TREE DIVERSITY AND SPACIAL STRUCTURE OF PINE-OAK FORESTS OF IXTLÁN DE JUÁREZ, OAXACA

Abstract

This paper presents a comparative analysis of tree species diversity and the spatial structure of several pine-oak forest stands in Ixtlán de Juárez. Following a randomly-stratified sampling design, a forest survey was applied on 91 circular sites of 500 m2. Using the importance value and the dominance level for all present pine species, eight phytosociological communities were defined; Pipat, Pioax, Pipse, Pidou, Piaya, Piteo, Pirud, and Pilei. Diversity was assessed through the Shannon Index (H’). Three indexes were used to describe the forest structure according with species mixture (Mingling Index, Mi), pattern of horizontal distribution (Uniform Angle Index or Contagion Index, Wi), and differentiation of tree sizes (Dominance Index, Ui). All recorded individuals (n) were split into 2,612 structural groups, which were compound by the five nearest neighbors. According to H’, the Pipat and Piaya communities had the broadest trees diversity (2.19 and 2.10 with 26 and 19 tree species, respectively). The Pipat, Piaya, Pilei and Pioax communities showed the greatest mixture of species (0.591 < Mi < 0.687), while for all the eight phytosociological communities, the horizontal spatial arrangement was random (0.530 < Wi < 0.569). The tree diameter size analysis, defined by the Ui at the community level, showed that 46-51% of trees were thinner than reference trees. It was concluded that tree species diversity and stand structure are different and dependant on the phytosociological community.