Seed dynamics and seedling establishment of woody species in the tropical savannas of Central Brazil (Cerrado)

Studies of seed dynamics and seedling establishment at the community level in savanna ecosystems are scant, particularly in the tropical savannas of Central Brazil (cerrado). Cerrado vegetation types differ from closed savannas with high tree density and a nearly continuous canopy cover to open savannas with few scattered short trees. Whether or not dynamics of seeds and seedlings of woody species are consistent with variations in tree density and tree canopy cover across the cerrado landscape has not been determined, but could potentially influence such spatial patterns of tree density and canopy cover.The objective of this study was to evaluate seed dynamics (i.e., seed rain, soil seed banks, and seed characteristics) and seedling establishment of woody species in three major cerrado vegetation types (closed, intermediate and open savannas) which are located along topographic gradients of approximately 30 m in elevation and 1000 m in length.I conducted field and laboratory work from June 2005 to August 2008 at the Instituto Brasileiro de Geografia e Estatistica (IBGE) reserve, a field experimental station located 35 Km south of Brasilia, Brazil (15° 56' S, 47° 63' W, altitude 1100 m). The results of this study indicate that cerrado vegetation types are limited in seeds and seedlings of woody species, but these limitations are higher in open than in closed savannas. Most woody species do not form persistent soil seed banks along cerrado vegetation types because their seeds are short-lived, predated and quickly removed from the ground. Timing of seed dispersal and dormancy appear to control timing of seed germination in the field. In the absence of fire, establishment of woody seedlings is higher in closed than in open savannas because canopy cover mitigates the stressful environmental conditions of open savannas and thus facilitates establishment of woody seedlings. Fire, however, reduces seedling establishment, increases vegetative reproduction, and changes the composition of woody and herbaceous species.This dissertation study indicates that interactions of early life-history stages of woody species with environmental and disturbance factors appear to play a more significant role in maintaining woody cover variations along cerrado topographic gradients than previously recognized.