Physiological responses to extreme hydrological events in the Pantanal wetland: heterogeneity of a plant community containing super-dominant species.

Aims: We tested two mechanisms of adaptation to extreme hydrological stresses (flooding and drought) of species making up a tropical wetland plant community by measuring leaf gas exchange and water potential. We hypothesized that anoxic conditions that occur during flooding will decrease leaf gas exchange when compared to the dry season, and that 'super-dominant' specie swill have a distinctive physiological advantage when compared to other plants within the community. Location: Northern Pantanal wetland, Private Natural Heritage Reserve of the Brazilian Social Service of Commerce (RPPN-SESC Pantanal), Mato Grosso, Brazil. Methods: Two periods representing typical extreme hydrological conditions in the Pantanal wetland were selected based on historical soil and meteorological measurements: (1) a drought period when plants experience stress due to soil moisture deficits during a dry season that persists for several months (May to Sept), and (2) a flooding period when oxidation-reduction potential is negative for 30 d or more (Mar or Apr), indicating anoxic stress. Measurements of gas exchange and leaf water potential were made on seven species in drought and flood stress conditions. The seven species represent the majority of the plant community. Results: As a whole, the plant community showed significantly lower potential net photosynthesis (P_N) during flooding when soil oxidation-reduction potential reached close to -900mV when compared to the dry season, but the magnitude of the decline in P_N was species specific. Not all super-dominant species showed higher P_N compared to non-dominant species, but they did demonstrate higher stomatal conductance and transpiration leading to lower water use efficiency. The combination of higher P_N despite low soil water content suggests that the plant community had access to deep water resources. This access was also confirmed by the midday leaf water potential, which was similar for the flood and dry seasons. Conclusions: Results suggest that the plant community may have high physiological performance under a wide range of soil oxidation-reduction potentials. Higher P_N rates of super-dominant species indicate a physiological advantage of these species in the different hydrological conditions. [ABSTRACT FROM AUTHOR]