Contrasting responses of two Amazonian floodplain trees to hydrological changes

Two common evergreen tree species of Amazonian floodplains, Laetia corymbulosa (Flacourtiaceae) and Pouteria glomerata (Sapotaceae), were investigated. Their phenological and physiological reactions to the changing hydrological environment during the annual cycle was analyzed. Leaves were present during the whole year in both species, but a strong leaf loss and significant reduction of new leaf production was observed in L. corymbulosa in the non-flooded phase in a period of unusually strong drought, which was accompanied by a significant decrease in predawn leaf water potential, which fell from -0.03 MPa to below -2 MPa. New leaves were flushed with the onset of rain a few weeks later. P. glomerata showed a complete inhibition of new leaf production in the period of highest water levels. Predawn leaf water potential was more or less constant at -0.5 MPa throughout the year. Stress reactions to drought were not detectable in this species. Photosynthetic activity in L. corymbulosa was highest in the terrestrial phase (13 µmol CO2 m-2 s-1). In the first weeks after inundation, net CO2 exchange rates were comparable with those of the terrestrial phase, but dropped drastically to 3 µmol CO2 m-2 s-1 after approximately six months of waterlogging. In contrast, assimilation rates of P. glomerata were nearly constant during the whole annual cycle, ranging between 8 and 12 µmol CO2 m-2 s-1. The results show that L. corymbulosa reacts more sensitively than P. glomerata to environmental changes in the annual cycle, such as long-term waterlogging and drought. The two species show different morphological adaptations, such as aerenchymatous tissues and adventitious roots, while different leaf development (senescence), which is linked to the hydric conditions of the environment, is responsible for reductions and subsequent recovery (by new leaf flush) in photosynthetic rates. Thus, different phenological performances lead to different ecophysiological responses to the same environmental constraints to which the two species are subjected