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

Abstract

The vegetation of the Paraná 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 Paraná River downstream from the confluence of the Paraguay and Paraná 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 Paraná River floodplain died when the flood period extended for more than 1 year.

Distribution Pattern of Trees in a Hydrological Gradient below the Paraná-Paraguay River Confluence

“…In a more recent study, [16] documented the differences in the distribution of two pioneer species compared with the results reported by [51] 20 years earlier. Another study by [52] on the Missouri River showed that forests are sensitive to hydrological changes and can modify their distribution and diversity.…”

“…These small differences in the range of distribution frequencies are related to the type of sediments that form the soil on each bank of the river, as indicated [16] in a study of pioneer forests of the Paraná River in this area. The studied species usually have normal distribution curves, as indicated by the frequencies reported by [48] in a study prior to the disturbances that have occurred since the dam was built on the Upper Paraná.…”

“…Riparian and riverine aquatic plant species have evolved within the context of a flowing water habitat [9] for which flooding and droughts are forcing factors in shaping the community features, through either a positive or negative effect on the ecosystem's function, according to the frequency, magnitude, amplitude, and timing of such events [10][11][12][13][14][15][16].…”

“…Riparian and floodplain environments are known to support disturbance-adapted species assemblages [17,18], frequently leading to defined vegetation zonation in which the species' lower distributional limits are determined by the specific tolerance to flooding [19][20][21][22] and droughts [10,16].…”

“…In the Paraná floodplain landscape, topographic position is a crude indicator of the position along the complex gradient, but it also includes information about flood/drought periods and the resilience of trees to extreme hydrological phases [16].…”