Ecohydrologic Functioning of Combretum lanceolatum Under Flooded and Dry Season Conditions in the Pantanal: an Investigation at Soil, Plant and Ecosystem Scales

Abstract:

The Pantanal is the largest floodplain in South America, and is comprised of a mixture of savanna vegetation and seasonally flooded forests. The Northern Pantanal experiences seasonal flooding in phase with the rainy season (October-April) with floodwater depths of up to two meters in some areas. Soil moisture and water level are important variables that influence soil oxidative-reductive potential, plant net photosynthetic capacity and ecosystem CO₂ fluxes. In this study, we investigated how these variables affect gas exchange of the Combretum lanceolatum species located within the footprint of an eddy covariance tower. Our goal was to identify the influence of this locally dominant species on ecosystem scale CO₂ fluxes during different hydrological conditions. CO₂ fluxes have been measuring since 2013 using a CO₂ eddy covariance system installed on a tower in an area with more than 80% C. lanceolatum. Leaf gas exchange measurements for C. lanceolatum were made during the dry and flooded periods, while water level and soil conditions were monitored continuously with sensors at the base of the tower. Net photosynthetic capacity rates showed no significant difference between the dry (12.90 ± 2.29 µmol m^-2 s^-1) and the flooded period (12.42 ± 2.6 µmol m^-2 s^-1), even after flooding created anoxic conditions when the soil oxidative-reductive potential values were less than -400 mV. During the flood period, we identified a net uptake of CO₂ with mean CO₂ fluxes of -4.12 ± 3.34 µmol m^-2 s^-1 for 2014 and -4.14 ± 2.62 µmol m^-2 s^-1 for 2015 flooded periods. These values were significantly different from the dry season, which exhibited net fluxes from the ecosystem to the atmosphere of +0.32 ± 0.27 µmol m^-2 s^-1 during the dry period of 2015. Despite the hydrological changes in the Pantanal, our data indicate that CO₂ fixation is more prominent than ecosystem respiration through large net photosynthetic capacity rates, leading to an annual CO₂ balance of -2.11 ± 2.97 µmol m^-2 s^-1.