Fruit phenology and seed germination strategies of woody and herbaceous species from Cerrado

Seed germination and seedling establishment are the initial and crucial stages of a plant’s life cycle and are important in determining plant community dynamics and plant distribution and diversity patterns. In seasonal environments, water availability in the soil and environmental heterogeneity are the main factors that limit seed germination and seedling establishment. In these environments, seed dispersal season, germination niche, and dormancy synchronize germination with the onset of the rainy season, which maximizes the favorable period for seedling establishment and increases the chances of seedlings surviving the subsequent dry season. Plants respond to environmental heterogeneity by distributing germination through space and time. Distributing seeds throughout different areas (spatial dispersal) increases the chances that at least some will arrive in sites adequate for germination and establishment. On the other hand, dormancy increases the variation in germination time, distributing germination into multiple reproductive events (temporal dispersal), thus decreasing the likelihood that all seedlings die due to unpredictable events. However, most studies addressing the mechanisms that synchronize germination with the onset of the rainy season in seasonal tropical environments do not consider germination niche or species evolutionary history and are mainly focused on forest ecosystems. Therefore, a clear understanding on the relative importance of the mechanisms controlling germination time, on the interaction between these mechanisms, and on how they might be affected by life history traits and evolution is still lacking in seasonal tropical environments. Moreover, the role dormancy plays as a mechanism to face environmental heterogeneity has not been thoroughly explored in seasonal environments. Thus, in this thesis, I evaluated the mechanisms that cerrado plants use to synchronize germination with the onset of the rainy season and to face environmental heterogeneity. I also analyzed how such mechanisms vary in relation to plants functional and evolutionary characteristics. For such, dormancy and the thermal germination niche of 82 cerrado plant species, belonging to 26 families and including several growth forms, were determined from germination experiments. Additionally, dispersal syndrome, dispersal season, and seed mass were also determined for each species. Our results show that dormancy is an adaptative characteristic in cerrado as it increases the chances of seedling establishment by synchronized germination with the onset of the rainy season and by distributing the risk of recruitment of species with limited spatial dispersal (i.e., authochorous). Our results also demonstrate that dormancy is evolutionarily correlated with spatial dispersal, but not with seed mass or dispersal season, indicating there is a trade-off between spatial and temporal dispersal. Furthermore, dormancy and germination niche are alternative mechanisms used to face seasonality, and the relative importance of these mechanisms varies with dispersal season. Hence, dormancy is key to understanding the great plant diversity found in cerrado, not only because it increases the chances for seedling establishment, but also because it is crucial for the successful recruitment of autochorous species in sites that are unfavorable for the establishment of zoochorous and anemochorous species. Finally, our results show that plant species from closed to open cerrado physiognomies show similar germination traits and proportion of dormancy, suggesting that seasonality is a strong filter for both germination and seedling establishment throughout the cerrado plant communities and other seasonally dry ecosystems. (AU)