Leaf litter of Bombax ceiba L. threatens plant cover and floristic diversity in a new urban ecosystem

Bombax ceiba L. (Bombacaceae) is a common deciduous tree introduced to Egypt for ornamental purposes. This study was conducted to describe the impact of B. ceiba leaf litter on plant cover and species diversity in a new urban ecosystem. In general, the sites under B. ceiba canopy attained lower species richness, diversity, plant cover and biomass than in adjacent open areas. Winter species were more adversely affected than summer ones. Both Amaranthus viridis and Trigonella hamosa were completely absent from the sites involving Bombax, whereas the cover of Euphorbia prostrata, Lolium temulentum and Paspalum dilatatum were stimulated. Moreover, the litter-affected soils were more abundant in organic matter and nutrients. Mulching with B. ceiba leaf litter in greenhouse reduced the emergence and growth of Amaranthus viridis, Melilotus indicus and Trifolium resupinatum, whereas the negative effect of litter amendment was pronounced for the former species only. The HPLC analysis confirmed the presence of some free phenolics and flavonoids. Gallic acid was abundant amongst phenolics (90.3 μg g−1 leaf dry weight) whereas quercetin and apigenin were common amongst flavonoids (8.7 and 6.7 μg g−1 leaf dry weight, respectively). In conclusion, the reduction in species diversity and plant cover in the understorey vegetation was attributed to the effects of the tree litter. The test due to litter amendment into soil denotes that allelopathy may have function at least for some species. Besides, the statistical differences between mulching and litter amendment indicate that reduction in plant cover and diversity could be related to the mechanical effect of the litter. These findings suggest that B. ceiba affects species diversity and plant cover and, therefore, represents a threat to the plant communities in the new urban areas. The severity of the litter impact was correlated with seed size, litter amount and the season coinciding with leaf abscission.