Distribution of Herbaceous Species in the Soil Seed Bank of a Flood Seasonality Area, Northern Pantanal, Brazil

To identify factors affecting the spatial distribution of soil seed banks of herbaceous species in the Pantanal floodplain, Brazil, the aims of this study were: to characterize the seed bank in terms of the abundance, richness and composition of germinated seeds; to relate these characteristics to flood duration, elevation, chemical and physical properties of the soil and to examine the seed bank's spatial pattern. Soil samples were collected at 14 points and were then placed in a greenhouse to allow germination to occur. Each sample point had the flood level monitored, the elevation measured and the soil properties identified. A total of 1710 seedlings from 26 species were recorded, of which Echinodorus tenelus was the most abundant (24.9%). Cyperaceae and Poaceae were the richest floristic families (5 species each), with Alismataceae the most represented in terms of number of individuals (36% of the total). Duration of flooding plays an important role in determining Pantanal soil seed banks. The flood pulse influences the abundance (r = 0.79; P = 0.006; partial), richness (r = 0.61; P = 0.02; partial) and composition of the soil seed bank (Pillai trace = 0.552; P = 0.027), carrying the seeds to areas where the duration of flood is longer. Except for aluminium, the soil characteristics expressed by the first principal component of PCA exert indirect positive effects on the seed bank. This is because this component was correlated with the duration of inundation (r = 0.76). Elevation and the toxic effect of aluminium do not vary sufficiently to be able to influence seed bank characteristics. The correlograms show that soil seed banks have no discernible spatial pattern, even though most species are dispersed hydrochorically. This suggests that, at the scale of the study, the tendency for flooding to cause homogeneous dispersion has no influence on seed-bank spatial structure, because of the complexity of flood-plain geomorphology.