Continental-scale patterns of canopy tree composition and function across Amazonia.

The world's greatest terrestrial stores of biodiversity and carbon are found in the forests of northern South America, where large-scale biogeographic patterns and processes have recently begun to be described. Seven of the nine countries with territory in the Amazon basin and the Guiana shield have carried out large-scale forest inventories, but such massive data sets have been little exploited by tropical plant ecologists. Although forest inventories often lack the species-level identifications favoured by tropical plant ecologists, their consistency of measurement and vast spatial coverage make them ideally suited for numerical analyses at large scales, and a valuable resource to describe the still poorly understood spatial variation of biomass, diversity, community composition and forest functioning across the South American tropics. Here we show, by using the seven forest inventories complemented with trait and inventory data collected elsewhere, two dominant gradients in tree composition and function across the Amazon, one paralleling a major gradient in soil fertility and the other paralleling a gradient in dry season length. The data set also indicates that the dominance of Fabaceae in the Guiana shield is not necessarily the result of root adaptations to poor soils (nodulation or ectomycorrhizal associations) but perhaps also the result of their remarkably high seed mass there as a potential adaptation to low rates of disturbance.

Changes in structure and composition of evergreen forests on an altitudinal gradient in the Venezuelan Guayana shield.

There have been several ecological studies in forests of the Guayana Shield, but so far none had examined the changes in structure and composition of evergreen forests with altitude. This study describes and analyzes the structure, species composition and soil characteristics of forest stands at different altitudinal zones in Southeastern Venezuelan Guayana, in order to explain the patterns and the main factors that determine the structure and composition of evergreen forests along the altitudinal gradient. Inventories of 3 948 big (>10cm DBH) and 1 328 small (5-10cm DBH) woody stems were carried out in eleven plots, ranging from 0.1 to 1.0ha, along a 188km long transect with elevations between 290 and 1 395masl. It has been found that 1) hemiepihytes become more dominant and lianas reduce their dominance with increasing altitude and 2) the forest structure in the study area is size-dependent. Five families and 12 genera represented only 9% of the total number of families and genera, respectively, recorded troughout the gradient, but the two groups of taxa comprised more than 50% of the Importance Value (the sum of the relative density and the relative dominance) of all measured stems. Moreover, the results suggest that low species richness seems to be associated with the dominance of one or few species. Stand-level wood density (WD) of trees decreased significantly with increasing elevation. WD is an indicator of trees'life history strategy. Its decline suggests a change in the functional composition of the forest with increasing altitude. The Canonical Correspondence Analysis (CCA) indicated a distinction of the studied forests on the basis of their altitudinal levels and geographic location, and revealed different ecological responses by the forests, to environmental variables along the altitudinal gradient. The variation in species composition, in terms of basal area among stands, was controlled primarily by elevation and secondarily by rainfall and soil conditions. There are other interacting factors not considered in this study like disturbance regime, biological interactions, productivity, and dispersal history, which could affect the structure and composition of the forests in the altitudinal gradient. In conclusion, it appears that the structural and floristic variability observed in the studied transect is produced by a combination of different climates and randomly expressed local processes interacting across a complex physical landscape.

Relationships between species richness and ecosystem services in Amazonian forests strongly influenced by biogeographical strata and forest types.

Despite increasing attention for relationships between species richness and ecosystem services, for tropical forests such relationships are still under discussion. Contradicting relationships have been reported concerning carbon stock, while little is known about relationships concerning timber stock and the abundance of non-timber forest product producing plant species (NTFP abundance). Using 151 1-ha plots, we related tree and arborescent palm species richness to carbon stock, timber stock and NTFP abundance across the Guiana Shield, and using 283 1-ha plots, to carbon stock across all of Amazonia. We analysed how environmental heterogeneity influenced these relationships, assessing differences across and within multiple forest types, biogeographic regions and subregions. Species richness showed significant relationships with all three ecosystem services, but relationships differed between forest types and among biogeographical strata. We found that species richness was positively associated to carbon stock in all biogeographical strata. This association became obscured by variation across biogeographical regions at the scale of Amazonia, resembling a Simpson's paradox. By contrast, species richness was weakly or not significantly related to timber stock and NTFP abundance, suggesting that species richness is not a good predictor for these ecosystem services. Our findings illustrate the importance of environmental stratification in analysing biodiversity-ecosystem services relationships.

Estimating the global conservation status of more than 15,000 Amazonian tree species.

Estimates of extinction risk for Amazonian plant and animal species are rare and not often incorporated into land-use policy and conservation planning. We overlay spatial distribution models with historical and projected deforestation to show that at least 36% and up to 57% of all Amazonian tree species are likely to qualify as globally threatened under International Union for Conservation of Nature (IUCN) Red List criteria. If confirmed, these results would increase the number of threatened plant species on Earth by 22%. We show that the trends observed in Amazonia apply to trees throughout the tropics, and we predict that most of the world's >40,000 tropical tree species now qualify as globally threatened. A gap analysis suggests that existing Amazonian protected areas and indigenous territories will protect viable populations of most threatened species if these areas suffer no further degradation, highlighting the key roles that protected areas, indigenous peoples, and improved governance can play in preventing large-scale extinctions in the tropics in this century.

Hyperdominance in the Amazonian tree flora.

The vast extent of the Amazon Basin has historically restricted the study of its tree communities to the local and regional scales. Here, we provide empirical data on the commonness, rarity, and richness of lowland tree species across the entire Amazon Basin and Guiana Shield (Amazonia), collected in 1170 tree plots in all major forest types. Extrapolations suggest that Amazonia harbors roughly 16,000 tree species, of which just 227 (1.4%) account for half of all trees. Most of these are habitat specialists and only dominant in one or two regions of the basin. We discuss some implications of the finding that a small group of species--less diverse than the North American tree flora--accounts for half of the world's most diverse tree community.

Changes in structure and composition of evergreen forests on an altitudinal gradient in the Venezuelan Guayana Shield

There have been several ecological studies in forests of the Guayana Shield, but so far none had examined the changes in structure and composition of evergreen forests with altitude. This study describes and analyzes the structure, species composition and soil characteristics of forest stands at different altitudinal zones in Southeastern Venezuelan Guayana, in order to explain the patterns and the main factors that determine the structure and composition of evergreen forests along the altitudinal gradient. Inventories of 3 948 big (>10cm DBH) and 1 328 small (5-10cm DBH) woody stems were carried out in eleven plots, ranging from 0.1 to 1.0ha, along a 188km long transect with elevations between 290 and 1 395masl. It has been found that 1) hemiepihytes become more dominant and lianas reduce their dominance with increasing altitude and 2) the forest structure in the study area is size-dependent. Five families and 12 genera represented only 9% of the total number of families and genera, respectively, recorded troughout the gradient, but the two groups of taxa comprised more than 50% of the Importance Value (the sum of the relative density and the relative dominance) of all measured stems. Moreover, the results suggest that low species richness seems to be associated with the dominance of one or few species. Stand-level wood density (WD) of trees decreased significantly with increasing elevation. WD is an indicator of trees’life history strategy. Its decline suggests a change in the functional composition of the forest with increasing altitude. The Canonical Correspondence Analysis (CCA) indicated a distinction of the studied forests on the basis of their altitudinal levels and geographic location, and revealed different ecological responses by the forests, to environmental variables along the altitudinal gradient. The variation in species composition, in terms of basal area among stands, was controlled primarily by elevation and secondarily ...

A pesar de los diversos estudios ecológicos realizados en los bosques del Escudo de Guayana, ninguno de ellos había analizado hasta ahora los cambios en la composición y estructura de bosques siempreverdes que ocurren al incrementar la altitud. Con el fin de identificar patrones y factores determinantes de la estructura y la composición de bosques en un gradiente altitudinal (290-1 395msnm) en el sudeste de Venezuela se realizaron inventarios de 3 948 fustes grandes (>10cm DAP) y 1 328 pequeños (5-10cm DAP) en once parcelas a lo largo de un transecto de 188km. Con el incremento de la altitud, el área basal aumenta en las hemiepífitas y disminuye en las lianas. Las familias y los géneros más importantes representaron una reducida proporción del total de taxones, pero abarcaron más del 50% del Valor de Importancia. Bajos valores de riqueza se asocian con la dominancia de pocas especies. La densidad de madera decrece significativamente a mayor altitud, y sugiere un cambio en la composición funcional. Existen diferentes respuestas ecológicas de los rodales ante cambios físico-ambientales. La variación de la composición de especies en el gradiente fue controlada por altitud, lluvia y condiciones edáficas; pero se desconoce el efecto de otros factores no considerados como régimen de perturbaciones, interacciones biológicas, productividad e historia de dispersión. La variabilidad observada en la estructura y la composición de bosques en el gradiente parece ser un resultado del efecto combinado de diferentes climas y procesos locales aleatorios que interactúan en un complejo paisaje.

Crecimiento diamétrico arbóreo en bosques de sierra de Lema, Guayana Venezolana: primeras evaluaciones / Tree diameter growth in forests at sierra de Lema, Venezuelan Guayana: initial evaluations

Con el fin de describir y analizar el crecimiento diamétrico arbóreo de bosques en un gradiente altitudinal (290-1395msnm), fueron realizados censos arbóreos sucesivos en 22 parcelas permanentes de 0,1ha localizadas en 6 sitios durante períodos variables de monitoreo de 1 a 5 años. Este gradiente está ubicado en La Escalera en la Sierra de Lema, al sudeste de Venezuela, y comprende los bioclimas: húmedo tropical, húmedo premontano bajo y húmedo premontano alto. Fueron identificados dos patrones unimodales de distribución de fustes por clases de incremento corriente anual del diámetro (ICADAP): uno con centro en la clase de -0,2 a 0,3cm y otro con centro en la clase 0,3-0,6cm. Los valores promedios del ICADAP en los diferentes bioclimas y sitios oscilaron entre 0,39 y 0,21cm. A pesar de tal intervalo, el ICADAP no presentó diferencias estadísticamente significativas entre bioclimas, entre sitios ni entre períodos de monitoreo; solamente las tasas de ICADAP entre las clases de iluminación de la copa y las clases diamétricas, presentaron diferencias significativas (P<0,0001). Las mayores tasas de crecimiento (>0,5cm/año) en el gradiente correspondieron a árboles emergentes, generalmente pertenecientes a clases diamétricas superiores (>30cm DAP). La iluminación de copas fue un indicador útil para explicar la diferencia de crecimiento arbóreo. Finalmente son discutidas las limitaciones y perspectivas del estudio

Relaciones alométricas y patrones de crecimiento para especies de árboles de la reserva forestal Imataca Venezuela / Allometric relations and growth patterns for tree species in the Imataca forest reserve Venezuela

Las relaciones alómetricas diámetro-altura fueron analizadas en 34 especies arbóreas de la Reserva Forestal Imataca, según la ecuación exponencial de Chapman y Richard, utilizada por los modelos de base individual ZELIG y FACET, que simulan la dinámica de crecimiento del bosque. Los coeficientes de la relación alométrica para cada especie fueron calculados por regresión lineal aplicada a la transformación logarítmica de la ecuación exponencial. Se determinó que la relación exponencial explica una proporción elevada de la variabilidad de altura para las especies estudiadas aunque se utilizaron datos de parcelas en localidades diferentes. Las relaciones resultantes serán incorporadas al modelo FACET en trabajos posteriores. Se evaluó la existencia de patrones alométricos, agrupando las especies según los requerimientos de luz (intolerantes o pioneras y tolerantes a la sombra) y por clases de altura (medianas con 10-30m, grandes de >30m), conformando cuatro grupos funcionales: pioneras medianas, pioneras grandes, tolerantes medianas y tolerantes grandes. Se encontraron diferencias entre las alometrías de los cuatro grupos utilizando análisis de varianza y análisis discriminante. Las diferencias en los coeficientes alométricos, tomados uno a la vez, son más significativas entre pioneras y tolerantes que entre las grandes y medianas. La combinación de ambos coeficientes, b2 y b3, permite diferenciar todos los grupos, especialmente usando una función discriminante, que además permitirá predecir la pertenencia de otras especies a cada grupo dados sus valores de coeficientes alométricos

Desarrollo cognoscitivo en niños y jóvenes del Estado Vargas / Cognitive development of children and youth in Vargas State

Este estudio analizó al desarrollo psicológico en 1386 niños y jóvenes de 2,7,11 y 15 años del Estado Vargas y comparó su desempaño según estrato social. Se administró la encuesta de datos evolutivos generales (solo a niños de 2 años), las pruebas de conservación de sustancias y peso de Piaget (sólo 7 años), las subpruebas de información y vocabulario de WISC-R y una encuesta de conocimientos básicos (7, 11 y 15 años). Los niños de 2 años presentaron un adecuado desarrollo psicológico general, independientemente del estrato socioeconómico. Adicionalmente, el rendimiento de los niños y jóvenes de 7, 11 y 15 años, fue bajo en todas las evaluaciones, especialmente en el estrato V. Los niños de 7 años mostraron mejor rendimiento que los de 11 y 15 años en sus capacidades cognitivas, intelectuales y académicas