Ecological responses of two pioneer species to a hydrological connectivity gradient in riparian forests of the lower Paraná River

Abstract

The vegetation of the Parana´ River floodplain has physiognomic, structural and ecological characteristics that are distinct from those of the surrounding landscapes across the river (Forest, Chaco Savannahs, Pampean steppe). Few species are able to live in the alternately flooded and dry soil. The distribution of each species is strongly conditioned by the water regime in each area of the floodplain, and its location in the topographical gradient of the islands allows us to determine the possibility of colonising the bars and islands of the river course to determine their tolerance to hydrological variations and to foresee the changes that can take place as a result of disturbances in the hydrological regime. Here, we present the occurrence of willow (Salix humboldtiana Willd.) and South American alder (Tessaria integrifolia Ruiz et Pav.) in bars and islands of different topographies in a section of the Parana´ River downstream from the confluence of the Paraguay and Parana´ rivers. The results indicate that both species have similar niches in relation to hydrological fluctuations. However, willow was significantly more frequent at of 48.49 m a.s.l., while palo bobo reached its highest frequency at 49.29 m a.s.l. The difference between the modes of the distribution curves of each species was 0.80 m. Canopy trees such as willow and palo bobo are adapted to flooded soil conditions for 77.8 and 40% of their respective lives and survive with a longlasting inundation phase (306 and 366 days, respectively). However, many trees in the Parana´ River floodplain died when the flood period extended for more than 1 year.