This paper compares the non-deltaic, riparian-flooded forests of the Orinoco and Amazon River basins. Ecological relationships between these forests and their environments that can be useful in establishing schemes for biodiversity conservation are identified. Adaptations of species to flow seasonality, flooding intensity, sedimentation pattern and nutrient depletion are described. The variability and diversity of riparian-flooded forests is related to (i) landscape evolution (regional-scale, long-term), (ii) water quality (basin scale, long-term) and (iii) hydrology and geomorphology (sector-scale, medium-term). The floristic analysis has produced a preliminary list of 242 tree species common to the riparian-flooded forests of both basins. This relatively high number of species is related to connectivity between the riparian corridors of both basins and the effective operation of dispersal mechanisms. Highly oligotrophic environments add uniqueness at the regional scale through the evolution of endemic species presenting adaptations not only to flooding but also to nutrient depletion. The process of genetic diversification and the evolution of genotypes adapted to flooding are suggested to explain longitudinal gradients at tributary junctions and floodplain-upland ecotones where current fluvial dynamics are unpredictable over ecological time scales. The paper presents information that may be used to devise appropriate measures to evaluate sites for riparian biodiversity conservation and management.

KEY WORDS: ORINOCO RIVER, AMAZON RIVER, RIPARIAN FORESTS, FLOODED FORESTS, IGAPÓ, VÁRZEA.

 This article presents and discusses two complementary approaches, one qualitative and the other quantitative, for describing the riparian forests of the lower Caura and for evaluating their importance to the Ye’kwana people. The qualitative approach is based on systematic reconnaissance walks and interviews with a Ye’kwana collaborator during exploration of the different forest types found along the Caura in the vicinity of the Ye’kwana village Boca de Nichare. The resulting preliminary classification distinguished 11 vegetation types according to flooding pattern, soil texture and color, characteristic plant species and utility for the Ye’kwana. The qualitative approach also demonstrated the generally higher utility of the non-flooded forests as sources of useful plants and as agricultural sites. However, some very important useful plant species were found only in the flooded forests.The quantitative approach is based on a set of 28 inventory plots of 0.1 hectare each, representing the vegetation zones found along 9 transects perpendicular to the main channel of the river between Los Raudales de Mura and Salto Para. The vegetation in these inventory plots was classified based on presence-absence data (>5 cm DBA, including palms and sinusia of tall herbs) and based on the Importance Value (> 10 cm DBA) using the software Twinspan.Stepwise Multiple Discriminant Analyses were used to relate the qualitative and quantitative classifications with soil variables. According to these analyses, 6 different riparian forest communities occur in the study area with differentiation based on degree of flooding and position along the main river channel. Comparison of the quantitative and qualitative approaches revealed similar results: the distinction of qualitative classes was related to soluble phosphorous, percent organic matter, magnesium and leaf litter depth; while the quantitative classes were most closely correlated with soluble phosphorous, pH, percent organic matter and exchangeable aluminium. Further univariate analyses with the soil data without considering the interactions between the variables, showed the importance of sand and clay percent for the qualitative classes. For the quantitative forest classes, percent silt, root-mat depth, effective CEC and exchangeable acidity were also significant for explaining differences between he classes. The number of species and the indices Shannon-Wienner H' and J showed differences between the classes in both approaches. Basal area and density were significantly related only with the quantitative data.The main vegetation types were related with flooding condition; species richness in the vegetation types seemed to increase with decreasing inundation.The number and percentage of directly useful plant species in the riparian forest types (between 48 and 91 %) indicated a high importance of the riparian forests for the Ye'kwana people. The presence of useful plants was higher in the non-flooded forests confirming the results of the qualitative approach. It decreased downstream of the confluence of the Nichare River with the Caura.

KEY WORDS: RIPARIAN FORESTS, ETHNOECOLOGY, YE'KWANA ETHNIC GROUP, CAURA RIVER, TROPICAL FORESTS, VENEZUELAN GUAYANA, ORINOCO BASIN

This article presents a historic overview of the botanical explorations made in the high mountains of the Caura river basin (Estado Bolívar, Venezuela). Firts, the results obtained between 1900 and 1996 on the 1000-2200 m tall summits of the sandstone massifs ("tepuis") Guanacoco, Sarisariñama, and jaua are described and commented. Next, we present the results of the first botanical and ecological explorations carried out starting in 1988 in the summit region (1500-2400 m asl) of the Sierra de Maigualida, of granitic litholology. Finally, we discuss the floristic and phytogeographic differences found between the highland ecosystems of these two interesting mountain systems of the Venezuelan Guayana.

KEYWORDS: HIGH MOUNTAIN ECOSYSTEMS, FLORA, PHYTOGEOGRAPHY, CERRO GUANACOCO, CERRO SARISARIÑAMA, CERRO JAUA, SIERRA DE MAIGUALIDA, VENEZUELAN GUAYANA.

This article presents a preliminary description to the diversity of wild plant resources utilized by the Ye’kwana in the lower Caura river and to the variety of uses the Ye’kwana realize for their plant resources. It documents 358 plants of diverse life forms (representing 70 plant families) that are either utilized by the Ye’kwana for basketry, the fabrication of dugout canoes, wood carving, construction, miscellaneous technology, consumption, medicinal and magic or spiritual purposes, or that provide food for game and fish. Each of these categories is described in detail, including observations on changes in plant uses and potential impacts of the Ye’kwana harvesting activities on the resources. We provide an analysis of the plant families that are most useful to the Ye’kwana and highlight those species that provide irreplaceable resources for subsistence and commercial uses.We conclude that an important part of the Ye’kwana’s subsistence and commercial activities do involve plant materials from wild growing forest plants. Therefore, development and conservation planning in the Caura should take into account the current natural resource use patterns of its inhabitants.

KEYWORDS: ETHNOBOTANY, YE’KWANA, FOREST USE, CAURA RIVER, GUAYANA REGION, VENEZUELA.

We evaluated the ecological and reproductive characteristics of plant species occurring in severely disturbed areas that were revegetated with exotic grasses. We identified those species with the best combination of attributes that increase their probability of success in degraded lands. Fifteen degraded areas were studied in two different bioclimatic regions, a high premontane humid bioclimate and a low premontane humid bioclimate. The frequency of native colonizing species and the presence of arbuscular mycorrhizal colonization in their roots were evaluated. The sexual and breeding system, pollination mode, fruit set, and dispersal syndrome of ten of the most frequent colonizing species were also studied. The floristic survey of the colonizing species revealed a similarity to the reported flora of the treeless savannas that are dominant in the region. Bioclimatic conditions prevailing in the degraded lands seem to be an important factor for the presence of colonizing species and for species richness. All colonizing species studied were mycorrhizal, and for this reason the restoration program in these degraded areas should take mycorrhizae into account, reintroducing them or manipulating the soils to increase the mycorrhizal inoculum. We suggest Scleria cyperina and Trachypogon plumosus to start or promote the natural succession in the degraded areas from La Gran Sabana. Because their frequency is high and their reproductive system is less dependent on biotic factors, these species stand out in the studied areas.

KEYWORDS: ARBUSCULAR MYCORRHIZAL FUNGI, REPRODUCTIVE-BIOLOGY, SHRUBLAND COMMUNITY, GUAYANA HIGHLAND, POPULATIONS, SUCCESSION, INFECTION, FITNESS, FRUITS, PLANTS

Although these forests cover only a relatively small area of the Caura River basin, they do play an important functional role as the source of energy and nutrients for the aquatic ecosystems. Also, most of the human population living in the watershed depends directly or indirectly on riverine forests.The largest extent of riverine forests is represented by the inundated forests that are associated in the Caura with two main geomorphological environments: the large floodplain of the Lower Caura below the rapids of La Mura, and the river channels and floodplains of variable width in those areas where the river is structurally controlled, which is the case for most of the Caura river above the Rapids of La Mura and its tributaries.The floodplain of the Lower Caura represents the largest area of inundated forest in Venezuelan Guayana. This forest gets flooded to a depth of 12 m for several months per year, its canopy height varies from 12 to 25 m, the number of species is 10-14 per 0.01 hectare and the flora represents a mix of species typically found in white water floodplains and those typically found in blackwater areas.The inundated forests above the rapids of La Mura grow mostly in floodplains found near the mouth of the tributaries of the Caura river, in depressions of variable size behind dikes, on alluvial or colluvial banks, and in some areas directly adjacent to the main river cchannel. The inundation varies from 2-5 m in the Lower, and from 1-2 m in depth in the Middle Caura. The number of species averages 18-20 per 0.01 hectare; several of the species of this forest type are also found in the large floodplains of the Lower Caura, but those species typically associated with eutrophic waters do not occur above the rapids. Two additional inundated forest types are found in this part of the Caura mostly on very sandy soils. One seems to develop in areas of common and severe disturbance by the river, and is characterised by an open canopy of typical successional species and an understoy dominated by bamboos. The other is formed by low, monospecific associations of woody species that show strong basal ramification and often form a narrow band along the shore line.Non-inundated riverine forests occur on the higher shores of the Lower and Middle Caura, have a canopy height of 20-25 m and are composed of a set of typically riverine trees accompanied by species belonging to the respective tierra firme forests. 60-70 % of the species found in the riverine forests above the rapids La Mura are directly useful to the Ye'kwana.The extensive floodplain forests of the lower forest seem to be utilized by the criollo population mostly indirectly via their contribution of nutrients to the fish population.

KEY WORDS: RIPARIAN FORESTS, CAURA RIVER, TROPICAL FORESTS, VENEZUELAN GUAYANA, ORINOCO BASIN

El estudio aquí presentado muestra los resultados de un muestreo realizado en una serie de áreas de préstamo de La Gran Sabana, a fin de evaluar la presencia de especies autóctonas en los mismos. Asimismo se hace un análisis de su diversidad en términos de riqueza de especies colonizadoras y de la relación con los tratamientos de recuperación de áreas efectuados previamente entre los años 1987 y 1991 por EDELCA y la empresa Dell Acqua, con el objetivo de conocer el grado de éxito de los mismos. Estos resultados son básicos para la ejecución de las futuras programaciones que se realizan en el Programa de Recuperación de Áreas Degradadas de La Gran Sabana que lleva a cabo la Autoridad Gran Sabana en la Estación Científica de Parupa y que incluirán los ensayos de recuperación a través de plantaciones de especies autóctonas.

KEYWORDS: RESTAURACIÓN DE ÁREAS DEGRADADAS, RECUPERACIÓN DE ÁREAS DEGRADADAS, ESPECIES AUTÓCTONAS, GRAN SABANA, ÁREAS DE PRÉSTAMO.

Riparian forests in the Guri Dam region are diverse in relation to their phenological pattern, vertical structure and floristic composition. In this paper, a general description and classification of these forests is made: 1. Flooded riparian forests along the borders of the Caroní River, upstream and downstream of the Guri Dam; 2. Non-flooded riparian forests (inside the forests matrix) and 3. Gallery forests (inside the savanna matrix). In the immediate surroundings of the Guri Dam, the floristic composition of the flooded forests is related to that of similar riparian ecosystems of oligotrophic waters in the Orinoco and Amazon basins; however the flooded forests of the Caroní basin are structurally distinct, are lower in height and distributed in discontinuous patches along the channel borders. On the other hand, the flora associated with non-flooded riparian and gallery forests is made up of species that are common in other macrothermic riparian forests of lowland Venezuela and also in non-riparian tropophillous forests of the Lower Caroni basin. The importance of conservation of these forest ecosystems is stressed as well as a need for a functional restoration in degraded areas, in order to ensure the preservation of the biological diversity in the region.

KEYWORDS: RIPARIAN FORESTS, GALLERY FORESTS, MAURITIA FORESTS, ECOTONES, CARONI RIVER BASIN, FOREST REFUGIA, GURI DAM. ORINOCO BASIN

Tree dispersion was evaluated in a Trachypogon savanna by measuring the distance from individual trees to their nearest conspecific neighbour. The vegetation is a closed bush island savanna with coexistent isolated trees and small patches of semideciduous forest. The isolated trees were in clumps similar to trees growing in savannas and tropical forests. In our study, adults were less clumped compared to juveniles. Environmental and biotic differences related to edaphic heterogeneity, life history, and reproductive strategy could account for the clumped pattern. Different spatial patterns emerged for stems in groves. Thus, uniform spatial distribution was evident in the individuals of the fire resistant species (Curatella americana, Byrsonima crassifolia and Bowdichia virgilioides) and a gap filling species (Casearia decandra). Changes in the demographic components between different age groups provide evidence for density dependent mortality. A hypothesis mainly based on two scales of disturbance (human impact and gap formation) acting synergetically with a large scale factor (substrate patchiness) is put forward to explain the dynamics of tree invasion and the maintenance of the Trachypogon savanna.

KEYWORDS: SPATIAL PATTERNS, TROPICAL SAVANNAS, VENEZUELA

High local densities of tropical forest plant species have been found in riparian forests of Belizean and Venezuelan savannas. Soil and light-intensity heterogeneity within the forests is limited and species coexistence may be mediated more by specialised disturbance regimes than by micro-habitat specialisation. Provided that this local diversity is complemented by high regional diversity within these systems, they could provide plausible refugia for tropical forest biotas during arid climatic phases. The results demonstrate that a reduced species density is not an inevitable consequence of forest fragmentation, and have important implications for the future conservation of tropical forests.

KEYWORDS: RIPARIAN FORESTS, GALLERY FORESTS, DIVERSITY. CONSERVATION,

FOREST FRAGMENTATION, TROPICAL SAVANNAS