Environmental determinism and stochasticity in an undertory community of the Atlantic Forest

Author(s)

Bruno Almozara Aranha

• Document type: Master's Dissertation
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Biologia
• Defense date: 2008-02-22

Examining board members

Fernando Roberto Martins; Adriana Maria Zanforlin Martini; Marco Antonio Portugal Luttembark Batalha

• Advisor: Fernando Roberto Martins

Abstract

Both determinism and stochastic process are important to community organization and composition, although environmental determinism increasing in harsh habitat has been recently hypothesized. We aim to test this hypothesis in an understory community of the Atlantic rain forest in Ubatuba, São Paulo State, SE Brazil. We sample plants taller than 30 cm, with perimeter at breast height up to 15 cm, in 25 plots of 3 x 3 m placed in concave site and 25 plots placed in convex site. We measured variables related with micro-relief (elevation, slope, and index of convexity) and luminosity (canopy openness). We classified species into two components: residents (those completing life cycle in the understory) and transients (young individuals of species of upper strata). Through simple and partial Mantel tests and partial multiple regression, we confront the distance matrices of these two components and the total community with the distance matrices of the micro-relief variables, luminosity, and space (geographic distance). We also generated richness curve for both components and total community in different environmental situations. To evaluate whether environmental determinism increases in harsher sites, we split up the range of each environmental variable into three matrices, with 20% of the low values, 60% of the middle values, and 20% of the extreme values. Also, we split up the range of slope into 40% of the low values, 20% of the middle values and 40% of the extreme value and we rerun the analysis. Elevation and slope showed a higher explanation power of community composition than canopy openness. However, most of community variation remained unexplained. We found higher environmental determinism only on steeper sites. In these sites, the richness of the total community and transient component was smaller, whereas the richness of the resident component was higher. Our results pointed out that resident species had more success on steeper sites, whereas transient species were better colonizer of the flatters sites. We suggest that the ability of resident species to occupy steeper sites could be afforded by higher ratios of vegetative reproduction. We corroborated the hypothesis that environmental determinism increase in harsher sites and concluded that environmental determinism has a role in the community assembly coupled with stochastic processes (AU)