Can germination explain the distribution of tree species in a savanna wetland?

The relationship between seed germination and ecological niche is determined by matching germination characteristics with environmental features. In this study, we selected tree species occurring in the largest savanna wetland in South America - the Pantanal. Very few species are endemic or exclusively found in savanna wetlands, and the majority of tree species occurring in the Pantanal are also found in the neighbouring Brazilian Cerrado, a drier vegetation type that does not flood. We investigated the relationship between germination characteristics and occurrence of savanna trees in wetlands testing the hypothesis that such seeds are tolerant to flooding. We also addressed the question of whether seed tolerance to flood, assessed by survival analysis, explains tree distribution along a gradient of flooding intensity. In this flooding gradient, widely distributed species are those that occur in areas subjected to low as well as to high flooding intensity whereas restricted distributed species are those that occur only in areas subjected to a low level of flood. Seeds from tree species occurring in areas subjected to different flooding intensities were collected. Seed tolerance and germination during and after both one and two months of simulated flood were evaluated. Our results show that seeds of most of the studied savanna species tolerated submergence, which helps to explain their occurrence and wide distribution in wetlands. Nevertheless, germination behaviour checked by survival functions (i.e. how germination is distributed over time) partially explained tree species distribution along a flooding gradient. We conclude that seed tolerance to flooding is one of the components of the regeneration niche that determines tree occurrence and distribution at the regional scale, from savanna to wetland, but not at a local scale along a flooding gradient.