Composição florística e ciclagem de macronutrientes em diferentes seres sucessionais nas margens de reservatório de hidrelétrica no Oeste do Paraná

Abstract

The water reservoir of Salto Caxias hydroelectric power plant is located at the lower course of Iguaçu river, southwestern Paraná State (25º 32’ 35’’ S; 53º 29’43’’W). The climate is classified as Cfa according to KÖPPEN and the vegetation is characterized as an ecotone between the seasonal semideciduous and the mixed ombrofhylous forest. Vegetation structure and floristc, soil chemical and granulometric characteristics, as will as litterfall and accumulation over the forest floor were studied in five secondary succession seres (early stage, two intermediary and one advanced stage) located along the margin of the reservoir. Experimental units consisting of four 25 x 10 m replications were set in each sucessional sere. Sub units in each replication were used to sample individuals of different diameter classes, including natural regeneration. Soil was classified and samples at depths of 0,0 – 2,5; 2,5 – 5,0; 5,0 – 10,0 and 10,0 – 20,0 cm were collected and analyzed for macronutrients and C content. Within each plot there litter traps each with collecting area of 0,336 m2, were randomly located. Litterfall was collected monthly for one year, and after drying at 60º C for 24 hours on more the litter, was sorted into twigs annual branches, leaves and miscellaneous material (flowers, fruits, bark, etc). Composed samples of each type of litterfall were analyzed for macronutrient (N, P, K, Ca and Mg) content. Accumulation of litter at the forest floor was estimated collecting 2 circular (0,042 m2) random samples in each plot. Vegetation survey showed dominance of the seasonal semideciduous forest formation, growing over eutrophic Entisols. Nutrient concentration decreased with depth. Soil C and acidity increased with succession. Litterfall also increased with the sucessional process (2,2 ton.ha-1 to 11,7 ton.ha-1). Leaf litter was the greater faction in all seres. Total litterfall showed annual periodicity, correlated with maximum deposition correlated with lower temperatures following a period of high rainfall. Quantity of nutrients returned with litterfall increased with succession and followed the order N > Ca > K > Mg > P except in one of the Ca was higher than N. Accumulation of litter at the forest floor tended to increase with successional process.