Composición de las comunidades arbóreas de la Isla Martin Garcia en relación a un gradiente ambiental

Abstract

Species composition of Martín García Island forests was studied in relation to a topographic gradient and the distance to the river. An index of species abundance was calculated on the basis of all individuals with diameter equal to or greater than 10 cm, using a sampling method based on distance measures. A direct ordination determined the importance of topographic level; distance to the river, and top soil texture as explanatory variables of species variations. A cluster analysis was performed to obtain groups of sites with similar composition. All environmental variables were mutually related and associated to the first ordination axis which explained 65% of vegetation heterogeneity. Flooding probability decreased with coast distance because higher sites were farther away from the river. Higher sites had higher sand contents than lower sites, but texture effect was not dicriminated due to the high degree of correlation among environmemtal variables. There were foristic similitudes among the island and other evironmentally similar forests of the lower basin of the Río de la Plata. The highest forests of the island were similar to other unfooded sites near the Uruguay river (El Palmar National Park) and Río de la Plata (talares de Magdalena). The lowest sites, with high flooding frequency, shared many species with subtropical humid forests (selva paranense) and were similar to the lowest sites near the Uruguay river (El Palmar) and Río de la Plata (selva marginal de Punta Lara). Intermediate topographic positions at the island showed similar composition to equivalent sites at Uruguay river and shared species with both ends of the gradient.