SOIL-VEGETATION RELATIONSHIP IN FOREST ISLANDS AND ADJACENT SAVANNA AT THE NORTHEAST RORAIMA, AMAZONIA

Abstract

O estudo de solos em ilhas florestais inseridas em \ue1reas de dom\uednio de savanas \ue9 fundamental para compreender os processos de forma\ue7\ue3o da paisagem. Este trabalho caracterizou propriedades morfol\uf3gicas, f\uedsicas e qu\uedmicas de solos em quatro fragmentos naturais de florestas de ocorr\ueancia no mosaico savanafloresta em Roraima, norte da Amaz\uf4nia. O m\ue9todo consistiu em transectos atravessando as ilhas nos sentidos leste-oeste e norte-sul, onde foram abertas cinco trincheiras para a coleta de amostras e estudos de solos. Nas \ue1reas de savana cont\uedguas a cada ilha foram estabelecidos transectos de 100 m de comprimento e abertas cinco trincheiras equidistantes para caracteriza\ue7\ue3o qu\uedmica e f\uedsica do solo comparativa. Os Latossolos foram as classes de solos predominantes nas quatro ilhas investigadas, seguida de Argissolos e Plintossolos, todos predominantemente oligotr\uf3ficos (distr\uf3ficos, de baixa CTC, \ue1cidos). Condi\ue7\uf5es qu\uedmicas e f\uedsicas melhores foram verificadas nos solos das ilhas florestais em rela\ue7\ue3o \ue0s \ue1reas de savana circundantes, numa mesma classe de solo. Assim, embora a classe de solo n\ue3o tenha variado entre diferentes fitofisionomias em um dado gradiente, caracter\uedsticas qu\uedmicas e f\uedsicas espec\uedficas variaram e podem exercer influ\ueancia positiva no estabelecimento de vegeta\ue7\ue3o florestada. Embora florestas e savanas ocorram lado a lado, em mosaico, o clima atual com longa esta\ue7\ue3o seca \ue9 concordante com a exist\ueancia da savana em relevo plano e a ocorr\ueancia das ilhas florestais parece condicionada a varia\ue7\uf5es f\uedsico-qu\uedmicas sutis dos solos, sem necessidade de invocar uma rela\ue7\ue3o com oscila\ue7\uf5es paleoclim\ue1ticas. Estudos mais aprofundados posteriores poder\ue3o testar a hip\uf3tese de contra\ue7\ue3o ou expans\ue3o florestal durante o Quatern\ue1rio, buscando evid\ueancias de que tais ilhas possam representar rel\uedquias paleoclim\ue1ticas imersas em dom\uednio sav\ue2nico.Studies on soils of forest islands within the savanna domain are key for understanding processes of landscape formation and evolution. We characterized the morphological, physical and chemical properties of soils at four different forest fragments that occur in the savanna-forest mosaic of northeastern Roraima, north Amazonia. The methodology was based on transects crossing the entire island, from east-west and northsouth direction, digging up five soil profiles for sampling and classification. In addition, the neighboring savannas were also sampled following the same strategy, at 100 m long transects departing from the border, allowing comparisons to be made. Latosols were the dominant soil class in all four islands, followed by Ultisols and Plinthosols. All soils were dystric, with low CEC and acid. Better chemical and physical conditions were observed in forested soils compared with surrounding savannas, in a given soil class. Thus, in spite of no variation on soil class at different phytophysionomies at a given gradient, specific chemical and physical attributes were significantly varied, exerting a positive effect for the establishment of forest vegetation. Despite their occurrence side-by-side in the savanna-forest mosaic, the present-day climate agrees with the existence of savanna in the flat landforms, whereas forest islands are conditioned by subtle, yet significant, soil physico-chemical variations, with no need to invoke any paleoclimate for explaining this relationship. Further in depth studies may contribute for testing the hypothesis that Quaternary shifts of the expansion and contraction of forest may represent paleoclimate relicts isolated within the savanna domain