Crack willow changing riverine landscapes in Patagonia

In Patagonia (Argentina) in recent decades, the exotic species and hybrids of Salix alba-Salix fragilis complex has spread over wide areas along watercourses, taking advantage of it is sexually and vegetatively propagation. It outcompetes with native vegetation, facilitated by stochastic events related to large floods of long-lasting duration higher than the average maximum of 186 m(3)/s for more than a day. We analysed the exotic willow's increase along the braiding Azul River, and the hydrological regime between 1966 and 2012. We evaluated the links between regime and tree density over time using dendrochronological data. Results showed an increase of the forest's cover from 2.1% to 70.8% over the last 46 years. The densities of exotic willows grew exponentially between 1966 and 1973, during a period which was free of higher floods. The initial native populations declined as willow's population increased. Up to 1973, there was an increase in the native populations too. Then from 1974 on, the willow showed a gradual growth coupled with a decline in the native trees. Up to 1977, the willow expansion could be linked to an extreme flood that reached 392 m(3)/s (210.75%) greater than the average of the maximum flows). We concluded that plant succession in the Azul River follows a Red Queen effect whereby the exotic willow and native trees repeatedly reorganized the system after pulses and periods of extreme flooding events.