Soil seed bank at an abandoned Afromontane arable site

Species diversity, density and spatial distribution of the soil seed bank was investigated at an Afromontane forest site abandoned after clearing and cultivation in eastern Ethiopia. At least 89 plant species were identified in the litter and soil layers down to 9cm with a total mean density of 11870 viable seeds/m 2 (standard error = 3674 seeds/m 2 ). Herbs accounted for 89% of the species and 93% of the total soil seed bank. The share of woody species in the soil seed bank was very low, probably due to the continuous weeding associated with cultivation of crops at the site. Most of the species represented in the soil seed bank are successional species with hard seeds that can withstand and regenerate after disturbances. There were large differences in the spatial distribution between species, suggesting differences in mode of dispersal, seed longevity and predation pressure. Out of the 89 species, 46 species were recorded from only one sample, 41 species from two to nine samples and only two species from all the ten samples. The density of seeds recovered from the ten samples ranged between 2090 and 37070 seeds/m 2 . The overall vertical distribution showed a similar trend to earlier studies with the highest densities occurring in the upper three centimeters of soil and gradually decreasing densities with increasing depth; relatively high densities also occurred in the litter layer. Principal Component Analysis revealed that the soil seed bank at the abandoned site is more similar to that of the adjacent dry Afromontane forest than to the soil seed bank at the arable land in the surrounding. Results from the present study and casual observation indicate that succession at the abandoned site is progressing towards the composition and structure of the nearby dry Afromontane forest vegetation. However, the regeneration of woody species is being slowed down or arrested due to human impact and pressure from grazing animals.