of the vegeta tional formations used in this account is strongly patterned afte r the one established by Deard (1944). Certain modification s a nd additions have been made in o rder to better describe the formations as t hey occu r in Chiapas. None of the available treatments of vegetation types o f Mexico which are applicable to Chiapa~ (Miranda, 1952 ; Miranda and Hernandez X., 1963; Wagner, 1964 ; Gomez Pompa, 1965 ; Pennington and Sarukhan, 1968) adequately take in to account Beard 's Fo rmatio n Series concept. This concept is essential to the classification presented here and it is felt that it g ives the simplest, most easily un derstood terminology for the tropical forest types of Chiapas. Where it is possible and clearly understanda ble, the forest types referred to by other authors for Chiapas a re listed after the appropriate formations. The form ations of Chiapas are arranged into fo ur formation-series and are listed diagrammat ically (Fig.2). Second-grow th fo rmations will be discussed briefly at the efi{j. Optimum formatiom occur on well-drained lands and have a habitat characterized by significant preci pitation in every month of the year. The four formati ons in this series can be arranged on an altitudinal grad ient from 300 to 3,500 111 and have broad areas of intergradatio n between them. Seasonal formations also occur on weli-

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Description of the formations

T hese fo ur formatio ns are all quite similar physiognomi cally, being composed of many layers of evergreen trees a nd having abundant epiphytes and !ian as. A dry season is absent or at most only a few weeks i n length and the forest floor never dries out. Deciduous leaved trees are quite uncommon and mostly occur in the zones o f intergradation with Seasonal formatio ns. B etween orie fi fth and o ne fourth of the forest a rea of Chiapas is covered with an Opt imum formation .

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d rained la nds bu t have a habitat characterized by alternate wet and dry seasons with the dry peri od lasting between 2 a11d 6 mont hs. The fo ur lowland for mations in this series can be arra nged on a gradient o f increasing arid ity and broad areas of intermediate forests occur. The two highland formations are also in response to greater aridity. T he remaining two formation-se ries arc admittedly artificial groupings, made for the sake of conveni.ence, and sharing only physiognom y in t!1e case of the treeless formations and standing water in the wet-land forest fo rmations. The formations within these series have litt le or no no ristic relationships and a re for the most part quite locally distributed and. dependen t on some special feat ure of the environ ment.

Optimum formations

e OPTI MUM FORMATIONS

!55

•VI'ULANO fOil Sf f OIMAttOHS UMPUtAU I *'AI.IAN fOifSf SWAII\P & lOWlAND II:IPAJt iAN ,OII:ESJ

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Fig.2. Diagrammatic representation of the vegetational formations of Chiapas.

{!) Tropical rain forest . (Selva Alta Siempre Verde in part : Miranda, 1952, 1957 ; Selva Alta Perenn ifolia in part: Miranda and Hernandez X ., 1963 ; Gomez Pompa, 1965; Pennington and Sarukhan, 1968.) Only in a few locations in the valleys bordering the Rio U sumacinta does a forest occur of t he proportions great enough to be called tropical rain forest. Here the forest is t hree-storied with the uppermost story composed of t rees with straight, unbranched, buttressed trunks ri sing as much as 50- 60111 from the forest fl oor (Miranda, 1961). The second story forms a continuous canopy and is between 25 and 40 m tall. The third story is composed of branched small trees from 10 to 20m tall. Small sh rubs and herbaceous cover are p ractically absen t and epiphytes are only in the up per stories. T hese forests are completely surrounded by lower montane rain fo rest, and intergrade into it. Some of the most common canopy trees include : AJpidosperma megalocarpon Muell. Arg., Brosimum a!icastrum Sw., Dialiwn guianense (Aubl.) Sandw., Erblichia xylocmpa Standi. a nd Steyerm., Guatteria anomala R. E. Fries, Manilkara ac!tras (Mill.) Fosberg, Poulsenia armata (Miq.) Standley, Swietenia macrophylla King, Terminalia amazonia (G111el) Exell. The understory trees and the lower trees of the canopy that are most abundant are Alchornea latifolia Sw., Alibertia edulis (Rich.) Rich., Belotia cambellii Sprag., Bume!ia persimilis Hems!., Bursera simaruba (L.) Sarg., Cassia grandis L.f., Blepharidium mexicanum Standi., Gaurea exce!sa HBK., Hasse/tia dioica (Benth.) Sleumer, Licaria peckii (I.M. Jo hnst.) Kosterm., Orthion subsessile (Standi.) Standi. and Steyerm.,

!56

D. E. BREEDLOVE

Pithecelfobium arborewu ( L.) Urb., Quararibea funebris (La Llave) Visch., Sickingia sah·adorensis (Standi.) Standi., Wimmeria bartleffii Lundell, Zue/(llria guidonia (Sw.) Britt. and Millsp. (2) L ower montane rain forest. (Selva Alta Sicmpre Verde in part: Miranda, 1952, 1957; Selva Alta Pcrennifolia in part: Miranda and Hermindcz X., 1963; Gomez Pompa, 1965; Pennington and Sarukhan, J968'.) Inasmuch as most of Chiapas is momitainoLtS, this formation is much more co mmon than tropical rain forest. Most of the Eastern Highlands and large sections of the Northern Highlands along with a narrow band on the western base of the Sierra Madre are covered with this formation. The Sierra Madre forests have been cut or altered for the production of coffee, while those of the Eastern and Northern Highlands remain mostly virgin. This formation is physiognomically quite similar to tropical rain forest but lacks the uppermost story extending above the canopy. The canopy layer is 25-45 m high. Lianas and epiphytes are much more coinmon and epiphytes are spread throughout all of the forest layers. The forest floor is covered with a dense thicket ofspinescentpalms, cycads, low-branched shrubs and lianas making travel qn anything but established trails impossible. Most of the canopy and understory trees present in the tropical rain forest are present here. There are many additional species, incl uding the following canopy trees: Be!otia mexicana (DC.) Sch um., Calophyllum brasi/iense Camb., Chaetoptelea (Ulmus) mexicana Licbm., Licmria platypus (Hems!.) Fritsch, Nectandra sinuata Mez., Ocotea rubrif/ora Mez., Quercus o!eoides Cham. and Schlect., Quercus skinneri Benth., Sebastiana longicuspis Standi., Ta/auma mexicana (DC.) Don, and Vochysia hondurensis Sprague. Some of the more common understory and canopy trees include in addition to those cited for tropical rain forest : Chrysophy!lum mexicanum Brandeg., Cleidion oblqngifolium (Standi.) Croizat, Cymbopeta/um pendulif/orum (Dun.) Baiu., Faramea occidenta/is (L.) A. Rich., Pseudo/media spuria (Sw.) Griesb., Sloanea terniflora (M oe. and Sesse) Standi., Stemmadenia donnel!-smithii (Rose) Woods, and Troplris racemosa (L.) Urb. (3) Montane rain forest. (Selva Mediana y Baja Siempre Verde, in part: Miranda, 1952, 1957; Selva Mediana o Baja Perennifolia, in part: Miranda and HermindezX., 1963; G6mez Pompa, 1965; Pennington and Sarukhan, 1968.) Along the upper slopes and crests of all the ridges in the Eastern and Northern Highlands, along the eastern escarpment of the Central Highlands and along both slopes of the southern part of the Sierra Madre, occurs this formation composed of three or sometimes two layers of trees and a dense shrubby understory. Epiphytes including a profusion of mosses are characteristic of all levels of this formation. Epiphytic ferns, orchids, bromeliads, aroids, and many other groups are best developed here of all forest formations. Herbaceous plants are quite common on the forest floor. Montane rain forest can occur between 900 and 2200 m and is often associated with steep slopes. T he canopy

I'HYT EGEOGRA I'H Y AND VEGETATIO N OF CHIAPAS (MEXICO)

!57

is not nearly as continuous as that of lower montane rain forest and is often only ?_5- 35 m high although occasional taller trees arc common ly encountered. T he second layer is composed of shrubs and small trees between 5 and 15 m tall. The more common trees of the canopy layer are : Ardisia alba Lundell, Bnmel/ia mexicana Standi Hedyosmum mexicalll/111 Cordemoy, !vfatudea trinerria Lundell, Meliosma matuda; Lundell, Mosquitoxylon ja111aicense Krug. and Urban, Nectandra reticula/a (R. and Pav.) Mez, Oecopetalum mexicammr Grecnm. and Th oms., Oreopanax sanderianus Hems!., Oreopanax xa!apensis (HBK.) Dcnc. and Pl., Quercus peduncu!aris Nee, Platanus c!riapensis Standi., Rinorea guatemalensis (Wats.) Ba rth., Synardisia 1•errosa (Mast.) Lundell, Turpinia occidentalis Don. T he more common trees and shrubs of the understory are: Aca/yplra skwchii l. M. Johnst., Billia lrippocastammr Peyr., Cenlropogon cord(rtus Benth., Cephaelis axil!aris Sw., Cephaelis e/ata Sw., Clramaedorea concolor Mart., Eugenia rm·e11ii Lundell , Ham pea longipes Miranda, M iconia !auriformis Naud., Mo/linedia guatemalensis Perk., Oreopanax !iebmanii Marchal, Palicourea spp., Parathesis microca!yx Donn. Sm., Psychotria spp., Troplris mexicana (Liebm.) Bur. , Urera a!ceifolia Ga ud. (4) E••ergreen cloud forest. (Cloud Forest: Leopold, 1950; Selva Mediana y Baja Siempre Verde, in part: Miranda 1952, 1957; Selva Mediana o Baja Percnnifol ia, in part: Miranda and Hernandez X., 1963; Gomez Pompa, 1965; Pennington and Saru khan, 1968.) This formation consists of one to two layers of closely spaced trees and a dense shrubby understory. The canopy can be as tall as 40 m with straight smooth trunks in protected locations but it is often much lower especially on windswept ridges where it resembles the Elfm Woodland described by Beard (1944). Tree ferns are most common here although fl1ey a lso occur in montane and lower montane rain forests. There are a great many narrowly endemic species in t his fo rmation. Well-developed examples of this forest are scarce and rapidly disappearing from pressure of man. At present it occurs only about the crests of the highest peaks or the Central Highlands between 2,000 and 2,900 m and also in the Sierra Madre between 1,900 and 3,200 m. Lianas are al~ost completely absent in this formation; herbaceous vines, however, are quite abundant. Epiphytic angiosperms such as orchids and bromeliads are uncommon a nd limited to a few species. Mosses hang in sheafs from the branches and epiphytic ferns are common. Fog and clouds seem always to be hanging over these forests and account for much of the precipitation that falls here. Frost is common in January on cleared or exposed land adjacent to these forests, although the temperature rarely, if ever, drops to 0° centigrade on the forest floor. Common trees of the canopy are: Abies guatemalensis Rehder, Acer negundo L., Chiranthodendron pentadactylon Larreatequi, Clethra lana/a Mart. et Gal., Clet!rra oleoides Wms., Drimys granadensis L., Magnolia slrarpii Miranda, 0/medie/la betschleriana (Goepp.) Loes., Oreopanax capita/us (Jacq.) Dcne. and Planch., Persea donnell-smitlrii Mez ex Donn. Sm., Persea schiedeana Nees, Photinia matudai Lundell, Pinus ayacahuite Eh renb., Quercus acatenmrgensis Trel., fVeinmannia pinna/a L.,

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PHYTOGEOG RAPH Y AND VEGETATION OF C HIAPAS (MEXICO)

Wimmeria driapensis Lundell. Common shrubs of the understo ry incl ude: Carendishia guatemaleusis var. cl!iapensis (Brand.) L. Wms., C!eyera tlteaoides (Sw.) Choisy, Deppea grcmdijfora Sc hlecht., Eugenia tonii Lunctcll, Fuchsia micropliy/la su bsp. aprica (Lundell) J3recd l., Fuchsia paniculata Lind!., Gent/ea micrantha (Donn.Sm.) Lundell, Jvfahonia paniculata Forst., Miconia rarenii Wurdack, Rapanea juergensenii Mez, Saurauia oreopl!i/a Hems!., Symplocos limoncil/a H. ct B., Vaccinium conferlum 1-1BK., Viburnum blandwn Morton.

incl ude: Amyris cliiapensis Lundell, Ardisia escallonioidC's S. and C., Bourrerialruanita (-la Llavc ct Lex.) Hems!., Clusiaf!am Jacq ., Eugenia acapulcemisStead., Ca/yptrantiles chiapensis Lundell , Gentlea renosissi111a (R. and P.) Lundell, Kanvinskia ca/deronii Stand i., P~>ychotria erytilrocarpa Schlec ht.

Seasonalfornrations Well over hal f of the forest area of C hiapas consists of Seasonal formations. These format ions are q uite diverse physiognomically but are fl oristically related and form bands of intergradation. Characteristically, they are open fore sts wit hout a closed canopy. Some percentage of deciduous trees occur in all of the formations. The forest floor does not have a continuous understory ; however, shrubs are common. Lianas are uncommon or totally absent in most of these fo rmations, whereas epiphytic a ngiosperms are abundant and can become more prominent than jn the optimum formations. !3piphytie mosses are poorly developed. ' ( 5) E1â&#x20AC;˘ergreen and semi-el'ergreen seasonal forest. (Selva Alta S u bdec"id ua: Miranda,

1952, 1957; Selva Alta o Mediana Subperennifolia: Miranda and Hernandez X., 1963; Gomez Pompa, 1965; P ennington and Sarukhan, 1968; Selva Alta o Mediana Su)Jcauducifolia: Miranda and Hernandez X., 1963; Gomez Pompa, 1965; Pennington and Sarukhan, 1968.) Although often split into two formations, the zones of intergradation are so large as to make the distinction meaningless. This is the first in a transitional series of forest types from tropical and lower montane rain fo rests to thorn woodland. Superficially, it is quite similar to the rain forest fo rmations but differs in havi ng only two and sometimes one layer of trees. The canopy is discontinuous and contains some percentage of deciduous trees, depending on the aridity of the locality. The fores t attains a height of 25- 35 m. The dry season is very marked and the forest floor becomes quite dry. There is a great seasonal variation in herbaceous plants. Lianas and epiphytes are usually quite abundant. This formation is common on the western and northeastern slopes of the Sierra Madre (up to 1,200 m), in the southern an d western portions of the Northern Highlands and on the Gulf Coastal Plain. Some of the common trees of this formation are: Astronium graveolens Jacq., Bernoullia flammea Oliver, Brosimum alicastrum Sw., Bumelia persimilis Hems!., Ceiba pentandra (L.) Gaertn., Calycophyllum candidissimum (Vahl) DC. , Cordia alliodora (R. et P.) Cham., Enterolobium cyclocarpum (Jacq.) Griesb., Ficus glaucescens (Liebm.) Miq., Guettarda combsii Urban, Hymenea courbaril L., Lafoensia punicaefolia DC., Licania arborea Seem., Platymiscium dimorphandrum Donn. Sm., Sapium macrocarpum Muell. Arg., Sterculia mexicana R.Br. , Tabebuia chrysantha (Jacq.) N ichols, Vatairea lundellii (Sta ndl.) Killip. Common shrubs and small trees

159

(6) .Tropica~ deciduous forest. (Deciduous Seasonal Forest: Beard, 1944; Selva Baja DCCJdua : M1randa, 1952, l 957; Selva Baja Cauducifolia: Mi randa and Hermindez x 1963; Penni nglon and Sarukhan, 1968; Bosque Tropica l Dcciduo: Rzedowski an~ McVaugh, 1966.) This is a diverse association of decid uo us and semi-deciduous trees which is common in dry situations from Sonora to Panama. This associat io n is normally between 10 and 20m high with a dense, thicket-like understory. H owever, in some protected semi-riparian situations the trees can be as tall as 40- 50 m with straight trunks and a low understory. T here arc many species of trees common in th is association and the dominants vary with the environment of the specific locality. Lianas and epiphytes are present but qu ite reduced in number. The dry season is very long (4-6 months) and severe. Almost all of the trees and understory pla nts lose their leaves. Many trees flower at th is time and a re qui te showy, but the general appearance is of complete dormancy. A few weeks after the rai ns begin this once-desert looks like a verdant jungle. Many hundreds of species of herbaceous plants sprout and cover the gro und, often producing a stand of 1-2 m high. In Chiapas one find s tropical deciduous forest prevalent along the Coastal Plain and the lower hills of the Sierra Madre. It is also common throughout much of t he Central Depression. Some of the common trees are: Bursera excelsa (HBK.) Engl., Bursera simaruba (L.) Sarg., Calycoplryl/um candidissimum (Vahl.) DC. , Cecropia pf!..llata L., Cedrela oaxacensis C. DC. and Rose, Coclrlospermum l'itifolium Willd. ex Spreng., Cordia a!liodora (R. and P.) C ham., Eysenlrardtia adenostylis Baill., Gliricidia sepium (Jacq.) Steucl ., Godmania aesculifolia (HBK.) Standi. , Hauya e/egans HBK., flura polyandra Bail!., L11elrea candida (DC.) Mart., Lysiloma aurita (Schl.} Benth., Pseudobombax e/liiJticum (HBK.) Dugand, Spondias mombin L., Tabebuia chrysantha (iacq.) Nichols., Tabebuia rosea (Bertol.) DC., Triplaris melaenodendron (Bert.) Standi. and Steyerm.

(7) Short-tree smâ&#x20AC;˘anna. (Sabana: Mirand a, I 952, I 957; Miranda and Hernandez X., 1963; Gomez P ompa, 1965; Pennington and Sarukhan, I968; Selva Baja Subperennifolia : Miranda and Hernandez X., I963; Vegetacion Sabanoide: Rzed owski and McVaugh, 1966; Savanna: Beard, 1953; Leopold, 1950; Lundell, 1934, 1937.) Confusion as to the application of the term "savanna" and the questioning of the nature of this type of vegetation concerning whether it represents a climax or a dis-climax in response to the activities of man, along with the great variety of associations and situations which fa ll under this term, has led to the adoption by tbe author of the above term to refer to a very abundant vegeta tional association of grasslan d and spaced low trees usually occurring on deep, poorly drained, soils. It is commonly found on gradual sloping plruns or on flat bottom lands. The trees are low and gnarled,

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D. E. l3REEDLOVE

and usually well spaced; Lhcy rarely attain a height of more than 20 m. In some situations (described by Miranda and .Hermindez X., 1963 as Selva Baja Subpercnnifolia) the trees will form a low fo rest with a grass unclerslory. Four species of Lrees, Byrsonima crassifolia (L.) DC., Crescell/ia cujete L. , Crescentia a/ala HBK. a nd Curate/la americana L., are Lhe most characterist ic of this formation. Piscidia piscipula (L.) Sarg., Ah·aradoa amorplwides Licbm., Acacia pennatula (S. and C.) Benth., and occasio nal species of palms also can be fou nd here, either singly, or in association with the other mentioned species.

(8) Thorn woodland. (Selva Baja Espinosa Cauducifolia: Miranda and Hern{mdez X., 1963; Pennington a nd Sa rukluin, !968; Bosque Espinoso: Rzedowski and MeVaugh, 1966.) This is a dense association of shrubs and low trees which is often quite uniform in appearance. The majority of the species present are armed wi th an assortment of spines, thorns and prickles. This fo rmation usually occurs on poor, rocky soils on slopes adjacent to stands of short-tree savanna of tropical deciduous fo rest. Thorn woodland is poorly developed in Chia pas, limited to the northern parts of the Central Depression and the Pacific Coastal Plain. It is, however, quite prominent in the Isthmus of Tehuantepec , just no rth of the state line. The more common associates are: Acacia farnesiana (L.) Willd. , Acacia pringlei Rose, Acacia cornigera (L.) Willd., Bauldnia a/biflora Britt. and Rose, Bauhinia pau/etia Pers., Casearia nitida Jacq., Cordia curassal'ica (Jacq.) R oem. and Schult., Croton guatema/ensis Lotsy, Diphysa fioribunda Peyr., Ja cquinia aurantiaca Ait., Piptadenia jfat•a Benth., Pithecel/obium dulce (Roxb.) Benth., and Randia aculeata L.

(9) Pine-oak-Liquidambar forest. ( Deciduous Forest: Miranda and Sharp, 1950; Bosque D eciduo : Miranda, 1952 ; Pine-Oak-Liquidambar Forest: Carlson, 1954; Bosque : Miranda, 1957; Bosque Cauducifolio: Miranda and Hernandez X., 1963, Pennington and Saruklu1n, 1968.) On the moist north and east escarpments of the Central Highlands and a t scattered localities in the Northern Highlands and on the eastern escarpment o f the Sierra Madre, occurs a diverse association with many species of deciduo us and semi-decid uous trees. Epiphytes are very common, with numerous species, and oaks are often so heavily laden that one can barely see the branches. The understory is variable, ranging from a dense association with many species of shrubs, subshrubs and vines, to a tall grassy expanse with scattered shrubs. The t rees are commonly between 15 a nd 35 m tall and can be widely spread; but usually are close enough for their crowns to form a continuous can opy. Many of the trees are quite broad-crowned. This association is always diverse and one never finds pure stands of one or two species. Some of the common species of trees are: BruneIlia mexicana Standi., Carpinus caroliniana Walt., Citharexy/um donnell-smithii Greenm., Clethra suaveolens Turcz., Cornus disciflora DC., Cupania dentata DC., Cassia oxyphylla HBK., Erythrina chiapensis Krukoff, Fraxinus uhdei (Wenzig.) Lingel, Liquidambar styraciflua L., M ontanoa hexagona Rob. and Greenm., Nyssa sylvatica

I'H YTOG EOGRAJ'HY AND VEGETATIO N OF CH I A PAS (MEXICO)

16 t

Marsh, Ostrya guatemalemis (W ink!.) Rose, Pinus clriapensis (Martinez) Andersen, Pinus montezumae Lamb ., Quercus candicans Nee, Quercus oocarpa Schicde, Quercus po/ymorpha S. a nd C., _Quercus sapotaefo/ia Liebm., Quercus segoriensis, Quercus skinneri Benth., Rhus striata Ruiz and Pavon, Saurauia scabrida Hems!., Styrax argemeu.1· Pres!, Turpinia occidentalis (Sw .) D. Don. The following sh rubs are common in the understory: Aplre/andra gigantiflora Lindau, Bacclraris trinenis (Lam.) P ers., Bocconia gracilis I-1 utch., Cestrum gualemalensis var. gracile Morton, Firclrsia micropll);lla s ubsp. quercetonrm Drecd l. , Gaultheria odorata Willcl ., Guamatela tuercklreimii Do nn.Sm. , Hibiscus bijirrcatus var. pi/osus Gurke, Liabum g/abrum var. hypo/eucum Cogn., Parathesis clriapensis Fern., Plrenax rnexicwws Wedel., Rapanea myi'icoides (Schlech t.) Lundell, Rlrus terebinthijo/ia S. and C., Solanum brachystaclrys Mart. and G al. , Symplocos /ongipes Lundell, Triwn{ella grandiflora Yah!, Verbesina perymenioides Sch. Bip., ViburnumJrartwegii Bent h.

( 10) Pine-oak forest . (Pine-Oak F orest : Leopold, 1950; Pinares and Encinares : Mirand a, 1952; Miranda a nd Hernandez X., 1963; .Bosq ue de Pino y Encino : Rzedowski and McVaugh, 1966.) On the dryer south and west slopes of the Central Highlands, and on the eastern escarpment of the Sierra Mad re, an open forest associat ion occurs, comprised of relat ively few species of trees. Mixed pines and oaks are most common; however, stands of p ure pine and pure oak occur in specialized edaphic situat ions. Trees range in height from 15 to 40 m a nd can be quite variable in their spacing. Epip hytes are sparse to common, but only heavy in canyon situations. The understory is usually herbaceous with occasional shrubs and often only low grassy patches between the trees. T his formation is predominan t between I ,300 and 2,500 m, with occasional associations occurring as low as I ,000 m. T he common trees are: Arbutus xalapensis HBK., Buddleia skutchii Morton, Crataegus pubescens (HBK.) Steud., Pinus michoacana M artinez, Pil1us oaxacana Mirov., Pinus oocarpa Schiede, Pinus pseudostrobus Lind!., Quercus acatenangensis Trel., Quercus corrugata H ook., Quercus crassifolia H. and B., Quercus mexicana H . and B., and Quercus rugosa Nee. T he followin g sh rubs are common in this format ion : Buddleia crotonoides A. Gray, Ceanotlrus coeruleus Lag., Clriococca p/raenostemon Schlecht., Garrya laurifo/ia Hartw., Holodiscus argenteus (L.f.) Maxim., Lippia chiapensis M o ldenke, Litsea neesiana (Schauer) Hems!., Mailonia l'ulcania Standi. and Steyerm., Monniria xa/apense HBK., Myrica cerifera L., Rhus schiedeana Schlecht., Senecio cristobalensis Greenm., Solamoil nudum HBK. , Solanum lrispidum Pers., Viburnum jucundum M orton. Wet-landforest formations (11) Swamp and lowland riparian forest. (Manglar, in part: Miranda and Hermindez X., 1963; Canacoital: Pennington and Sarukhan, 1968.) Along all of the lowland rivers, especially where the water is slow moving, in flat, periodically inundated ateas where water stands for some months of the year, and in fre sh-water swamps directly

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D. E . B RE EDLOVE

behi nd the mangrove swamp, this ve ry distinctive fo rest type occurs. The trees ca n often attai11 a heigh t of 40 m wit h a contin uous canopy, and blend into the surrou nding fo rest, but can also be as low as 15- 20 m and occur in discontin uous patches of promi nen t green in a sea of dryer vegetat io n. T he trees arc characteristically buttressed and often have st ilt roots and aerial roots resembling mangrove swamp. Stands of single or few species arc common in this formation. The understory is sparse and usually herbaceous. Some of the common trees found here are: Andira galeolfiana Standi., Bravaisia integerrima (Spreng.) Stand i. , Bucida brrceras L., Calophyllrrm brasiliense var. rekoi Standi., Haematoxylum campechiamm1 L. , Pachira aquatica Aubl., Salix chilensis Mol., Taxodium mucronatum Ten.

(12) Mangrore swamp. (Mangrove Swamp: Standley and Steyermark, 1945; Steyermark, 1950; Manglar: Miranda, 1952, 1957; Miranda and Hermindez X., 1963; Rzedowski and MeVaugh, 1966; Pennington and Saruk h1in, 1968.) Forming almost a continuous belt of low forest cover, th is formation inhabits the entire Pacific coast of Chiapas in brackish, swa mpy conditions. This swamp is several kilometers wide in the north, in the region of the Mar Muerto, but only a few hundred meters wide along the rest of the coast. The vegetation is uni form and dense with the-shrubs and trees with stilt roots and simple, thickly cutinized leaves. T he stands are usually 4-7 m high but occas i o ~ a ll y attain a height of 10-20 m. This association is widespread along the coast of tropica l America as far north as Sonora and Baja California. The dominants are: A ricennia nitida, Jacq., Conocarpus erecta L., Laguncu/aria racemosa (L.) Gaertn ., Rhizophora samoensis (Hochr.) Salv. (13) Pa!m.forest. (Palmar: Miranda, 1952, 1957; Miranda and Hermlndez X., 1 963~ Rzedowski and McVaugh, 1966.) Along sandy alluvial flats and terraces of the upper portion of the Central Depression and along the Rio Usumacinta and its tributaries in the Eastern Highlands and in scattered localities in the southern portion of the Pacific Coastal Plain, often in poorly drained soils, a forest occurs in which palms are the predominant tree with a sparse understory. (A much more dense u nderstory is Âˇcommon in these forests in the Eastern Highlands.) The palms reach between 24 and 40 m in height, and can occur qui te closely spaced. Some of the common savanna trees a re often associated with the palms. The dominant trees are: Saba/ mexicana Mart., Schee/ea liebmannii Becc., Schee/ea preussii Burrel. (14) Temperate riparian forest. Along streams above 1500 min the Central Highlands and the Sierra Madre, a number of trees and shrubs are common which do not occur in abundance in the surrounding forest. The understory is dense and often thicketlike. Trees are mostly between lO and 25 m tall and can occur in very pure stands. The most common species are : Acer negundo var. mexicanum (DC.) Standi. and Steyerm., Alnus arguta (Schlecht.) Spach, Alnus ferruginea HBK., Baccharis heterophyl/a HBK., Berchemia scandens (Hill) Trel., Cornus excelsa HBK., Crataegus pubescens (HBK.) Steudl., Cuphea hyssopifolia HBK., Platanus chiapensis Standi., Salix bonplandiana HBK ., S alix taxifolia HBK.

PHYT OGEOGRA PH Y AND VEGETATI ON OF C HIAPAS (MEXICO}

163

Treelessfonnations (15) Bunch grassland (Zacatonal : Miranda, 1952, 1957; Mi randa and HermlndezX., 1963.) At high elevations in the Central Highlands and in the southern Sierra Madre, there occur large stands often up to severa l hectares, of tall bunch grasses. The clumps are closely spaced and can be as tall as 2 m. The origins of this association may be secondary, but it appears to be quite stable. Common species are: Briza rotundata (HBK.) Hitchc., Bromus carinatus H. and A., Festuca ainpli.ssima Rupr., M1ddenbergia gigantea (Fourn.) Hitchc., Muh/enbergia macroura HBK., Muhlenbergia robusta (Fourn.) Hitchc., Stipa iclw (R. and P.) Kunth. , Stipa rirescens .HBK., Trisetum irazueme (Kuntze) Hitchc.

( 16) Paramo. (Paramos de Altura: Mira nda 1952, 1957; Miranda and Herm\nclez X., 1963) This is a low association of grasses and herbaceou s perennials foun d only around the summit of the Volcan de Tacana. Similar second growth associations occur in overgrazed areas at very high elevations in the Central Highlands. The associates arc: Alchemilla pinna/a R. and P., Arenaria bryoides Willd., Calamagrostis to/ucensis (H BK.) Trin., Draba ro/canica Benth ., Festuca tolucensis HBK., Gentiana pumilio Standi. and Steyerm., Gnap/wlium l'ulcanicwu I. M. Johns!., Haplopappus stolo11ijerus DC., Lobelia nana HBK., Luzula racemosa Desv., Pernellya tomasii Camp., Polenrilla heterosepala Fritsch, Viola na1mei Polak, Weldenia candida Schult., Werneria nubigena HBK. ( 17) Herbaceous marsh. (Tulare : Miranda and Hernandez X., 1963; Popal : Mi randa and Herm\ndez X., 1963.) This distinct herbaceous cover of shallow standing water has very different associations in temperate and lowland situations. ln the larger non-draining valleys of the Central Highlands and in local areas of the Sierra Madre a temperate marsh occurs which is often associated with bogs. Carex spp., Cfadium jamaicense Crantz, Cyperus spp., Juncus spp., Ludwigia peruriana (L.) Hara, Ly tlmon rulneraria Schrank, Rhynchospora spp., Scirpus californica (C.A. Meyer) Steudl., and Typha latifolia L. are common associates. Large tracts of the Gulf Coastal Plain and some smaller areas on the Pacific Coastal Plain are covered with a tropical marsh which in some areas becomes quite dry seasonally. In addi tion to many of the species p resent in the temperate marshes these lowland association s have the following common species: Calathea spp., Heliconia bihai L., Hymenachne amplexicaulis (Rudge) Nees, Leersia spp., Paspalum spp., Phragmites communis Trin., and Thalia genicula/a L.

(18) Coastal strand. (Cordon Littoral: Miranda, 1942.) On sandy portions of the Pacific Coast, there occurs a low, herbaceous association of dune plants. The dominants are : Distich/is spicata (L.) Greene, and Ipomoea pes-caprae L.

(19) Second-growth and successional forest, and shrub associations. Either for lumber,

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pasture, o r some crop, great ex panses of virgin forest arc presentl y being cut down in Chiapas. rn many areas strcccssional forests a r·c being cut for the plan ting of maize or for firewood and charcoal. These decimated lands recover slowly and it would take several centuries to replace the primary forest. Second-growth associations arc prominent and arc, in some areas, the dominant vegetation. T his account does not attempt to describe the myriad of possi ble successions and their component associations. One example to illustrate vegetation type is given here. When the pine-oak, pine-oak-Liquidambar and evergreen cloud forest associations are cut oiT, burned, planted to maize and, after a few yea rs, allowed to grow back, the rege neration begins with a scru b association which can last up to 10 years. This manperpetuated succession has bee n going on in this area for several thousand years. Many of the races of maize used arc narrowly restricted endem ics. This association is naturally quite variable in its composition depending upon the locality and the forest type which it is succeeding. £tis usually a quite un ifo rm cover of shrubs and herbaceous plants up to 3 m tall. Common species are: A butilon tridens Standi. and Steyerm., A cacia augustissima (Mill.) Kuntze, Arctostaphylos Iucida (Small) Standi., Bacc/wris I'Occinioicles H 13K., Cal/ia/l(fra grandiflora (L'Hcr.) Benth., C~l/iandra lwustoniana (Mill.) Kuntze, Ceanotlws coemleus Lag., Clibadiwn arboreum Donn. Sm., Coaxana ebracteata Rose, Lantana hirta Grah., LCIIztana lriJ1Jida HBK., Lippia hypo/eia Brig., Malrar·i scus arboreus Cav., Muelzlenbeckia tamnifolia (HBK.) Miessn., Paronia paniculata C-av., Polymnia maculata Cav., Rhamnus discolor (Donn.Sm.) Rose, Rhamrws nelsonii Rose, Rubus spp., Sahia spp., Tithonia diversifo/ia (H ems!.) A. Gray, Tithonia scaberrima Bent h., Vemonia leiocarpa DC., Vibumum /autum Morton.

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