Vegetation and its relationship with geomorphologic units in the Parana River floodplain, Argentina

Abstract

The Parana River is one of the most important fluvial systems of South America and its floodplain includes the most diverse subtropical ecosystem on the continent. However, the relationship between basic aspects, such as the vegetation and geomorphology of the river floodplain, has scarcely been investigated. In this paper, the annual dynamics of vegetation in relation to the geomorphologic and hydrological characteristics of a river floodplain around 31° 30′ S, are analyzed. The annual dynamics of vegetation was investigated using values of the Normalized Difference Vegetation Index (NDVI) obtained from satellite images at two scales of spatial analysis: the first, at the geomorphologic unit level, through several transects crossing the total width of each unit and, the second, through some transects selected from each unit. Our analysis considered variables of different temporal stability (such as geomorphology, hydrology, vegetation, precipitation, and ground temperature), using scenes corresponding to two hydrological cycles of the system (2009 and 2010), which represented relatively “dry” and “humid” years. Five main geomorphologic units were identified in the floodplain of this anabranching system, which were named considering the predominant landforms and the most important (or typical) water course of each area: Bars and Islands of the Main Channel of the Parana River (BI-MCH), Scroll Bars of the Colastine Branch (SB-C), Scroll Bars of the San Javier River Channel (SB-SJ), Crevasse Splays and Levees of the Malo–Mendieta minor channels (CSL-MM), and Crevasse Splays and Levees of the Santa Fe–Coronda river channels (CSL-SFC). These major units are assembled at different general levels and with variable slopes, which partially control the permanence and other characteristics of the flood flow. The crevasse splays and river levees units were predominantly characterized by herbaceous–bushy marshy vegetation, with low mean NDVI values, while SB-C and BI-MCH units showed two types of forests. The latter showed the highest NDVI values. Unit SB-SJ showed mixed characteristics between the two above-mentioned groups. Mean NDVI values were different among the varying geomorphologic units and during the studied years, showing extreme values of 0.25 and 0.80 after both hydrological cycles. According to our results, the main variations in the seasonal dynamics of vegetation show a higher dependence on the dynamics of the hydrological cycle than on surface temperature or precipitation. The hydrological dynamics of the Parana River floodplain are in turn regulated by its geomorphological architecture, so the annual dynamics of vegetation activity in this system are finally influenced by the geomorphologic unit's level, rather than by the short duration modeling processes (i.e., hydrological cycles or drought–flood pulses).