Stand dynamics of the drought-affected floodplain forests of Araguaia River, Brazilian Amazon

Background: The floodplain forests of Araguaia River, a clear-water river in the southeastern Amazon (Tocantins State, Brazil), are characterized by seasonal flooding up to 3.5 m height, low nutrient levels in the water, and seasonal drought periods of 4-5 months. Methods: We studied the forest dynamics (tree diameter growth, tree mortality and recruitment) of this unique forest ecosystem over a 5-year period by repeated censuses in 12 permanent plots established along a flooding gradient. Results: The cumulative basal area in the plots increased by 0.84 (+/- 0.45) m(2).ha(-1).yr(-1) (mean +/- SD) in the annually-flooded (AF) plots in lower terrain and by 0.69 (+/- 1.00) m(2).ha(-1).yr(-1) in the higher non-annually flooded (NAF) plots, corresponding to an aboveground biomass increase of 0.81 (+/- 0.57) and 0.69 (+/- 1.58) Mg.ha(-1).yr(-1) in the AF and NAF plots, indicating a recent carbon sink in the biomass. Mean diameter growth rate was 1.8 (+/- 0.44) mm.yr(-1) in the AF and 2.0 (+/- 0.56) mm.yr(-1) in the NAF plots (corresponding to a coarse wood production of 1.53 (+/- 1.29) and 2.02 (+/- 0.52) Mg.ha(-1).yr(-1)), indicating no flooding effect on radial growth. Mean mortality rates in the 5-year period were 1.9 (+/- 0.37)%.yr(-1) in the AF plots and 1.8 (+/- 0.87)%.yr(-1) in the NAF plots with no differences along the flooding gradient. Highest mortalities were registered in the AF plots for the 10-20 cm dbh class (2.4%.yr(-1)), likely as a consequence of flooding, and in the NAF plots for the 40-50 cm dbh class (3.0%.yr(-1)), probably mainly caused by ENSO-related droughts. Conclusions: We conclude that these drought-affected tropical floodplain forests have a lower standing biomass and aboveground productivity than central Amazonian floodplain forests in more humid climates, and the imprint of the flooding gradient on stand dynamics is relatively weak, which may result from the lower flooding height and the interaction of flooding with low nutrient supply and periodic drought.