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-AF 1 , AF 2 and AF 3 , 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 AF 1 , AF 2 , and AF 3 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 AF 3 having the highest number of seedlings at the 0-5 cm depth. Although AF 3 had the highest seedling abundance at the 0-5 cm depth, seedling diversity in the age-sequences decreased with increasing length of cultivation with AF 1 and AF 3 having the highest and lowest diversity, respectively. Longer period of cultivation in AF 3 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 AF 3. 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 AF 1 10-15 & AF 3 10-15 cm and AF 1 5-10 & AF 2 5-10 cm. The dominance of herbaceous plants in seed banks at AF 1 , AF 2 & AF 3, 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.