A relação entre a usina hidroelétrica de Balbina e a morte de árvores de Macrolobium acaciifolium (Benth.) Benth. (Fabaceae) nas florestas alagáveis a jusante do Rio Uatumã, Amazônia Central

Abstract

The monomodal flood pulse is the principal driving force in the Amazonian river floodplain systems, triggering productivity and interactions of the ecological processes of its species-rich biota. Trees respond to the long-term and predictable annual flooding by morpho-anatomical and physiological adaptations leading to cambial dormancy and the formation of annual tree rings in the wood. Macrolobium acaciifolium (Benth.) Benth. (Fabaceae) occurs over a wide geographical range of the Amazonian floodplains especially at the lower topographies. This species reaches ages of up to 500 years and forms regular and annual tree rings. At the lower topographies of the black-water floodplains (igapó) of the Uatumã River in Central Amazonia, downstream of the hydroelectric power plant of Balbina, huge populations of dead trees from this and other species have been observed. In this study, dendrochronological methods and radiocarbon dating (14C) were applied to date the year of death of trees from Macrolobium and relate the mortality to changes in the hydrological regime. Comparing the hydrological data of the Uatumã River downstream the Balbina dam, before and after its implementation, we observed a significant increase of the mean water level and a pronounced decrease of the duration of the terrestrial phase for the lower topographies. A chronology spanning the period 1638-2012 from living trees of M. acaciifolium (n = 37) of the Uatumã River floodplains was established comprising the period from 1638 to 2012 which is significantly correlated with the duration of the terrestrial phase calculated by water level data from the Port of Manaus comprising the period 1903-2012 (r = 0.49, p<0.0001). We sampled entire cross sections of 17 dead trees of M. acaciifolium, which still had the presence of the bark in the Uatumã River floodplains with a mean diameter of 71.56 ± 29.83 cm. The ring width of the carefully prepared stem disks was measured and the individual tree-ring series were cross-dated with the exactly dated tree-ring chronology (reference curve). To validate the dating of the trees we isolated the outermost ring to perform radiocarbon dating. The dendrocronological and 14C dating matched in 87.5% of the cases, and in 12.5%, the deviation in the dating was about only one year. All trees died in periods of inundations during consecutive years, up to two decades after the implementation of the hydroelectric dam. Possibly those impacts may be similar among other hydroelectric projects in the Amazonian floodplains. The application of dendrochronology used in this study is unique for tropical forests and may be a useful tool to study the impact of natural and anthropogenic disturbances in tropical forests affecting its structure, dynamic and functioning.