The study areas were located 1) along the middle course of the Napo River between Santa Maria, Santa Clotilde, and Puerto Huamán; 2) along the lower course of the Nanay River between Lagunas, Yarina, and Mishana; and 3) in the vicinity of Huaisi along the lower course of the Tahuayo River (Fig. 1). Observations were made from canoes and were concentrated along the river and the oxbow lake or "cocha" margins and in the proximity of small feeder streams.
The architecture of vegetation, the soil, and the effects of seasonal fluctuations in water level and human activities varied within the study areas. The different levels or strata in the Amazonian forest vegetation are readily apparent because different floristic elements have uniform physiognomic characteristics [Ducke & Black, 1954; Pires, 1974; Prance, 1979; Encarnación, 1985].
At Huaisi the soil is muddy and of recent alluvial origin. There is no topographic relief, and the area is flooded successively by black, mixed, and white water during a single flood stage. These characteristics determine the floristic composition of the "Tahuampa" forest [Encarnación, 1985], which includes among other plant species Hura crepitans, Couroupita subsessilis, Clarisia biflora, Chorisia insignis, Campsiandra spp., and Macrolobium acaciaefolium. These species attain heights greater than 25 m, and since they have commercial importance as lumber, both the structure and height of the plant cover have been altered by selective logging. It is possible, nevertheless, to distinguish several strata. There is a lower story of dense, woody understory between 8 and 10 m in height and a middle story between 11 and 15 m, with thin and tortuous trees supporting climbers and semiwoody vines. The uniform upper story is between 16 and 20 m high and consists of large trees and entanglements of climbers and vines. In places, this upper story is interrupted by an older, higher story between 21 and 25 m in height consisting of old, partially fallen senescent trees, supporting vines, hemiepiphytes, and woody epiphytes. Among this emergent level, some trees attain 40 m in height.
Along the Nanay River, the soil is claylike mud and is of older alluvial origin. The relief is slightly undulating, and low areas are inundated by both black and mixed water. The riparian forest is of the blackwater "tahuampa" type described by Encarnación [1985]; it is composed of Eschweilera spp., Campsiandra spp., Pithecellobium laetum, and other species of large size and height, which are without commercial value. The understory is dispersed and composed of shrub between 10 and 15 m high. The middle story consists of mature tree trunks between 16 and 20 m in height. The upper story presents a uniform aspect with mature, senescent, and dead trees between 21 and 30 m in height, which support lianas, hemiepiphytic vines, and some herbaceous epiphytes. The emergent trees reach almost 40 m in height and sometimes form a diffuse stratum that supports many epiphytes
Along the Napo River, the soil is muddy and of recent alluvial origin; the topography is somewhat undulating, and the terrain is inundated seasonally by white water. The forest is of the "tahuampa" type represented by Calycophyllum spruceanum, Naucleopsis glabra and Naucleopsis imitans, and Scheelea spp., in associations called "capironales," "tamamurales," and "shapajales." These species attain great height but have no commercial value. The understory is composed of thin and erect trees between 10 and 15 m high, without thickets. The middle story consists of straight and erect trees reaching from 16 to 20 m in height, with few vines or climbers. The upper story, between 21 and 30 m, consists of the foliage of low branches, which support dense entanglements of climbers and vines. Emergent trees reach over 40 m. In some places, stands or associations of mature, senescent, or dead emergents define a true upper story.
