By David A. Neill
Studies of the vegetation of Ecuador were initiated nearly 200 years ago. Indeed, it may be said that Alexander von Humboldt (1807) founded the scientific disciplines of vegetation ecology and phytogeography following his travels in Ecuador and other regions of tropical America with Aimé Bonpland during 1799–1804. Humboldt's famous illustration of the vegetation zones in the equatorial Andes, from the tropical lowlands to the páramos and glaciated peak of Chimborazo Volcano, is reproduced on the cover. His descriptions of the changes in vegetation as one ascends a tropical mountain, and his comparisons with the similar vegetation changes observed as one travels from the equator to the poles, were seminal concepts in the history of biogeography (Botting, 1973).
Since the time of Humboldt, a number of botanists have published descriptions of the vegetation and phytogeography of continental Ecuador, including L. Sodiro (1874), Diels (1937), Acosta-Solís (1969b, 1976), Harling (1979), and Cañadas (1983). A discussion of the vegetation and phytogeographical patterns in the high Andes above 2,400 m elevation is found in Jørgensen and Ulloa Ulloa (1994). Wiggins and Porter (1971) and van der Werff (1978) described the vegetation of the Galápagos Islands. Recently, a collaborative group, including geographer R. Sierra and botanists C. Cerón, W. Palacios, and R. Valencia, devised a new classification system for the vegetation of mainland Ecuador, using floristic and climatic information that has become available in the past 20 years, as well as remote-sensing (Landsat) images and Geographic Information Systems (GIS) technology. The Ecuadorian group has produced a draft version of the vegetation system, including a vegetation map (Sierra, 1997) currently being revised for publication. The classification system is hierarchical and generally follows the terminology for vegetation units, as well as the hierarchical concepts, used by Huber and Alarcón (1988) for a vegetation map of Venezuela, and by Huber (1995) for the vegetation of the Venezuelan Guayana.
The new vegetation classification system and map for Ecuador by Sierra and collaborators are being adopted by the Ministry of the Environment for purposes of conservation planning, gap analysis, and management of national parks and reserves. For such purposes, the new system is an improvement over earlier vegetation studies in Ecuador because at the lowest hierarchical level, it recognizes differences in regional floristic composition among vegetation types of similar physiognomy and structure. Once published, the system will provide a new framework for studies on the vegetation of Ecuador, and the vegetation map will be widely used. Since it is a work in progress, we cannot use the vegetation system of Sierra and collaborators in this Catalogue.
For purposes of this Catalogue, we use Harling's (1979) account of vegetation formations of Ecuador and reproduce his map on the right back endpaper. Harling's scheme is simple, with 16 main vegetation types, and is sufficient as a framework for the brief discussion of vegetation types in Ecuador that follows.
Mangrove formations occur in the salt- and brackish-water tidal zone of river estuaries and bays along coasts. The largest extent of mangrove is found in northern Esmeraldas, in the area around San Lorenzo, in the large estuary of the Santiago and Cayapas Rivers; another large area of mangrove is in the Gulf of Guayaquil. The mangrove stands are dominated by Rhizophora harrisonii and R. mangle; on the landward edge of the stands the other common species of neotropical mangroves are found: Avicennia germinans, Laguncularia racemosa, and Conocarpus erectus. Pelliciera rhizophorae is found in the northern mangroves, but not in the Guayas-area stands. The Esmeraldas mangroves are among the tallest and best developed in the Neotropics, with Rhizophora trees over 30 m in some stands (Acosta-Solís, 1959; Little & Dixon, 1969).
In the Esmeraldas area, landward from the true mangroves, are fresh-water swamps dominated by a few tree species such as Campnosperma panamense and Otoba gracilipes; also present are Pterocarpus officinalis and Mora megistosperma.
In the past 25 years, large areas of mangroves have been destroyed and converted to diked ponds for production of shrimp, most of which is exported. The mangroves in the Gulf of Guayaquil were the first to suffer extensive destruction, but in recent years the shrimp aquaculture industry has spread to the Esmeraldas mangroves, despite Ecuadorian laws that prohibit clearing of mangroves.
Coastal desert and semi-desert
Arid scrub vegetation occurs on the Peninsula of Santa Elena and adjacent areas of southwest Ecuador, where annual precipitation is generally less than 300 mm and the dry season lasts for nine months (see climate diagram, Figure 1). The vegetation is composed of scattered columnar cacti, mostly Armatocereus cartwrightianus, and low shrubs and treelets including Capparis crotonoides, Loxopterygium huasango, Maytenus octogona, Bursera graveolens, and Vallesia glabra.
A similar arid scrub vegetation occurs in the lowland areas of the Galápagos Islands, with some of the same species that are found on the mainland of Ecuador, such as Bursera graveolens and Maytenus octogona, and also tall cacti including Jasminocereus thouarsii and Opuntia echios (van der Werff, 1978).
Savanna and deciduous forest
Savanna and deciduous forest occur in extensive areas of lowland coastal Ecuador where the annual precipitation ranges from about 800 to 1,200 mm and the dry season is about seven months long (Figure 1). The vegetation of these areas has been modified by human activities for so long that it is difficult to discern what the potential natural vegetation would be in the absence of human intervention, and the boundary between savanna and deciduous forest is not readily apparent. True savanna, in the sense of open grassland with scattered trees, is probably confined mostly to the alluvial plains with deep soils. Such areas were undoubtedly affected by human-induced fires, which helped to suppress woody vegetation and maintain the dominance of grasses, since the late Pleistocene. Deciduous forest with a relatively closed canopy and near absence of grasses is prevalent in the same region on the shallow, stony soils of the hillsides. These two vegetation types, therefore, occurred together as a mosaic, corresponding to the different edaphic types, in large areas of western Ecuador.
Historical records suggest that the lower Guayas River basin, between Babahoyo and Guayaquil, was originally a seasonally inundated grassland savanna, with forest on the flood-free hills that dot the plain (Hidalgo, 1998).
In the deciduous forest, virtually all of the trees and understory shrubs shed their leaves during the long dry season. Occasional individuals of Ficus with thick coriaceous leaves, usually found near watercourses, remain evergreen. The most conspicuous floristic element of the deciduous forest is the Bombacaceae tree Ceiba trichistandra with its grotesque, thick twisted limbs and trunk and green bark, which is photosynthetic through the dry season when the tree lacks leaves; the species is common throughout the dry forest zone from northern Manabí to Loja. Other Bombacaceae trees are also common, including Eriotheca ruizii, Pseudobombax guayasense, and west of Guayaquil, Cavanillesia platanifolia. Machaerium millei and Pradosia montana are other dominant trees of the deciduous forest. In the areas of savanna on the alluvial plains, scattered Mimosaceae trees are most common, including Samanea saman with its broad umbrella-shaped crown, and Pseudosamanea guachapele.
Descriptions of deciduous forest vegetation in western Ecuador are found in Kessler (1992) and Parker and Carr (1992). Josse and Balslev (1994) carried out quantitative forest inventories in this vegetation type, in one-hectare sample plots in Machalilla National Park.
Semi-deciduous forest formerly covered extensive areas of the central coastal plain in western Ecuador. This vegetation type has an annual rainfall ranging from about 1,500 to 2,500 mm (Figure 1), with a dry season of about three months; it is therefore intermediate in the moisture gradient between deciduous forest, which occurs mostly in southwestern Ecuador, and lowland rain forest in the northwest. This vegetation type corresponds approximately to "Tropical Moist Forest" in the Holdridge system (Holdridge, 1967; Cañadas, 1983).
In the Guayas River basin, with its fertile soils, the original forest was almost entirely cleared for agriculture, a process that was essentially completed during the 1950s and 1960s (Dodson & Gentry, 1991). A small remnant (about 130 ha) of semi-deciduous forest in the Guayas basin was preserved at the Jauneche Biological Station; a floristic study of Jauneche was published by Dodson et al. (1985). The largest remaining fragment of this vegetation type is probably a narrow strip, comprising several thousand hectares of undisturbed forest, along the coast between Jama and Pedernales in northern Manabí province (D. Neill, pers. obs.).
As the name of this vegetation type implies, some of the canopy tree species shed their leaves during the dry season while others retain them; among common species of the former group are Centrolobium ochroxylum, Erythrina poeppigiana, Gallesia integrifolia, Castilla elastica, and Pseudobombax millei; among the latter group of canopy trees are Brosimum alicastrum, Poulsenia armata, and species of Ficus. The canopy palm Attalea colenda and the understory palm Phytelephas aequatorialis are ubiquitous in the semi-deciduous forest. Both these palms are economically important, and in large areas of coastal Ecuador where this forest type has been cleared, they are virtually the only tree species left standing in pastures and agricultural plots.
Lowland rain forest
Lowland rain forest, in Harling's vegetation classification, covers the northern Pacific coastal lowlands below about 700 m elevation, including most of Esmeraldas and adjacent parts of Pichincha provinces, and small areas of northern Manabí and Los Ríos. This vegetation type also includes virtually all of the Amazonian lowlands east of the Andes. Climatically, lowland rain forest is characterized by annual rainfall in excess of 3,000 mm (Figure 1) and lack of a distinct dry season (i.e., generally, no more than one month with less than 100 mm precipitation). This is the most extensive vegetation type in the country, covering more than one-third of continental Ecuador.
The lowland rain forest is tall, dense, and evergreen, with a canopy height usually 30 m or taller, and high species diversity. Alpha diversity of trees, as sampled in one-hectare forest plots, is much higher in Amazonian Ecuador than in the rain forest area of the northern Pacific coast (Valencia et al., 1998). In lowland Esmeraldas, 110–120 tree species (with a minimum sampling diameter of 10 cm DBH) are typically found in one-hectare forest plots (Palacios et al., 1997). In lowland Amazonian Ecuador, upwards of 200–240 tree species (Balslev et al., 1987; Korning et al., 1991; Cerón & Montalvo, 1997; Palacios, 1997) and in one case over 300 species (Valencia et al., 1994, 1997; Balslev et al., 1998) are found in equivalent one-hectare samples. Density and diversity of epiphytes, however, are probably equally as high or even higher in the forests of northwestern Ecuador, as compared with lowland Amazonia; Gentry and Dodson (1987) recorded exceptionally high epiphyte diversity at Río Palenque in western Ecuador, with 127 epiphyte species in an area of only 0.1 hectare. In Amazonian Ecuador, Balslev et al. (1998) recorded a total of 172 epiphyte species in one hectare.
The lowland rain forest of northwestern Ecuador is coterminous with that of the Colombian Chocó region of the Pacific coast and shares many species with the Chocó, but there is also a significant element of endemic species that are not known north of the Colombian border. Many species are also shared with the rain forests of Amazonia and/or those of Central America. Floristic studies in this region include Dodson and Gentry's (1978a) flora of the Río Palenque Science Center, near the southernmost extent of this vegetation type, and Little and Dixon's (1969) work on the common trees of Esmeraldas province. An earlier forestry survey (Holdridge et al., 1947) described the lowland rain forest in the northwest as well as the other forest types in western Ecuador and the Andes; another early general description of forest vegetation and flora is found in Rimbach (1932). The flora, vegetation, and conservation status of the region are summarized in Neill (1997). Common canopy tree species in the region include Brosimum utile, Humiriastrum procerum, Dacryodes cupularis, Nectandra guadaripo, Virola dixonii, and Otoba novogranatensis; the palm Wettinia quinaria is abundant in the subcanopy.
Intensive floristic inventories in lowland Amazonian Ecuador during the past 20 years have greatly increased our knowledge of that region; much of the inventory work has been associated with petroleum development activities. Checklists of the trees of the region (Neill & Palacios, 1989) and of all flowering plants (Renner et al., 1990; see also Balslev & Renner, 1989) have been published, but are far from complete; species new to science and new records for Ecuador continue to accumulate each year for lowland Amazonian Ecuador as well as for other regions of the country. Vegetation studies in the region, besides the one-hectare forest plot samples cited above, include ecophysiological studies by Grubb et al. (1963) and Grubb and Whitmore (1966a, 1966b); inventory of ground vegetation beneath the forest canopy (Poulsen & Balslev, 1991); and forest dynamics (growth and mortality of trees) in permanent sample plots and transects (Korning & Balslev, 1994). A pilot study on mapping palm species distributions correlated with climatic parameters, using GIS technology, was carried out by Skov and Borchsenius (1997).
Several large-scale vegetation studies are currently being conducted in the Yasuní National Park region: a 50-hectare permanent plot in mature-phase forest (preliminary data on a two-hectare subsample in Romoleroux et al., 1997); mesoscale patterns of tree species diversity on different substrates (J. F. Duivenvoorden & collaborators) and a large series of one-hectare forest sample plots (N. Pitman). A general overview of the flora and vegetation of the Yasuní Park region is found in Herrera-MacBryde and Neill (1997).
Cedrelinga cateniformis is a common canopy emergent tree throughout lowland Amazonian Ecuador on well-drained sites; Ceiba pentandra is a frequently observed emergent on richer alluvial soils. Common tree species of the forest canopy in the region include Parkia multijuga, Hymenaea oblongifolia, Schizolobium parahyba, Dussia tesssmannii, Sterculia colombiana, Otoba parvifolia, Pseudolmedia laevis, Pouteria multiflora, and Erisma uncinatum. Subcanopy palms, especially Iriartea deltoidea and Oenocarpus bataua, are abundant. Extensive areas of swamps are dominated by nearly monospecific stands of the palm Mauritia flexuosa. Black-water inundated areas in the Cuyabeno region and the lower Yasuní River are dominated by a distinctive suite of tree species including Macrolobium acaciifolium, Astrocaryum jauari, and Coussapoa trinervia. The "white sand" podzolic soil type that is characteristic of parts of the Guayana and Brazilian Shield areas does not occur in Amazonian Ecuador, as far as is known, so the distinctive, oligotrophic Amazonian "white sand" vegetation (Pires & Prance, 1985) and its component taxa are not recorded from Ecuador. Extensive deforestation of the lowland rain forest has taken place in both western and eastern Ecuador, especially during the past several decades. Rudel and Horowitz (1993) carried out a sociological study of forest clearing by small farmers in Amazonian Ecuador. Sierra (1996) described and quantified deforestation in the northwestern lowlands using remote sensing data.
Lower montane rain forest
In Harling's vegetation classification, lower montane rain forest occurs on the western and eastern Andean slopes between about 700 and 2,500 m elevation. Following common usage of the term "cloud forest" elsewhere in the Neotropics, this zone as well as the forest above 2,500 m elevation may appropriately be termed "cloud forest" along with the upper montane rain forest (see discussion in Webster, 1995). The climatic and physiognomic hallmarks of cloud forest are present in this vegetation type: nearly constant high atmospheric humidity, frequent fog- and mist-associated precipitation, and dense loads of vascular epiphytes as well as bryophytes on tree branches and trunks. On the summits of the Pacific coastal range in western Ecuador, physiognomically typical cloud forest vegetation occurs as low as 400 m (Parker & Carr, 1992).
In general, alpha diversity of tree species, as well as general height of the forest canopy, decreases with increasing elevation on both sides of the Andes. On the eastern slopes, 132 tree species (DBH³ 10 cm) were recorded in a one-hectare plot at 1,200 m near the Sumaco volcano, and 45 species in a comparative sample at 2,000 m near Baeza (Valencia, 1996; Valencia et al., 1998). Density and diversity of vascular epiphytes in the lower montane rain forest zone are undoubtedly higher than in the lowland rain forest, but quantitative data are lacking.
Floristic studies of lower montane rain forest on the western Andean slopes have been carried out most thoroughly at the Maquipucuna forest reserve in Pichincha province (Webster & Rhode, in press). Common canopy tree species on the western slopes include Ruagea glabra, R. pubescens, Dussia lehmannii, Meriania tomentosa, Cinchona pubescens, Roupala obovata, and Nectandra acutifolia.
For the eastern Andean slopes, an overview of the flora and vegetation in the Sumaco volcano region of Napo province is described in Neill and Palacios (1997). Common canopy tree species in this vegetation type include the tall palm Dictyocarym lamarckianum as well as Erythrina edulis, Clethra fagifolia, Hyeronima macrocarpa, Ruagea glabra, Dacryodes cupularis, Metteniusa tessmannii, Meriania hexamera, and Ocotea javitensis.
Floristically distinct facies of lower montane rain forest occur on the non-volcanic substrates of the "third cordillera," east of the main Andean chain. These areas include the limestone massifs of the Cordillera de Galeras and Cordillera de Cutucú, and the mosaic of shale, limestone, and sandstone substrates of the Cordillera del Cóndor. All three ranges are, as yet, poorly known botanically, but each supports some endemic taxa. The Cordillera del Cóndor on the Ecuador-Peru border is probably the most diverse and probably has the most local endemics (Schulenberg & Awbrey, 1997); the Cordillera del Cóndor flora also includes several genera, including Pterozonium, Stenopadus, Phainantha, and Bonnetia, that are essentially disjunct from the "tepuis" of the Guyana Shield (Berry et al., 1995).
The moist woods on the volcanic slopes of the Galápagos Islands, for purposes of this review, are included in the lower montane rain forest vegetation type. The "Scalesia zone" and "Miconia zone" vegetation of the Galápagos (Wiggins & Porter, 1971; van der Werff, 1978) are certainly not "rain forests" like the lower slopes of the Andes, but the vegetation is equally influenced by fog-associated precipitation; the trees support mostly epiphytic bryophytes. The evolutionary radiation of the endemic tree genus Scalesia on different islands is one of the remarkable features of the flora of Galápagos.
It is ironic that Cinchona pubescens, a common tree of the western Andean slopes that was exploited in past centuries to the point of depletion in its native range for the extraction of quinine to treat malaria (Acosta-Solís, 1961), was introduced to the equivalent vegetation zone of the Galágapos and has become an aggressively invasive threat to the native Galápagos flora (MacDonald et al., 1988). Several papers on introduced plant species in Galápagos and their impact on the native vegetation are included in a volume edited by Lawesson et al. (1990).
Harling also uses the term "upper montane rain forest" for this vegetation type, which is in agreement with the terminology for Neotropical montane forests suggested by Webster (1995). Upper montane rain forest occurs on the high Andean slopes from 2,500 m elevation to the upper limit of closed forest, which is variable but frequently at 3,400–3,600 m elevation. The Spanish term ceja andina ("eyebrows of the mountains") is often used for the "elfin forest" near the upper limit of forest. With increase in altitude, the height of the tree canopy becomes lower, the trees are more twisted and gnarled and tend to be multiple-stemmed, and alpha diversity of trees also decreases (Valencia et al., 1998).
Quantitative vegetation studies of high-altitude Andean forests were carried out in a series of four one-hectare plots at sites ranging from 2,700 to 3,300 m elevation: two sites in northern Ecuador (Valencia & Jørgensen, 1992) and two in southern Ecuador (Madsen & Øllgaard, 1994; the four plots compared in Jørgensen et al., 1995). Tree species richness was much higher in the two southern plots (75 and 90 species) than in the northern plots (32 and 39 species); the northern sites may have been subjected to more recent disturbance. Structural characteristics, including density, canopy height, and basal area, differed considerably among the four study sites.
Characterization of forest types above 2,800 m elevation was carried out at 140 sites throughout the Ecuadorian Andes, with multivariate analysis of tree species composition and frequency (Fehse et al., 1998). Four main forest types were recognized, with 8 subtypes: 1) Forests in the central and southern Andes of Ecuador between 2,800 and 3,200 m, dominated by: 1a) Myrcianthes in Loja province; 1b) Myrsine, Ilex, and Weinmannia glabra; 1c) Clusia flaviflora, Weinmannia glabra, and Ruagea hirsuta; 2) forests throughout the Ecuadorian Andes at 2,800–3,600 m elevation dominated by: 2a) Weinmannia pinnata, Schefflera sodiroi, and Myrcianthes rhopaloides; 2b) Hedyosmum cumbalense, H. luteynii, and Oreopanax ecuadoriensis; 3) forests at intermediate altitudes (3,200–3,700 m) dominated by Hesperomeles ferruginea and Weinmannia fagaroides; 4) high-altitude forests (3,600–4,300 m) dominated by: 4a) Gynoxys acostae, Escallonia myrtilloides, and Buddleja; 4b) forests dominated by Polylepis.
A description of the montane forests and páramos of the Sangay National Park region in the eastern cordillera is found in Mena et al. (1997); montane forests of the "Huancabamba region" in extreme southern Ecuador and northern Peru are described in Young and Reynel (1997).
Montane forests in many areas of the tropical Andes occur on very steep slopes that are geologically unstable, being subjected to frequent landslides caused by earthquakes and other natural disturbances. Stern (1996) described vegetation succession on earthquake-triggered landslide sites in the eastern Ecuadorian Andes.
North Ecuadorian grassland and quebrada vegetation
This vegetation type occurs in the densely populated inter-Andean valleys, where the original vegetation was almost entirely removed during past centuries and replaced by agricultural plots and pastures. Remnants of the original vegetation are now found only in steep ravines (quebradas in Spanish) and other agriculturally marginal sites. These remnants are composed mostly of shrubs and small trees, often spiny, such as Barnadesia arborea, Mimosa quitensis, Hesperomeles obtusifolia, and Duranta triacantha.
The landscapes of the inter-Andean valleys today are dominated visually by stands of Eucalyptus globulus, introduced from Australia in the 1860s, which line roadsides and field borders and are also grown in silvicultural plots for timber production. In some areas there are large stands of Pinus radiata and Pinus patula, introduced from California and Mexico, respectively, around the turn of the 20th century. A study of the ecologial impact of the pine plantations indicated that on moister sites, in the northern valleys, the planting of pines results in reduced soil organic matter and moisture, but in drier sites, in valleys of the central Ecuadorian Andes, where pines were planted on eroded soils, the plantations protect the sites from further degradation (Hofstede, 1997). Large areas of the inter-Andean valleys are dedicated to grazing by dairy cattle, and the introduced African grass Pennisetum clandestinum, among other introduced grasses, predominates in most of the pastures.
Historical information, including municipal records and traveler's descriptions, allows a partial reconstruction of the original vegetation of the inter-Andean valleys at the time of European contact during the 16th century, and the changes that have occurred since then (Hidalgo, 1998). For example, the upper Guayllabamba River basin south and southeast of Quito (present-day Machachi area and Valle de los Chillos) was covered with a tall, dense montane forest at least until the 18th century. The floristic composition of these forests is not known, but probably included such canopy tree species as Cedrela montana, Juglans neotropica, Symplocos quitensis, Myrcianthes rhopaloides, and Inga insignis, which are still found in the area as isolated trees. The protected forest of Pasochoa volcano, south of Quito, is one of few remnants of inter-Andean forests. The valley floor of the upper basin of the Mira river in Carchi province was densely forested well into the 20th century; a remnant forest patch near San Gabriel, dominated by Myrcianthes rhopaloides, is still standing.
The Patate River basin—the region of Latacunga and Ambato—was not forested at the time of European contact, according to the historical records of Hidalgo (1998), but rather was characterized by low, open scrub vegetation, due to frequent disturbance from mudslides resulting from eruptions of the Cotopaxi volcano.
South Ecuadorian shrub vegetation
This vegetation type, like the previous one, has been profoundly altered by human activities. This montane scrub occurs in the inter-Andean valleys of southern Ecuador between 2,000 and 3,000 m elevation. The climate is generally drier than in the northern valleys, and the soil, derived from Tertiary volcanic substrate rather than Quaternary volcanic ash as in the north, is more highly weathered, poorer in nutrients, and in many areas has been heavily eroded. The vegetation is characterized by a discontinuous cover of shrubs and small trees, generally with bare ground between the woody plants. Common species include Oreocallis grandiflora, Lomatia hirsuta, Hypericum laricifolium, Bejaria aestuans, and Cantua quercifolia. Some species that are endemic to this vegetation type, especially Streptosolen jamesonii and Chionanthus pubescens, are frequently cultivated in the cities of northern Ecuador as ornamentals. Quantitative ecological studies of this vegetation type do not exist. It is not clear, from the information available, whether the original vegetation of the southern valleys and slopes was a closed-canopy forest as in the northern valleys, or if the open shrubby vegetation is in fact the "climatic climax" for these areas.
Dry scrub vegetation of southernmost Ecuador
This vegetation type is confined to the arid intermontane valley of the Chinchipe river, in southern Loja and Zamora-Chinchipe provinces. The vegetation is dominated by scattered low, thorny shrubs or small trees such as Acacia macracantha, Anadenanthera colubrina, Cercidium praecox, and Prosopis juliflora, as well as some columnar cacti and Opuntia.
Inter-Andean desert and semi-desert
This vegetation type occurs in the lower portions of most inter-Andean valleys, where precipitation is reduced due to the "rain shadow" effects of the surrounding high cordilleras. Annual rainfall in these deep, arid valleys is generally less than 300 mm (Figure 1). Among the valleys that have this type of vegetation are those of the Chota, Guayllabamba, Patate, Chanchán, and León rivers, and the Catamayo valley. In most sites, the vegetation is dominated by scattered low shrubs of Acacia macracantha. Other shrubs include Croton wagneri, Dodonaea viscosa, and Caesalpinia spinosa. The rosette-plant Agave americana is common on some slopes, as well as the introduced Aloe vera. Cacti are frequent on some sites, including Opuntia soederstromiana, O. pubescens, and O. tunicata. Species of epiphytic Bromeliaceae that are adapted to long periods of drought, including Tillandsia recurvata and T. secunda, occur frequently on the branches of the Acacia macracantha shrubs. On more humid sites, such as along watercourses, are small trees of Salix humboldtiana and Schinus molle.
This vegetation type, which occurs throughout the Ecuadorian Andes from about 3,400 to over 4,000 m elevation, is dominated by bunch- or tussock-forming grasses, mostly species of Calamagrostis as well as Festuca and, in the drier páramos of southern Ecuador, Stipa. Taller tussocks of Cortaderia are frequent at the edges of páramo, where it borders with patches of forest or shrubs, and in disturbed areas such as along roadsides. In between the grass tussocks grow a diverse assemblage of herbaceous plants, some prostrate and some erect, including species of Halenia, Gentiana, Gentianella, Ranunculus, Geranium, Castilleja, Lupinus, and Valeriana. Scattered small shrubs such as Chuquiraga jussieui, Baccharis caespitosa, and Lupinus pubescens also occur amid the bunch-grasses.
The giant stem-rosette plant Espeletia pycnophylla forms extensive populations covering thousands of hectares that dominate the bunchgrass páramos of Carchi province in northern Ecuador near the Colombian border, on both the western and eastern cordilleras. Very small disjunct populations of Espeletia pycnophylla, not more than 5 hectares in extent, occur 200 km to the south, in the Llanganates region of the Eastern Cordillera. Rosette plants of the genus Puya are more widespread throughout the bunchgrass páramos, especially on moister sites. The vegetation of the Chiles volcano, on the Ecuador-Colombia border, and nearby volcanoes in southern Colombia, is described by Rangel and Garzón (1997).
In very wet areas of the Eastern Cordillera, instead of Calamagrostis bunchgrass páramos are dense thickets of the erect dwarf bamboo Neurolepis aristata. Such thickets most notably cover extensive areas in the Llanganates region.
Most grass páramo areas are burned annually, or at least every few years, by fires set deliberately by the local inhabitants in order to maintain pasture for beef cattle and sheep. All of the páramo plant taxa, therefore, possess adaptations that enable them to survive the frequent fires (Lægaard, 1992); such adaptations include the ability to resprout from fleshy roots or rhizomes, seeds that germinate after fires, and in the case of rosette plants, protection of the apical bud. As Lægaard (1992) pointed out, these adaptations must have evolved long before anthropogenic fires began to have an impact on páramos, within the past 10,000 years or so. The morphological and physiological characteristics that enable páramo plants to survive frequent fires probably evolved as adaptations to other factors such as drought and diurnal temperature fluctuations.
In many areas, the grass páramo is interspersed with copses of small trees up to 4,100 m elevation. These fragments of upper montane forest are composed mostly of Polylepis and also include a few other tree taxa such as Oreopanax, Buddleja, Clethra, Baccharis, and Gynoxys. These copses occur generally on microsites that appear to be protected from fires, such as on scree and boulder slopes and in small hollows. The occurrence of tree patches does not appear to be related to any differences in soil structure or topography from the areas of grass páramo (Lægaard, 1992).
The ecological dynamics of the lower altitudinal limit of grass páramo and the upper limit of montane forest is a matter of great interest that has not been entirely resolved as yet. Most authors agree that the grass páramo is highly influenced by human activities, particularly by the frequent human-caused fires. Lægaard (1992) argued that since copses of trees occur up to about 4,100 m elevation, the "true timberline," i.e., the physiological upper limit of continuous tree cover, is at that elevation, and virtually all of the grass páramo below about 4,100 m is a fire-induced disclimax. If fires were suppressed, woody vegetation would invade the grass páramo and replace it with a continuous upper montane woodland of Polylepis and other small trees. This hypothesis appears reasonable but has not yet been put to a rigorous experimental test in Ecuador. Circumstantial evidence for a lower natural limit of grass páramo may be found in Løjtnant and Molau (1982) who surveyed a "virgin" páramo on the isolated summit of Sumaco volcano. This remote site has never been subjected to human intervention—burning or grazing. The páramo community on Sumaco extends from the upper limit of elfin forest at 3,300 m, to the summit above 3,700 m. The flora of the Sumaco páramo is depauperate, dominated by Cortaderia nitida and Blechnum loxense. Although this site has not been subjected to human-caused disturbance, lightning-induced fires may affect its vegetation dynamics.
Shrub and cushion páramos
Shrub and cushion páramos occur at elevations above the grass páramo, generally at 4,000–4,500 m. Bunch-grasses begin to decrease in density at about 4,000 m and are replaced by cushion plants, acaulescent rosette plants, and low shrubs. These sites are mostly on the slopes of the Quaternary stratovolcanoes of northern and central Ecuador. The vegetation cover is generally not continuous; bare sandy soil is exposed between the individual plants.
The cushion plant life-form is a very notable feature of this vegetation type. Cushion plants have very small sclerophyllous leaves, and are densely branching with short internodes, so that a dense, pillow-like mound is formed. The cushion plant form is evidently an adaptation to the nightly frosts; the surface of cushion plants is less exposed to temperature extremes than adjacent bare soil (Hedberg, 1992). A number of genera form cushion plants; among the most common taxa in the Ecuadorian Andes are Azorella aretioides, A. corymbosa, A. pedunculata, Plantago rigida, Draba aretioides, Distichia muscoides, and Xenophyllum humile.
Scattered small shrubs of a number of taxa including Chuquiraga jussieui, Pernettya prostrata, Baccharis latifolia, and Gynoxys buxifolia occur in this type of páramo, and also rosette plants including Lupinus alopecuroides and L. nubigenus.
This type of páramo is found at the highest elevations of vascular plant growth on the slopes of the major volcanoes, from about 4,500 to 4,900 m, nearly up to the lower limit of glacial ice. Plant growth at this elevation is probably not limited so much by arid conditions, as the name of the vegetation type implies, but rather by the nightly freezing temperatures. Plant cover is sparse, and most plants have rather deep tap roots that anchor them in the loose sandy soil. High winds frequently blow the soil about. Most plants are prostrate or nearly so, and are either herbaceous or woody at base. Some cushion-forming species are present. Characteristic species include Nototriche phyllanthos, Astragalus geminiflorus, Azorella pedunculata, Culcitium nivale, Calandrinia acaulis, Ephedra americana, and Xenophyllum rigidum.
The glaciers of the Andean volcanoes have been steadily retreating during the 20th century, leaving newly exposed areas available for colonization by plants (Clapperton, 1993). A study of primary succession was carried out on Cotopaxi volcano by Stern and Guerrero (1997), which included an area newly exposed by glacial retreat at 4,600–4,800 m, and a lava flow at 4,100 m.
Seasonally inundated areas
This vegetation type, as mapped by Harling, occurs in the lower portion of the Guayas River basin. This low-lying region, mostly between Babahoyo and Guayaquil, is subject to flooding during the rainy season. Historical records (Hidalgo, 1998) indicate that the original vegetation of this region was a seasonally inundated savanna, dominated by tall grasses, with deciduous forest on the scattered non-inundated hills. Most of the region has now been partially drained and is dedicated to cultivation of rice and sugar cane.