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Home , Agenium, Aristideae, Bambuseae, Bothriochloa, Deschampsia elongata, Homolepis

TAXON 24(I): 53-66. FEBRUARY 1975

EVOLUTION OF GRASSESAND IN

Arturo Burkart*

Summary

This is a discussion of the South American grasslands from the standpoint of their evolution and composition. The tribes are considered in relation to climate, and grasses are classified as mega-, meso-, or microthermic with respect to their temperature requirements. The principal regions are three: (A) Tropical and Subtropical, which include the Llanos of the Orinoco River system and the Campos Cerrados of Central ; (B) Temperate, including the Pampa of and the Campos of ; and (C) Cold Country Grasslands, which are the of the high Andes and Patagonia, and also the Pairamos of and . Some attention is given to the floristic composition of each of these regions. The subject of endemism is dealt with, as well as the problem of disjunct distribution. Included is a discussion of changes brought about by agriculture and ranching in historic times, and what may be expected in the future.

INTRODUCTION The Gramineae, with about 6oo genera and some 6ooo species, is one of the largest families of flowering . It is a truly cosmopolitan group, and remarkable because of the capacity of its members to form the domi- nant vegetation over large areas of the earth's surface. The terms steppes, , prairies, pusztas, campos or pampas all refer to vegetation types in which grasses are dominant. To quote Ronald Good (1953; p. 53) "Pride of place must certainly go to the Gramineae . . ., the great family ... Not only do the grasses reach to the furthest land in the north and to the borders of Antarctica in the south, but their degree of distribution is usually particularly complete and continuous. Almost alone among the families of flowering plants they form the dominant element in vegetation over great areas of the world, and nearly everywhere else too, except perhaps in some forest regions, the proportion of these plants is high." South America, as we shall see, is no exception, having rich and extensive grasslands. If we include both pure and mixed types, or parkland, this subcontinent possesses nearly 3,500,000 square kilometers of grassland. Approximately a quarter of its entire area is, or was originally, natural grassland (with, of course, many herbaceous dicotyledons and members of other groups associated). In the other vegetation types: Tropical Rain- forests, Subtropical Forests or Woods, Pairamos, Puna, Catingas, Chaco, Monte, and Deserts or Semi-deserts, grasses are well represented, but not dominant. An exception is, however, the , which are very conspicuous in hot humid or mountainous forest regions, and even in the

* Instituto de Botinica Darwinion, Lavarden 0oo, Correo Martinez, San Isidro, prov. Buenos Aires, Argentina.

FEBRUARY1975 53 humid temperate forests of south-central (Parodi, 1945). Along with the Leguminosae, Rosaceae, Vitaceae, Solanaceae, Cruciferae, and Cucurbitaceae, the Gramineae are closely connected with history, pre- history, and the development of human civilization. Long before man appeared on earth, grasslands must have played a decisive role in the evolution of such important and diversified animals as the ruminants, equidae, and other herbivores - mainly in the Old World. In South America, grasslands developed extensively without supporting large herds of vertebrate herbivores - except perhaps during the Tertiary, with great extinct mammals - up to the time of the Spanish and Portuguese conquests. The influence of prehistoric man also was very limited. The natural grass- lands of South America are, therefore, a nearly undisturbed result of the great biological processes: evolution, competition, and migration, since their origins in the second half of the Mesozoic. During the entire Tertiary period, i.e. some 6o,000,000 years or more, the development of the grass- lands was governed by the principal environmental factors: climate, geol- ogy, soils, topography, and diverse biological influences (parasites, polli- nators, symbionts, etc.). In spite of the great number of publications devoted to grasses, innu- merable questions posed by the morphology, , genetics, cytology, anatomy, embryology, physiology, ecology, etc., of these plants are still unanswered. They inspired Agnes Arber's (1934) admirable book, "The Gramineae;" a work of timeless value. Modern grass systematics has changed substantially the schemes of Nees, J. C. Doell, Bentham & Hooker, E. Hackel, and A. S. Hitchcock, superseding the classifications based purely upon exomorphology and characters. It exemplifies the high- est development of an integrated or synthetic taxonomy, being a model not yet attained in other families. The modern system for the Gramineae (cf. Potztal in Melchior, Engler's Syllabus ed. 12. 1964) is the result of a combined effort of workers in the special fields mentioned above, and makes use of basic contributions by authors such as: N. P. Avdulov, R. J. Roshevitz, H. Prat, J. R. Reeder, L. R. Parodi, F. A. McClure, G. L. Stebbins, T. Tateoka, and others. New lines of research are developing, as for example the recent discovery of a number of entomophilous olyroid grasses in the tropical rain forest of northern South America (Soderstrom & Calderon, 1971). This changes the traditional picture of grasses which were thought always to be wind pollinated, and, moreover, breaks down the no less traditional concept of a correlation between petaliferous, showy flowers, with zoophily against perianth reduced "amentiferous" flowers and anemophily. Electron micros- copy, applied to plastids, pollen, and other cell structures, is able also to contribute substantially to a deeper understanding of the natural affinities or diversities.

PHYLOGENY AND TAXONOMY OF THE GRASS FAMILY With respect to origins, we may focus on the main processes of evolu- tion: mutation, hybridization, introgression, chromosome changes - the domain of genetics, the science of heredity and variation. Or we may seek to describe the actual historical developments and connections by which the different taxonomic groups of grasses (i.e. tribes, genera, species) origi- nated from extinct progenitors. This is properly phylogeny, but to arrive

54 TAXON VOLUME 24 at meaningful conclusions, we need a fairly complete fossil record. Unfortunately, this is not the case in the Gramineae. The oldest remains are dated, with doubt, as Jurassic (Bambusidium liasicum Heer; Poacites (?), according to Roshevitz, 1969). Several fossils are considered to be Cretaceous, and they include some genera recognized as present-day Arundo, Phragmites, (Gothan & Weyland, 1954). Two conclu- sions seem warranted: (I) the grass family appeared simultaneously with the early Angiosperms during the Cretaceous evolutionary explosion, and (2) their development and migration dates back to early and middle Tertiary times (Oligocene, Miocene, etc.). It continued vigorously during the Quaternary. From this later period, Roshevitz (1969) mentions many well known genera: Arundinaria, Setaria, Oryzopsis, Phragmites, , Festuca, Agropyron, Hordeum, Alopecurus, and Beckmannia. At the present time, however, paleontology has not revealed the multiple steps in the phy- logeny of the different grass groups. Even so, it is perhaps significant that the few earliest grass fossils, which have been identified with certainty, be- long to tribes or subfamilies considered primitive by modern agrostologists: Phragmitoideae (Arundo, Phragmites) and Bambusoideae (Bambusa, Chus- quea). In South America, several grass fossils have been identified. Menendez in Fittkau et al. (1969) reported Poacites from the Cretaceous, and Parodi & Frenguelli (1941) described a fossil of the from Chubut. Such remains demonstrate the very old and independent develop- ment of the grasses in our subcontinent. But again, the fossil record is so meagre that it is of slight help in reconstruction of the phylogenetic events. It remains for us, therefore, to utilize the indirect but classical method, for the construction of an acceptable phylogeny of the grass family. We must rely on comparative morphology in its broad sense, including anat- omy, dermatography, palynology, embryology, as well as cytogenetics, ecology, and phytochemistry, in our attempt to discover the natural affinities. To venture a "genealogical tree" is a risky enterprise, but I think the possibilities in this case are equal or better than in any other group. Roshevitz (I946, cf. translation, 1969) gives an interesting picture of grass phylogeny, beginning with the primitive Streptochaeteae and other tribes of the subfamily Bambusoideae. The ancestors of Gramineae can perhaps be defined as Proto-Commelinales (Farinosae, pro parte), especially from pre-Restionaceous and Flagellariaceous stock. Roshevitz' conceptions, revolutionary at the time, are still valid in the main lines, e.g., the prim- itiveness of "Phragmitiformes", (now Phragmitoideae) is generally con- sidered to be a connecting link between Bambuseae and the three (now four) great, more modern and herbaceous groups, at present called (Festucoideae), Eragrostoideae ("Eragrostiformes") and ("Pan- iciformes"). From the latter I prefer to separate as a parallel, perfectly equivalent group, the Andropogonoideae (Tribes , Maydeae, and probably Arthropogoneae), founded by R. Pilger and E. Potztal. The characters of leaf anatomy and karyology led to the foundation of the intermediate subfamilies Phragmitoideae and Eragrostoideae, formerly included in the Pooideae, based on their gross morphology. These are indeed new concepts which connect the widely separated Pooideae with the Panicoideae and Andropogonoideae, which are at present, the most nu- merous and widespread Gramineae of the world. Time does not permit a more detailed discussion of the new and very

FEBRUARY I975 5J 5 natural of system the Gramineae (Parodi, I96I). I would stress, however, that it is founded on such a high degree of correlation of different characters, that there seems little doubt of the correctness of the con- clusions.

MEGA-, MESO- AND MICROTHERMICGRASS GROUPS I come now to the interesting relation between natural tribes and climate. With our new delimitations of subfamilies and tribes, there is such a close agreement between temperature requirements or preferences of their members, that climatic factors can be used as a component of the diagnosis. The basic influence of temperature on vegetation led Alphonse De Candolle to formulate the distinction between mega-, meso-, and micro- thermic cited C. plants (1874, by Troll, 1935). Roshevitz wrote in 1946 (translation 1969, p. io): "Finally we must note that the subfamily Pooideae is common in temperate and cold zones, while the subfamily Panicoideae [including the Andropogonoideael is typ- ical of tropical countries." Hartley (i95o), as a result of a statistical study, reported that the main tribes of Gramineae show a high correlation to a particular geographical area, either tropical or cold to temperate, being above normal in numbers in their own climatic area, or below normal in the other areas. By normal he refers to the percentage of the total grass flora. In my work on the grasses of the Argentine province of Entre Rios (Burkart, 1969), I adopted this order for the tribes because I think it serves admirably to characterize areas, especially if combined with hydric pref- erences (hydrophilous, hygrophilous, mesophilous, xerophilous). Rosengurtt, et al. (1970) group the Uruguayan grass tribes into those of "winter cycle" and "summer cycle." As Uruguay lies in the mesothermic area, this allows them to distinguish the two groups of micro- and megathermic origins.

I. Megathermic groups: from equatorial and tropical areas with 200C or more mean annual temperature, and over I5OC mean winter temperature. In South America, the equatorial and tropical vegetational zones are most extensive in the lowlands, mesetas, and lower sierras, between ca. 13W northern and nearly 280 southern latitude. But the Cordillera de los Andes is a formidable barrier which alters the climate through altitude, with profound influence on vegetation. In the Tropical Andes, the megathermics reach ca. I 5oo m, the mesothermics 15oo-35oo m, and microthermics ca. 3500-5000 m elevation, often reaching the line of perpetual snow. Under tropical or subtropical, i.e. megathermic conditions, which prevail over three-fourths of the South America's surface, the temperature factor loses importance and hydric conditions control the phases of vegetation. This is especially true in the savannas (llanos, campos cerrados, palmares). There is a wet season ("invierno") which produces a great development of grasses; and a dry season ("verano") which marks a maturation, seeding, and dispersal of the plants. Whith more general rainfall, where forests do not thrive, a continuous herb and grass flora flourishes the year round. Nearly the whole of Brazil, the Guayanas and , and Northern Argentina, as well as the extra-Andean parts of , ,

56 TAXON VOLUME 24 Ecuador, and Colombia have this type of vegetation. The grass tribes here exclusive or dominant are: Streptochaeteae (forests); Bambuseae (forests, along rivers, humid moun- tains); (forest understory); (swamps); (forests, savannas, swamps, dunes); Andropogoneae (campos, savannas, swamps, etc.); Arthropogoneae; Maydeae; (campos); Eragrosteae (campos); Chlorideae (campos); (campos).

II. Mesothermic groups: these are intermediate in temperature require- ments, and grow mainly in areas with io-200C mean annual temperature and 5-I5OC mean winter temperature. The following tribes, in whole or in part, are representative: Bambuseae (Chusquea-South Central Chile and Argentina); (Arundo, Cortaderia); Aveneae; Agrosteae; Triti- ceae; Phalarideae; Danthonieae; Pappophoreae; ; Sporoboleae; ; Chlorideae; Eragrosteae; Aristideae; Paniceae; Andropogoneae. Conditions for these mesothermic grasses prevail in the temperate zone in large areas of central Argentina, Chile, all of Uruguay, and the extreme south of Brazil (), between roughly 280 and 400 south latitude. Because of the Cordillera's influence, the zonation is much dis- torted from the northwest to the southeast. Rainfall diminishes steadily from east (I200-900 mm per year) to the west, where semi-desert condi- tions prevail (5oo-Ioo mm per year). Here the grasslands disappear and are replaced by the xerophilous, shrubby "monte" vegetation. The principal grassland of this zone is the Pampa, covering more than a half million square kilometers of plains, with a character in the drier west, and prairie physiognomy in the east. To the north, the Gran Chaco has much grassland between woods and palm groves. The same is true for the Argentine Mesopotamia (Entre Rios, Corrientes) and part of the extensive Campos of Uruguay and Rio Grande do Sul, altogether another half million square kilometers of land.

Winter and Summer Phases in the Mesothermic Grassland Zone. The natural Pampas and Uruguayan Campos consist of a mixture of originally megathermic and microthermic grasses. The former are plants which flower in summer and autumn; the latter flower in spring, seeding in early summer. In various parts of the year, therefore, the Pampa presents very different aspects. In spring (September-November) the domi- nants are Poa, Bromus, , Briza, Piptochaetium, Hordeum, and Cala- magrostis; in summer (January-March) representatives of the following genera are in flower: Paspalum, , Bothriochloa, Schizachyrium, Digitaria, Setaria, Chloris, and Eragrostis. Many Paniceae, Andropogoneae, Chlorideae, and Eragrosteae of megathermic, neotropical origin are repre- sented by numerous adapted species. The mild winters with temperatures seldom falling below o0C permit them to complete their life cycles. There- fore, during the summer and autumn the vegetation has a subtropical aspect. Conversely, other mesothermic grasses of microthermic affinities find the winter temperature favorable for vegetative growth, and after flowering and seeding in the spring pass the hot season in a more or less dormant stage. In the annual species, this difference is even more pronounced, and we have two kinds of these grasses in the temperate zone, which extend farther

FEBRUARY 1975 57 from their original home than the perennials, except for some weedy ones: WINTER ANNUALS SUMMERANNUALS Poa annua Digitaria sanguinalis Vulpia sp. Setaria viridis Koeleria phleoides Setaria verticillata Loliummultiflorum Eragrostis pilosa Hordeummurinum Eragrostis virescens Hordeum euclaston adscensionis

Soil conditions or exposure may alter the general scheme. In Entre Rios, near the subtropical boundary, on the high shores of the Uruguay River, I found Andropogoneae flowering in spring (September), which is exception- ally early. In South Brazil, on the "campinas altas" or in humid forests, some species or the micro- or mesothermic genera (Bromus, Poa, Festuca, Stipa) flower during the summer.

III. Microthermic groups: from cold temperate or cold climates, with less than IooC mean annual temperature and less than 5OC mean winter temperature. Members of the following tribes, with a predominance of those belonging to the Festuceae, are to be found here. It will be noted that with the excep- tion of the Arundineae, all belong to the subfamily Festucoideae: Arun- dineae, Festuceae, , Aveneae, Agrosteae, Stipeae, Phalarideae. The distribution of these grasses is restricted in tropical and subtropical South America to the high Cordillera, at a considerable altitude above sea level. At more southern latitudes they are to be found in the phytogeo- graphical provinces called Andina, Chilena Central, Patagonica, and Subantarctica, with considerable transgressions into the neighboring Monte, Pampa, and even Uruguayan provinces (winter/spring vegetation). Only in South Patagonia, with lower temperatures, owing to latitude, do they form grasslands down to sea level. The dominants are species of Festuca, Poa, Bromus, and Stipa, with admixtures of Hordeum, Hierochloe, Trisetum, , Agropyron, Elymus, , Alopecurus, Pol- ypogon, Phleum, and other cold-climate, mostly widespread genera (Soriano, 1956; Parodi, 1941, 1949; Dimitri, 1972). In Patagonia, natural grasslands occur only in the restricted sections in which aridity is not extreme, mainly in the Subandean, Western, and Fuegian districts. The genus Stipa plays an outstanding role in South American meso- and microthermic grasslands, and is absent in the megathermic ones. With some 80ospecies in our subcontinent, Stipa is one of the largest grass genera, and is especially important because of having developed many xerophilous species, adapted to the prevailingly arid conditions in western mountainous and southern temperate regions.

THE THREE MAIN GRASSLAND DOMINIONS OF SOUTH AMERICA In accordance with the preceding climatic groupings of South American grass tribes and genera, and somewhat independent of current geobotanical subdivisions, we may now proceed to enumerate and to mark the limits of the main grass regions (see map): A. Tropical and SubtropicalGrasslands. This vegetation is of the savanna type

58 TAXON VOLUME 24 SOUTH AMERICA No. 103 80 70 60 so 40

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Map i. South America's main vegetational divisions and grassland regions.

FEBRUARY1975 59 and is dominated by megathermic grasses with a variable admixture of woody plants, shrubs, trees, or palms, which never form dense groves. 1) The Llanos of the Orinoco River system in Colombia and Venezuela. 2) The Campos Cerrados of Central Brazil, when of the more open type. 3) The Swamp grasslands with hydrophilous or swimming grasses (such as Hymenachne amplexicaulis, Panicum elephantipes, and Paspalum repens); the Marajd Island area at the mouth of the Amazon; the Gran Pantanal in Matto Grosso, where the Paraguay River has its sources, and the Delta of the Parana' (Burkart, 1957). 4) The South Brazilian Campos, northern parts, including the humid lower Campinhas or small grasslands surrounded by the forest, mainly with acid soils. 5) The Tropical and Subtropical East-Andean Grasslands, which extend on a narrow tract at medium altitudes on the humid eastern slopes of the Andes, above timberline, from Venezuela and Colombia to Bolivia and northwestern Argentina. Everywhere in this domain there is strong competition between woody plants, tending to the natural forest climax, and grasses and herbs, which build up the savanna. Climatic or edaphic conditions have determined the existence of sa- vannas. Grasses are better adapted to a long dry period than are trees, and the latter require better and deeper soils. Through human activities (grazing, burning, etc.) the natural savannas have been extended, favoring the grasses, and forests are becoming more and more restricted.

B. Temperate Grasslands. This is the dominion of the mesothermic grass groups. Geographically, they occupy much of the vast plains between the western bush- lands and the Atlantic Coast in Argentina and Uraguay. Ecologically, these plains seem to be an inmense ecotone where typical grasslands developed with genera and species of mixed origin: mega- and microthermics. Many species are endemic. This domain includes the La Plata River grasslands, composed of two regions: 1) The Pampa (sometimes called the Humid Pampa, to exclude the Monte bushlands), in the main Argentine provinces of Buenos Aires, eastern La Pampa, and southern parts of C6rdoba, Santa Fe, and Entre Rios provinces. 2) The Campos of Uruguay and southern Rio Grande do Sul. Together these constitute one of the richest regions in the world for Gramineae. Fortunately, this grass flora is now fairly well known. In recent years three Floras have appeared: Burkart, 1969; Rosengurtt, Maffei, & Izaguirre, 1970; Cabrera, 1971. These cover Entre Rios, Uruguay, and Buenos Aires, respectively. The number of different species growing wild or naturalized in the three territories is 553, many of which are common to the different political divisions.

C. Cold Country Grasslands. A region of microthermic Gramineae, which embraces from north to south the whole extension of the subcontinent, but in the tropics only a narrow strip at high altitude on the Andean chain: 1) The Paramos of Venezuela, Colombia and Ecuador, with Espeletia (Com- positae) dominating. This high altitude, cold, and humid formation is not properly grassland, but is rich in Gramineae. 2) The high Andean Mountain Steppes of Peru, Bolivia, and northwestern Argentina. 3) The Patagonian-Fuegian-Falkland Steppe, mainly in the subandean districts.

FLORISTIC COMPOSITION OF THE THREE GRASSLAND TYPES

The specific composition of the different grasslands, according to authors who have studied them, confirms the theory of temperature preferences of tribes and genera. Within each of the climatic zones, the presence of forest or grasslands (under conditions not disturbed by man) depends more on rainfall quantity and distribution, or on edaphic factors, than on tempera- ture.

60 TAXON VOLUME 24 Megathermics: Los Llanos (Venezuela and Colombia). A typical example of neotropical savannas. Mean Annual temperature27.50 C; annual variation ca. 2.5OC. Rainfall 800-i200 mm mean per year. Principal dominant Gramineae (cf. Luces, 1942; Blydenstein, i962; Velasquez, 1965). Figures following the generic names indicate the number of species when more than one. Oryzeae: Leersia, Luziola, Oryza (2). Arundineae:Cortaderia (3), . Eragrosteae:Eragrostis (5). Chlorideae: (2), Chloris (3), Gymnopogon (2), Leptochloa, Trichloris(2). Sporoboleae:Sporobolus (5). Pappophoreae:Pappophorum. : Aegopogon (2), Tragus. Aristideae: Aristida (4). Paniceae: Anthephora, Axonopus (3), Brachiaria,Cenchrus, Digitaria (2), Echinochloa, Echinolaena, Eriochloa (2), Homolepis, Hymenachne, Isachne, Leptocoryphium, Mesosetum (2), Oplismenus,Panicum (3), Paspalum (6), Pennisetum, Reimarochloa, Setaria (4), Thrasya. Arundinelleae:Arundinella (2). Andropogoneae: (4), Diectomis, Elyonurus (2), Erianthus, Eriochrysis, , Hyparrhenia (2), Imperata, Sorghastrum, (4). Maydeae: Tripsacum.

The Campos Cerrados (Central Brazil). Savannas. Brazil, occupying nearly one-half of the land surface of South America, has a rich grass flora which is still imperfectly known. The only attempt at a comprehensive treatment of the Gramineae is that of Doell in Martius' Flora Brasiliensis (1871-1878), which is badly out of date. Forests are extensive and varied, but through human influence - agriculture, grazing, and burning - many areas have been converted to artificial or seminatural grasslands. The original grass formations in Brazil extend over parts of the territory of , , Sao Paulo and Goias, but they are of the savanna type with trees, palms, or small woods interspersed. Modern authors (cf. Ferri, 1973) consider that many campos left without human influences revert to secondary natural forest after some 30 or 40 years. The grass flora is very rich, and therefore it is impossible to give here a list of species. The dominance of Andropogoneae and Paniceae is very apparent. Species of Panicum and Paspalum are very abundant, and more than 20 other genera of Paniceae occur in this region. Several African grasses are cultivated for forage and have become established: Melinis minutiflora, "capim melado"; Hyparrhenia rufa, "yaragui", Panicum maximum, "guinea" or "coloniao; Brachiaria mutica, "Pari", Rhynchelytrum repens, "capim natal", Pennisetum clandestinum, "kikuyu," etc. These naturalized exotics demonstrate ecological affinities between Brazil and tropical . Moreover, there are floristic similarities, mainly on the generic level (Jacques- Felix, 1962).

Guayana Highland Savannas These differ from Llanos in being tableland of up to 1000 m altitude and of very old geological structure. The grass flora is still poorly known. (cf. Lemee, 1955).

Mesothermics:

The large temperate plains grassland regions, Argentine Pampa and Uruguayan- South Brazilian Campos.

FEBRUARYI975 61 1. There are many descriptions of the Argentine Pampa grasslands. Parodi (1930) published the fi'rst comprehensive study on the more northern sector, enumerating 110 grass species, native and naturalized. He was aware of the seasonal cycle which I have already discussed. Cabrera (1971) describes 374 species of Gramineae for the entire province of Buenos Aires. A large proportion of these extremely complex natural prairies of the Pampa except in stony or very low soils have been destroyed through agriculture. Even in 1920-1930, Dr. Parodi had difficulty in locating examples in the virgin state. Due to the richness of the soil, the humid Pampa is the most productive agricul- tural land in Argentina. Today in these areas, one can travel for miles and see only fields of cereals, alfalfa, or European weeds. Natural reserves are the mountainous enclaves in southern Buenos Aires province, called by geologists Ventania and Tandilia (Sierras de la Ventana and Sierras de Tandil). Here a rich native grass flora persists, with the addition of endemics such as Poa iridifolia Hauman and Bromus bonariensis L. R. Parodi & Camara. Farther south we find -grass steppes with Stipa brachychaeta Godron dominating. To the north- east, the depression of the Rio Salado, recently described by F. Vervoorst (1967), has a natural grass cover of Distichlis spicata (L.) Greene and D. scoparia (Kunth) Arech. In these areas, plowing is not feasible but overgrazing has heavily affected these grasslands.

Microthermics:

The Cordillera and precordillera region along the Patagonian Andes is an im- portant zone for the study of the struggle, during recent geological periods, between forest (the Nothofagus communities) and steppe (mainly Festuca grass- lands). Through pollen analysis and other data it has been possible to demonstrate in Magallanes, Chile, that the steppe during the Pleistocene extended farther west, where today there is only Nothofagus forest. Where humidity is sufficient or irrigation possible, all of the best forage grasses of central and northern Europe grow well and are being established for the improvement of agriculture. In the high Andes of western Venezuela (cf. Luces, 1942), common genera are: Aciachne, Agrostis, Bromus, Calamagrostis, Cinna, Danthonia, Festuca, Helleria, Hierochlo?, Poa, Stipa, and Trisetum. In Ecuador, Peru, and Bolivia, also in the high Andes with moderate humidity, (cf. Hitchcock, 1927; Tovar, 1957), common genera are: Aciachne, Agropyron, Agrostis, Alopecurus, Anthochloa, Briza, Bromus, Calamagrostis, Danthonia, Dissanthelium, Elymus, Festuca, Hordeum, Muhlenbergia, Nassella, Poa, Stipa, Triniochloa, and Trisetum. Another region includes Patagonia, Fuegia, and the Southernmost Islands. According to Boelcke (1957), Soriano (1956), and Roig (1964), the majority of species are members of the following genera: Agropyron, Agrostis, Alopecurus, Bromus, Cortaderia, Danthonia, Deschampsia, Distichlis, Elymus, Festuca, Glyceria, Hierochlo,, Hordeum, Koeleria, Phleum, Poa, Polypogon, Stipa, Trisetum, and Vulpia.

GRASS GENERA ENDEMIC TO SOUTH AMERICA*

There are relatively few grass genera endemic to South America, most of them being common to two or more continents. Notwithstanding this fact, the endemic genera indicate that South America may be a very old center for the origin of Gramineae. The number of endemic species, however, is

* I acknowledge the generous help of the Argentine agrostologist, Dra. Elisa Nicora, in preparingthis list.

62 TAXON VOLUME 24 rather large. This suggests that the process of speciation has been very active for long geologic periods within our subcontinent.

Aciachne Bentham, high tropical Andes, Venezuela to Bolivia and Nw Argentina. A. pulvinata Benth. Tribe: Stipeae. Agenium Nees, South Brazil, Paraguay, Bolivia, and Argentina. One species: A. villosum (Nees) Pilger. Tribe: Andropogoneae. Anthaenanthiopsis Mez ex Pilger, Paraguay, Bolivia, Northern Argentina (Misiones, Jujuy). 4 species in diverse habitats. Tribe: Paniceae. Chusquea Kunth, humid forests, mainly tropics but also in the Patagonian Cordillera of Chile and Argentina, to Lake Buenos Aires (460 S. Lat.) in cold humid climate. C. culeou E. Desv. Tribe: Bambuseae. Erianthecium L. R. Parodi, known only from Uruguay, on hills and stony campos. One species: E. bulbosum L. R. Parodi. Tribe: Festuceae. Kunth, forests and riversides of tropical regions. Tribes: Bambuseae. Hemimunroa (L. R. Parodi) L. R. Parodi, Chile, Puna de Atacama, Bolivia, and adjoining Argentina, on arid sandy stony soils. One species with one variety: H. andina (Phil.) L. R. Parodi and var. breviseta (Hack.) L. R. Parodi. Tribe: Eragrosteae. Lamprothyrsus Pilger, 3 species in Argentina; 2 in Peru and Bolivia. Tribe: Danthonieae. Lorenzochloa J. & C. Reeder, one species: L. erectifolia (Swallen) J. & C. Reeder in the high Andes of Venezuela, Colombia, and Peru. Tribe: Stipeae. Spreng., several species. Tribe: Bambuseae. Nassella E. Desv., Andean genus of ca. 9 species in Ecuador, Peru, Bolivia, Chile, and Argentina; most species in Chile. Tribe: Stipeae. Neobouteloua Gould, one species: N. lophostachya (Griseb.) Gould, in west central Argentina (Tucuman to Mendoza), and Chile. Tribe: Chlorideae. Oplismenopsis L. R. Parodi, one species: 0. najada (Hack. & Arech.) L. R. Parodi, Rio de la Plata region. Tribe: Paniceae. Otachyrium Nees, mainly Brazil, several species, one in Argentina. Tribe: Paniceae. Relchela Steud., Chile and western Patagonia in Argentina. One species: R. panicoides Steud. Tribe: Festuceae.

BOREAL INFLUENCES IN THE GRAMINEAE OF SOUTH AMERICA The megathermic tribes and genera of grasses in this subcontinent show evident affinities with the paleotropical groups of Africa (cf. Jacques-Felix, 1962), which suggests a possible ancient land connection. If the Gramineae developed in the Mesozoic, as fossil remains seem to prove, the present distribution of so many nearly cosmopolitan genera in both the New and the Old World, is best explained by Alfred Wegener's classic theory of Continental drift, now modernized and accepted by most geologists. From the original stock, which was already considerably diver- sified, the present grass flora developed during the Tertiary and Quartenary. This resulted in more localized species both to the north and to the south of the equator. Long distance migration is another possible explanation for the similar- ities of the flora of South and ,and even with . In fact, what seem to be boreal influences along the Andes Cordillera, are very evident in the tribes classified as micro- or mesothermic in the present paper. Temperate or cold slopes and valleys along the Andes, at medium or high altitudes, allowed many genera the sort of migration sometimes called "mountain hopping." Whether this process proceeded only from north to south, and not inversely, is hard to say. For instance, Stipa, Festuca, Poa, Calamagrostis, Hordeum, and other genera probably

FEBRUARY 1975 63 of holarctic origin, developed very important secondary centers of poly- morphism in western and southern South America. The Andes may have been the indispensable migration bridge. Some examples are given below:

GRAMINEAE WITH DISJUNCT AREAS: UNITED STATES AND ARGENTINA a) Disjunct Genera MonanthochloW Engelm. Two halophilous species, one in North America: M. littoralis Engelm., surrounding the Gulf of and in ; the other M. acerosa (Griseb.) Speg., in central Argentina, surrounding the Salinas Grandes, a remnant of an ancient epicontinental sea, and down to . Blepharidachne Hackel. Two species in the United States: B. kingii (S. Wats.) Hack. in and California, and B. bigelovii (S. Wats.) Hack. in . B. benthamiana (Hack.) Hitchc. in Argentina (Mendoza, San Luis). Munroa Torrey. Four species: M. squarrosa (Nutt.) Torr., Rocky Mountains to Texas; M. decumbens Phil., Puna region in S Peru, N Chile, SW Bolivia, and NW Argentina. M. argentina Gris. and M. mendocina Phil. in NW and W central Argentina. Tridens Roem. & Schult. Fourteen species in the United States and Mexico; two in Brazil and eastern Argentina: T. brasiliensis Nees ex Steud. and T. hackelii (Arech.) L. R. Parodi. Erioneuron Nash. Five species in the United States and Mexico. In western Argentina 2 species with 5 varieties: E. pilosum (Buckl.) Nash and E. avenaceum (H.B.K.) Tateoka (Jujuy to Rio Negro). Scleropogon Phil. Monotypic: S. brevifolius Phil. Southwestern United States, northern and central Mexico; Argentina: arid zones of the Precordillera in Mendoza and San Juan Provinces. b) Disjunct Species Phalaris angusta Nees. United States: , , Texas, California. Brasil, Chile, and Argentina: Buenos Aires, Entre Rios, Rio Negro. Muhlenbergia torreyi (Kunth) Hitchc. United States: Kansas and to Texas and . Argentina: Cordoba, San Luis, La Pampa. Also in Mexico and Puerto Rico. Sporobolus cryptandrus (Torr.) A. Gray (= S. subinclusus R. A. Phil.). United States and Northern Mexico. Argentina: Mendoza, Cordoba, San Luis, La Pampa, Buenos Aires, and Rio Negro. Deschampsia atropurpurea (Wahl.) Scheele. Greenland, Labrador, Alaska, United States (Colorado, California). Argentina: from Neuquen southwards to the Magellan Strait. Habitat: alpine, humid prairies in cold regions of both hemispheres. Deschampsia flexuosa (L.) Trin. Greenland to Alaska; Eurasia; United States. Ar- gentina: Patagonia to the Islas Malvinas (Falkland Islands). Deschampsia caespitosa (L.) Beauv. Greenland to Alaska; United States. Argentina: along the southern Cordillera, Mendoza to Punta Arenas (not to Fuegia). An Arctic and antarctic cosmopolite. Deschampsia elongata (Hook.) Munro ex Benth. Western North America, Alaska, to Mexico. Andes of Chile and Argentina: Neuquen to Santa Cruz. Calamagrostis (Deyeuxia) neglecta (Ehrh.) Gaertn., Meyer, & Schreb. Eurasia, North America. Argentina, southern Andes, Rio Negro to Tierra del Fuego. Willkommia texana Hitchc. The var. texana endemic in Texas. In Argentina, prov. Buenos Aires, the var. stolonifera L. R. Parodi. Stipa speciosa Trin. & Rupr. California: Chile, semideserts in the North; Argentina: Patagonia. Bouteloua aristidoides (H.B.K.) Griseb. United States and Mexico, in the arid regions of the West: western Argentina. Bouteloua barbata Lag. United States, arid West, and Mexico; western Argentina. Trichloris crinita (Lag.) L. R. Parodi (= T. mendocina (Phil.) Kurtz). North America: Texas, Arizona, and Mexico; northwestern Argentina, western Chaco, very common and important for recovery of pastures in the arid zone.

64 TAXON VOLUME 24 Panicum urvilleanum Kunth. Typical psammophyte with long creeping rhizomes, in semiarid habitats: Southern California; Peru, Chile, western and central Argentina. Eriochloa gracilis (Fourn.) Hitchc. Southern United States and Mexico. Argentina: Cordoba, Salinas Grandes. Setaria leucopila (Scribn. & Merr.) K. Schum. Southwestern United States and Mexico. Argentina: southwestern C6rdoba, eastern La Pampa and southwestern Buenos Aires provinces.

Finally, "Grasses and Grasslands in South America" is a fascinating research field, and much is yet to be done. No doubt there are still unknown species to be discovered and described. Much can be learned concerning the evolution and relationships of the various genera and species. Our present knowledge of the ecology of tropical and subtropical grasslands is scanty, as is information which will permit us to determine, in many cases whether the vegetation is primary or secondary. The Gramineae are, indeed, an extremely successful group. In South America, as in other parts of the world, repre- sentatives are to be found in nearly every plant community.

Acknowledgement I thank Prof. John R. Reeder for his interest and for his critical reading of my manuscript.

References

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