Diversidade alfa, florística e fitossociologia na ARIE do Cerradão, na APA Gama e Cabeça de Veado, DF

Abstract

O cerradão é uma formação florestal do bioma Cerrado que compartilha espécies com formações florestais e savânicas. Devido sua topografia plana, seus solos profundos e aráveis e às espécies florestais de grande porte que comporta, e por ocorrer naturalmente em manchas, tornase uma das fisionomias vegetais mais vulneráveis do bioma. No Distrito Federal, a Área de Relevante Interesse Ecológico (ARIE) do Cerradão com 54,12 ha é uma das manchas mais representativas de cerradão e, além desta fitofisionomia, integra também, para o norte e para o oeste, um cerrado denso. O solo em toda a UC é do tipo Latossolo Vermelho e argiloso distrófico, com saturação por bases de 6% no cerradão e 8% no cerrado denso. A acidez do solo aumenta do cerrado denso para o cerradão, assim como a quantidade de matéria orgânica e ferro disponível. Este trabalho partiu da premissa de que o cerradão da ARIE é composto por uma mistura de espécies de cerrado e de florestas regionais sem apresentar uma flora exclusiva da fisionomia e sofre efeito de borda. O objetivo deste trabalho foi estudar a composição florística, a estrutura fitossociológica e os efeitos de borda do cerradão da ARIE. Para tanto, realizaram-se coletas florísticas quinzenais por toda a fitofisionomia, nas quais foram coletadas amostras de todos os indivíduos férteis presentes. Para o estudo fitossociológico, foram sorteadas dez parcelas de 20 m x 50 m, e mensurados os indivíduos lenhosos a partir de 5 cm de diâmetro a 30 cm do solo. Foram também sorteadas 40 parcelas de 1 m x 1 m no interior do fragmento de cerradão, a 175 m de distância das bordas, e 40 parcelas do mesmo tamanho na borda da mancha de cerradão, para avaliar os efeitos da degradação. Para a medida da cobertura do estrato herbáceo, foi utilizada a escala de cobertura de Braun-Blanquet, e a significância da diferença entre os ambientes borda e interior avaliada pelo teste não paramétrico Kolmogorov-Smirnov. Foi registrado um total de 282 espécies, distribuídas em 194 gêneros e 75 famílias botânicas, levando a uma proporção de 0,92 espécie herbácea e 0,53 arbusto para cada árvore. Fabaceae, Asteraceae, Rubiaceae, Poaceae, Myrtaceae, Malpighiaceae e Melastomataceae foram as famílias que apresentaram maiores riquezas em espécies. Houve mais espécies comuns ao cerrado sensu stricto que às formações florestais, porém, 72,3% das espécies foram comuns a ambas as formações. Apenas 60,43% das espécies presentes na ARIE são registradas em áreas de cerradão. Não houve espécie exclusiva da fitofisionomia. _______________________________________________________________________________ ABSTRACT

The cerradão is a forest formation of the Cerrado biome that shares species with savanna and forests. It is very threaned due to a flat topography, deep arable soils and also because it occurs as natural patches. In the Federal District, the ARIE - Area of the Relevant Interest Ecological (ARIE) of the Cerradão with 54,12 ha is is one of the most representative pacthes. Besides the Cerradão, the ARIE includes a dense cerrado physionomy too. The soil is Red Oxysoil dystrophic and clayey, with base saturation equal 6% in cerradão and 8% in cerrado denso. The soil acidity increases of dense cerrado to the cerradão, as well as the amount of organic matter and iron available. The premisse of this work was that the Cerradão at the ARIE comprises a mixed flora with species from the cerrado and from the regional forests and suffers edege effects. The objective was to study the floristic composition, phytosociological structure and the effects of edge. Floristic collections were conducted fortnightly, where all fertile individuals present in the cerradão were collected. A phytosociological survey was conducted for the woody vegetation from 5 cm diamter at 30 cm from the ground level. In this survey, ten (20 m x 50 m) plots were randomly sampled; Forty (1 m x 1 m) plots were also sampled within the fragment of cerradão, at a distance of 175 m from the edges, and 40 plots of the same size were sampled at the edge, to assess the effects of degradation. Coverage of herbaceous layer, was evaluated by the scale of coverage of Braun-Blanquet, given in percentage, and the significance of the difference between the environments was evaluated by the nonparametric test Kolmogorov-Smirnov. A total of 282 species, distributed in 194 genera and 75 families, leading to a ratio of 0,92 herbaceous and 0,53 shrub species for each tree was found in the floristic survey. Fabaceae, Asteraceae, Rubiaceae, Poaceae, Myrtaceae, Malpighiaceae and Melastomataceae, were the families that showed greater richness of species. There were more common species to the cerrado sensu stricto that to forest formations, however, 72,3% of the species were common to both formations. Only 60.43% of species present in ARIE are recorded in areas of cerradão. There was no exclusive specie of this phytophisiognomy. A total of 1.583 woody individuals, of which 125 dead standing, distributed in 74 genera and 107 species, belonging to 40 families were registered. The alpha diversity of the area was high (H'= 3,85 nats/ind.) as well as the uniformity (J' = 0,82), suggesting a relatively uniform distribution of plant species. Emmotum nitens, Qualea grandiflora and Ocotea pomaderroides were the most important species in the cerradão. The family with higher value of importance was Vochysiaceae. The diameter distribution of the community resulted in a curve of type J-reverse, indicating potential for self-regeneration and distribution of height marked a unimodal distribution. Within the the fragment, ten herbs species native and no exotic were found whereas at the edge, there were 13 native species and seven exotic demonstrating the existence of a edege effect. Echinolaena inflexa and the weeds Melinis minutiflora and Urochloa brizantha were the species more frequent of the edge of the cerradão, already in the interior the species that dominated in number of individuals and coverage were Echinolaena inflexa and Rynchospora exaltata, both native. The cerradão of the ARIE presented 95% of its flora native of the biome. The cerradão studied suffers from the effects of fragmentation, however, the interior of the ARIE is still conserved regarding this physiognomy as a priority for conservation in the Federal District where Cerradão is scarce.