Climate change will increase the occurrence of flash floods as a consequence of extreme rain events, creating alternate periods of drought and flooding during the growing season. We analyzed the responses of two willow clones with contrasting responses to flooding (clone B, Salix matsudana × Salix alba hybrid; clone Y, Salix alba) to different combinations of stress treatments: continuous flooding or drought for six weeks, or cyclic treatments of two weeks of stress separated by two weeks of watering at field capacity. Drought reduced growth, stomatal conductance, and total leaf area in both clones, but flooding did not. Flooding reduced the root to shoot ratio in both clones. The hydraulic conductivity of the main stem was significantly reduced by drought only in clone Y. The area of the vessels was decreased by both drought and flooding, but the number was increased only by drought. The occurrence of drought before flooding reduced the vessel area, but the opposite treatment did not. An episode of drought after one of flooding is more stressful than the opposite situation, especially for clone Y, which could not adjust its water transport capacity during the drought period.

SUMMARYFlooding tolerance should be included as a trait in breeding programs in forested areas where extreme flooding events lasting several months can occur. In this context, it is difficult to carry out controlled, long-term flooding experiments with big trees due to the large number of specimens to screen. The hypothesis in this work was that a relatively short-term flooding experiment in a greenhouse would be useful to select clones capable of enduring long term flooding events in the field. The survival of 4 willow clones in a long term, 12 years field trial that was subjected twice to long term flooding events, was compared with the survival, growth and gas exchange of those same clones subjected to flooding in the greenhouse for three months. There were differences in flooding survival in the field: clone AN4 showed a 100 % survival, followed by clone 395 (43 %), clone 131-27 (18 %) and clone 13-44 (14 %). In the greenhouse, all plants survived and only flooded plants of clone 13-44 experienced a statistically significant growth and stomatal conductance reduction. The flood tolerance ranking in the field and in the greenhouse only partially overlapped; the worst clone in the field could be identified in the short-term greenhouse experiment, though the best one could not. It was not possible to identify clones with tolerance to long-term flooding episodes in the field. Nevertheless, the greenhouse results were useful to identify clones showing flooding sensitivity and tolerance to short term flooding episodes.Key words: Salix spp., survival, growth, Paraná River Delta. RESUMENEs necesario incluir la tolerancia a la inundación como un carácter en programas de mejoramiento para zonas forestadas donde eventos extremos de inundación que pueden durar meses son comunes. En este contexto, es difícil llevar a cabo experimentos controlados de larga duración con árboles de gran tamaño, debido al elevado número de ejemplares que es necesario evaluar. La hipótesis de este trabajo fue que un experimento relativamente corto de inundación en el invernáculo sería útil para seleccionar clones capaces de soportar eventos de inundación de varios meses en el campo. Se comparó la supervivencia de cuatro clones de sauces en un ensayo a campo que experimentó dos episodios largos de inundación a lo largo de 12 años, y la supervivencia, crecimiento e intercambio gaseoso de los mismos clones inundados por tres meses en invernáculo. Se encontraron diferencias en la supervivencia en el ensayo a campo, del clon AN4 sobrevivieron el 100 % de las plantas, seguido por el clon 395 (43%), clon 131-27 (18%) y clon 13-44 (14%). En el invernáculo, todas las plantas sobrevivieron y sólo las plantas inundadas de 13-44 experimentaron una reducción significativa del crecimiento y la conductancia estomática. El ranking de tolerancia en el campo y en invernáculo sólo coincidió parcialmente, el clon menos tolerante en el campo pudo ser identificado en el invernáculo, pero no el clon más tolerante. Si bien no fue posible identificar clones con tolera...

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