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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.

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The uptake and transport of salt ions (Na + , Cl -), macronutrients (K + , Ca 2+ , Mg 2+ ) and abscisic acid (ABA) response to increasing soil salinity were examined in 2-year-old seedlings of Populus euphratica and a hybrid, P. talassica Kom × (P. euphratica + Salix alba L.). Leaf burn symptoms appeared in the hybrid after 8 days of exposure to salinity when soil NaCl concentration increased to 206 mM, whereas P. euphratica exhibited leaf damage after day 21 when soil NaCl exceeded 354 mM. Leaf necrosis was the result of excess salt accumulation since the injury followed an abrupt increase of endogenous salt levels. Compared with the hybrid, P. euphratica exhibited a greater capacity to exclude salt ions from leaves under increasing salinity, especially Cl -. Salt treatment altered nutrient balance of the hybrid, leaf K + , Ca 2+ and Mg 2+ concentrations significantly declined and the same trends were observed in roots with the exception of K + . Although K + levels decreased in salinised P. euphratica, increasing salinity did not affect the levels of Ca 2+ and Mg 2+ in leaves, but did increase the uptake of these nutrients when salt stress was initiated. NaCl-induced increase of ABA concentration in xylem sap [ABA] was observed in the two tested genotypes, however xylem [ABA] increased more rapidly in P. euphratica and a fivefold increase of xylem [ABA] was recorded after the first day of exposure to salt stress. Therefore, we conclude that the increase of Ca 2+ uptake may be associated with the rise of ABA, and thus contributes to membrane integrity maintenance, which enables P. euphratica to regulate uptake and transport of salt ions under high levels of external salinity in the longer term.