Summary0 Tree!ring analyses and dendrometer measurements were carried out on 26 tree species in a semi!deciduous forest of the Reserva Forestal de Caparo\ Venezuela\ where the mean annual rainfall is about 0699 mm and there is a dry season from December to March[ The main purposes of the investigation were to show the seasonality of cambial growth\ and the connection between precipitation patterns and tree!ring curves[ Long!term rates of wood increment were also estimated[ 1 Cambial markings in consecutive years showed that annual rings were formed by many species[ 2 The distinctiveness of growth zones was usually greater in deciduous species than in evergreen species\ although not all deciduous species had distinct rings[ 3 Dendrometer measurements showed that the annual growth rhythm was related to precipitation patterns[ Evergreen species tended to show only a short interruption of wood growth "during the later part of the dry season#\ whereas deciduous species stopped growth completely at the end of the rainy season[ 4 For deciduous species\ regression analyses showed close relations between tree!ring width and the sum of precipitation outside the rainy seasons "i[e[ November to April#[ Evergreen species reacted to the total annual amount of precipitation[ 5 Variation in longest available ring chronology "for Terminalia`uianensis# showed little correlation with the El Nin ½ oÐSouthern Oscillation e}ect[ 6 On average trees from natural forests showed relatively constant growth over the entire life span[ Plantation trees grew fast up to an age of 04Ð19 years\ but annual increments then decreased to values seen in natural forest trees[ Keywords] annual rings\ climateÐgrowth relation\ dendrochronology\ El Nin ½ oÐSou! thern Oscillation\ radial wood increment\ tree!ring analysis\ tropical semi!deciduous forest Journal of Ecology "0888# 76\ 280Ð392

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Aim: Attention has increasingly been focused on the floristic variation within forests of the Amazon Basin. Variations in species composition and diversity are poorly understood, especially in Amazonian floodplain forests. We investigated tree species composition, richness and alpha diversity in the Amazonian white-water (varzea) forest, looking particularly at: (1) the flood-level gradient, (2) the successional stage (stand age), and (3) the geographical location of the forests. Location: Eastern Amazonia, central Amazonia, equatorial western Amazonia and the southern part of western Amazonia. Methods: The data originate from 16 permanent varzea forest plots in the central and western Brazilian Amazon and in the northern Bolivian Amazon. In addition, revised species lists of 28 varzea forest inventories from across the Amazon Basin were used. Most important families and species were determined using importance values. Floristic similarity between plots was calculated to detect similarity variations between forest types and over geographical distances. To check for spatial diversity gradients, alpha diversity (Fisher) of the plots was correlated with stand age, longitudinal and latitudinal plot location, and flood-level gradient. Results: More than 900 flood-tolerant tree species were recorded, which indicates that Amazonian varzea forests are the most species-rich floodplain forests worldwide. The most important plant families recorded also dominate most Neotropical upland forests, and c. 31% of the tree species listed also occur in the uplands. Species distribution and diversity varied: (1) on the flood-level gradient, with a distinct separation between low-varzea forests and high-varzea forests, (2) in relation to natural forest succession, with species-poor forests in early stages of succession and species-rich forests in later stages, and (3) as a function of geographical distance between sites, indicating an increasing alpha diversity from eastern to western Amazonia, and simultaneously from the southern part of western Amazonia to equatorial western Amazonia. Main conclusions: The east-to-west gradient of increasing species diversity in varzea forests reflects the diversity patterns also described for Amazonian terra firme. Despite the fine-scale geomorphological heterogeneity of the floodplains, and despite high disturbance of the different forest types by sedimentation and erosion, varzea forests are dominated by a high proportion of generalistic, widely distributed tree species. In contrast to high-varzea forests, where floristic dissimilarity increases significantly with increasing distance between the sites, low-varzea forests can exhibit high floristic similarity over large geographical distances. The high varzea may be an important transitional zone for lateral immigration of terra firme species to the floodplains, thus contributing to comparatively high species richness. However, long-distance dispersal of many low-varzea trees contributes to comparatively low species richness in highly flooded low...

Cambial dormancy and annual rings in tropical trees are induced by annually occurring dry periods or flooding. Growth periodicity is indicated by the leaf fall behaviour and is connected with an annual periodicity of shoot elongation. Changes in stem diameter are measured with a dendrometer or by measurable differences in the electrical resistance of the cambium. Dendrochronological methods applied to carefully prepared samples can serve as proof of the annual periodicity of growth zones. For this purpose the following methods have been used: cambial wounding, radiocarbon dating, pointer year detection and regression analyses of ring width and climate data. Although X-ray densitometry and the analysis of stable isotopes in rings of tropical trees promise to provide interesting climatological information, the use of these methods remains difficult.

To study the impact of the annual long-term flooding (flood- pulse) on seasonal tree development in Amazonian floodplains, the phenology and growth in stem diameter of various tree species with different leaf-change patterns were observed over a period of 2 y. The trees of the functional ecotypes, evergreen, brevi-deciduous, deciduous and stem-succulent showed a periodic behaviour mainly triggered by the flood-pulse. Trees have high increment during the terrestrial phase. Flooding causes a shedding of some or all leaves leading to a cambial dormancy of about 2 mo and the formation of an annual ring. Studies carried out in tropical dry forests verify a strong relationship between the phenological development and the water status of the trees, strongly affected by seasonal drought. The comparison of the phenology and the diameter growth of the corresponding ecotypes in floodplain forest and a semi- deciduous forest in Venezuela shows a displacement of at least 2 mo in the periodicity, except for stem-succulent tree species. For stem-succulent trees it remains unclear which factors influence phenology and stem diameter growth

Investigations on growth zones of tropical trees were carried out and published since the beginning of our century.In tropical regions with severe annual dry seasons or inundation phases trees form annual rings. This is demonstrated for trees from Central Amazonian inundation forests and the Gran Sabana in Venezuela using a combination of several dendrochronological methods (wood anatomy, radiocarbon dating, ring width measurements).The occurrence of annual rings allows the determination of age and growth rate of trees. The growth rate depends on the growth strategy of the species and the growth conditions. At a given site growth rate shows a weak negative correlation with the specific gravity of the wood of trees from the upper story. Several reported ring width patterns are explained by the vegetation history of different forest stands.

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There is a limited knowledge about the El Niñ o-Southern Oscillation (ENSO) effects on the Amazon basin, the world's largest tropical rain forest and a major factor in the global carbon cycle. Seasonal precipitation in the Andean watershed annually causes a several month-long inundation of the floodplains along the Amazon River that induces the formation of annual rings in trees of the flooded forests. Radial growth of trees is mainly restricted to the nonflooded period and thus the ring width corresponds to its duration. This allows the construction of a tree-ring chronology of the long-living hardwood species Piranhea trifoliata Baill. (Euphorbiaceae). El Niñ o causes anomalously low precipitation in the catchment that results in a significantly lower water discharge of the Amazon River and consequently in an extension of the vegetation period. In those years tree rings are significantly wider. Thus the tree-ring record can be considered as a robust indicator reflecting the mean climate conditions of the whole Western Amazon basin. We present a more than 200-year long chronology, which is the first ENSO-sensitive dendroclimatic proxy of the Amazon basin and permits the dating of preinstrumental El Niñ o events. Time series analyses of our data indicate that during the last two centuries the severity of El Niñ o increased significantly.