Spatial distribution of an undergrowth palm in fragments of the Brazilian Atlantic Forest

摘要

The occurrence of disturbed sectors may be as important as microscale edge effects in tropical forests fragments. We considered the spatial distribution dynamics of life stages (youngs, immatures, and adults) of the acaulescent, endemic palm Attalea humilis Mart. ex. Spreng. in fire-prone fragments of the Brazilian Atlantic forest to investigate population responses to fragment size and fire occurrence. From 1996 to 1999, we recorded the number of individuals in different life stages in adjacent 10x10-m plots in two perpendicular transects across each of three fragments of different sizes (1.6, 6.4, and 9.9 ha) in the Reserva Nacional de Poco das Antas, Southeastern Brazil. A fire burnt the fragments studied in 1997. Application of Morisita's Index showed clumping at most scales, with a marked reduction in clumping degree from youngs to adults, a pattern not influenced by fragment size. After fire, clumping degree increased temporarily among the youngs. No association between life stages was detected using presence/absence data. Palm density concentrated in the most disturbed transect arms, a pattern very little affected by fragment size or fire. No consistent relationship between palm density and distance from fragment edge was detected for any stage, fragment, or year. We hypothesize that short-distance seed dispersal by scatterhoarding rodents results in discrete, low-density clumps of youngs dissociated from reproductive plants. The spatial pattern of later stages is probably conditioned by the distribution of canopy gaps in the largest, more closed fragment. In the other fragments, the presence of large disturbed sectors promotes the formation of large, continuous stands of palms. This pattern would be reinforced by fire, which is known to be recurrent on already disturbed sites. For the species studied, large-scale variations in the forest structure (degraded vs. preserved fragment sectors) seems to be more important than microscale edge effects.