Tree species determine soil seed bank composition and its similarity with understory vegetation in a semi-arid African savanna

Abstract

IntroductionThe presence of larger trees in semi-arid African savannas creates sub-habitats, which influences on herbaceous plant communities grown under their canopies differently from opened areas. Knowledge of seed banks accumulated in the soils over time beneath larger trees could facilitate the recovery of plant communities that might disappear due to sustained heavy grazing, prolonged fire, or other anthropogenic factors in semi-arid African savannas. However, the impact of larger trees on soil seed bank composition and its similarity with plant communities grown under their canopies are less understood in semi-arid African savannas. Therefore, we studied the effect of leguminous and non-leguminous tree species and their canopies on soil seed bank (SSB) composition and its similarity with understory vegetation (USV) in a semi-arid savanna of Ethiopia.MethodsWe selected 20 matured trees from 3 dominant tree species, representing one leguminous (Acacia robusta Burch) and 2 non-leguminous tree species (Ziziphus spina-Christi and Balanites aegyptiaca (L.) Del), found in isolation, a total of 60 trees for this study. Under each selected individual tree, the species composition of USV were recorded using 1-m2 quadrat in four directions (north, south, east, and west) under the inside and outside tree canopies during the flowering stage. Similarly, soil samples in a 1-m2 quadrat were also collected under the inside tree canopies and their corresponding outside canopies, in each individual tree, for the determination of SSB composition, using a seed emergence method. Then, the soil was thoroughly mixed after removal of all roots and plant fragments, and spread over sand in plastic pots to a depth of 20 mm. The pots were placed at random in a glasshouse, examined every 3 days, for the first 2 months, and thereafter weekly for 6 months. A total of 960 soil samples were used for the determination of SSB composition during this study.ResultsA total of 64 species were emerged from the SSB samples, of which 27 were grasses (19 annual and 8 perennial grasses), 35 annual forbs and 2 woody species. Acacia robusta had a higher seedling density in the SSB compared to other tree species, whereas Z. spina-Christi had higher species diversity in the SSB than other tree species. Moreover, seedling density and species diversity were higher under the inside canopies than outside tree canopies. The mean similarity in species composition between the SSB and USV was low. However, it was higher under the leguminous trees than non-leguminous trees, and under the inside tree canopies than outside canopies.ConclusionsWe found that mature tree species maintained a higher SSB species diversity and abundance under their canopies than the surrounding opened areas. Therefore, conservation of mature dominant tree species is of paramount importance for ecological stability and possible restoration of degraded semi-arid savannas under the changing climate and global warming.