Evaluation of Seed Bank in Three Age-Sequences of Arable Land within a Biosphere Reserve in Southwestern Nigeria

Abstract

Deforestation through shifting cultivation to feed the burgeoning population in Nigeria is intensifying without adequate knowledge of its impact on the self-repairing mechanisms of the forest ecosystem. This study examined soil seed bank in three age-sequences of arable land - AF1, AF2 and AF3, reflecting short, medium, and long period of cultivation respectively, at three depths - 0-5, 5-10 and 10-15 cm. Herbaceous plants accounted for 85% percent of plant species in the seed bank at all age-sequences and soil depths. Species richness dropped sharply with increasing soil depth in all the age-sequences, and was highest in AF1, AF2, and AF3 at the 0-5 cm, 5-10 cm, and 10-15 cm respectively. Seedling abundance decreased sharply with increasing soil depth in all age-sequences with AF3 having the highest number of seedlings at the 0-5 cm depth. Although AF3 had the highest seedling abundance at the 0-5 cm depth, seedling diversity in the age-sequences decreased with increasing length of cultivation with AF1 and AF3 having the highest and lowest diversity, respectively. Longer period of cultivation in AF3 seems to have favoured the populations of some seemingly more tolerant herbaceous species like Digitaria ternata and Spigelia anthelmia, both of which accounted for 75% of the total seedlings recorded at the 0-5 cm depth in AF3. Similarity in species composition between age-sequences was over 70% at the 0-5 cm depth but dropped to below 50% at lower depths, and between soil depths in the same age-sequence except between AF1 10-15 & AF3 10-15 cm and AF1 5-10 & AF2 5-10 cm. The dominance of herbaceous plants in seed banks at AF1, AF2 & AF3, and the cutting of trees associated with shifting cultivation, imply that the native woody species have slim chance of regenerating in the farmlands. Therefore, a more eco-friendly farming system like agroforestry using indigenous species is recommended for the restoration of the native woody flora in the degraded farmlands.