Floods and Droughts: effect on Schoenoplectus californicus from the Salado River sub-basin (Buenos Aires, Argentina).

DE CABO, L.; SEOANE, R.; ARREGHINI, S.; CASARES, M. V; CALLAÚ, A.

Barcelona, España

Congreso; 3rd Wetland Pollutant Dynamics and Control - WETPOL2009; 2009

Water depth changes temporally and spatially in wetlands and littoral zones The plants species inwetlands of Salado River Basin are affected by floods, droughts and high salinity in water and soil.During the mature phase of the ENSO (El Niño-Southern Oscillation) more water than usual is storedon the ground in the Pampas Plain (Forte Lay and Spescha, 2001). An ongoing period of reducedrainfall, connected with La Niña event, is the cause of the current deficit in the hydrological balanceand the most drought over the last 40 years in the region.This paper is meant to be a first approach tounderstanding the relationships between cold and warm phases of ENSO phenomenon andhydrological variables, as well as their effects over growth, biomass allocation and morphologicalpatterns in a native population S. californicus from the Salado river sub-basin (Lowland Pampas).Thornthwaite and Mather (1957) method was applied to determine soil water balance components.ENSO phases were defined according to NOAA. Sediments and rhizomes of S. californicus wereplaced in plastic containers.Seven treatments were performed for 350 days with different water levels:T0:0 cm; T1:10 cm; T2:15 cm; T3:20 cm; T4:27 cm; T5:40 cm and T6:70 cm. At the end of theexperiment, shoot height and dry biomass of plant parts were determined. The shoot height-weightrelationships were analysed using the Huxley-Teissier allometric equation. One factor ANCOVA(water level), was applied to test the equality of slopes between treatments. The allometric level (bvalue) of each treatment was tested with a t-test . ANOVA was used to detect the water level effect onshoots height and weight and final aboveground biomass. LSD analysis was used to determinesignificant differences between means.The mean rainfall of the El Niño events was higher than the mean values of the total series duringaustral winter (July to October). During growing season (August to December), the highest waterdeficit was registered during the La Niña events. In T6, no shoots were produced, but rhizomes were viable. With increasing water depth plantsinvested relatively more biomass per shoot. The significantly higher shoot weights were registered atT5 and T4. T0 showed a mean height significantly lower than the rest, Drought induced a considerabledecrease in S. californicus shoot height and biomass. The aboveground biomass per pot wassignificantly lower in T0 and significantly higher in T4 (Table 1). Water deficit produced a decrease inthe aboveground biomass probably associated with a decrease in photosynthetic activities. Likewise,if water level was deeper than 27 cm, the aboveground biomass would decreased, due to the newshoots production decreased in spite of higher shoot weight and height (Table 1). According to testedwater levels, the optimum water level for S .californicus aboveground growth was 27 cm.The slope of the allometric equations (ß) was significantly lower for T0 and T2. The higher slope was estimatedin T5 revealing a plastic response to water level. Others aquatic emergent macrophytes occurring inthe transition zone show similar plastic response.S.californicus appears to be a plant that exhibits plasticity to tolerate or survive to extremwater depth conditions. During winter water increases by the El Niño event would not affectbulrushes production. However, during La Niña event, water level decrease or drought in theflooded area at the beginning of the growing season would induce a considerable decrease inshoot biomass.