SEED BANK DYNAMICS AND ALLELOPATHIC EFFECTS OF AQUEOUS LEAF EXTRACT OF SOME TREE SPECIES ON Zea mays L. IN TWO FOREST RESERVES

Abstract

ABSTRACT Agroforestry practice has been identified as a potential means of improving food security. However, allelopathic influences of trees on crops are yet to be totally understood, especially in agroforestry systems in Nigeria. This study investigated seed bank dynamics in Olokemeji (Ogun State) and Onigambari (Oyo State) Forest Reserves and allelopathic effect of some tree species on Zea mays L. Study areas, 90 m x 90 m each were mapped out using systemic line transects. Soil samples were taken from different soil depths (0-10 cm, 11-20 cm, and 21-30 cm) in both locations, while seed bank density was evaluated for 6 months in the screen house. Simpson’s index, Shannon-wiener index, species richness and species evenness were calculated for all trees found at each location. Aqueous leaf extracts from three dominant tree species in each location (Senna siamea Lam., Anogeissus leiocarpus (DC.) Guill. and Perr., Millettia thonningii Schum and Thonn., Leuceana leucocephala Lam., Khaya senegalensis Desr. and Enterolobium cyclocarpum Jacq.) were prepared using standard procedures. Allelopathic influence of each extract was investigated on Z. mays arranged in a complete randomized design with five replicates, while treatment with addition of water served as the control. Agronomic parameters such as plant height (PH), number of leaves (NL), root length (RL), leaf area (LA), chlorophyll content (CC), and yield were measured. Crop growth rate (CGR), leaf area ratio (LAR) and net assimilation rate (NAR) were calculated. Data were analysed using One-way Analysis of Variance while means were separated using Duncan’s Multiple Range Test at p < 0.05. Results showed that Olokemeji and Onigambari had 553 and 266 tree species, respectively. Simpson’s index of Onigambari showed 0.90 diversity while Olokemeji had diversity of 0.85. Shanno-Wiener index and Species evenness of Olokemeji showed 2.13 and 0.83 respectively, while Onigambari had 2.41 and 0.89, respectively. The seed bank of Olokemeji produced 130, 78, and 40 seedlings at 0-10 cm, 11-20 cm and 21-30 cm depths, respectively whereas, Onigambari had 92, 49 and 37 seedlings at 0-10 cm, 11-20 cm and 21-30 cm, respectively. Both forest had the same Simpson’s index nevertheless, Shannon-Wiener index, species richness, and species evenness of Olokemeji Forest Reserve were higher at all depths than Onigambari Forest Reserve. Leaf extracts of five out of six dominant trees increased agronomic parameters of Z. mays. Among the six dominant trees, extract of S. siamea had increased PH, and NL of Z. mays by 8.1% and 8.8%, respectively compared with the control. Also, leaf extract of L. leucocephala increased RL, CC, LAR and NAR of Z. mays by 8.1%, 1.7%, 24.1% and 36.8% respectively, compared with the control. However, the PH, NL, CGR, NAR and yield of Z. mays treated with E. cyclocarpum leaf extract decreased by 13.3%, 19.2%, 45.9%, 42.1% and 50.5% respectively, compared with the control. The study showed that Olokemeji had a richer seed bank and higher tree frequency than Onigambari. Also, aqueous leave extracts of the studied tree species had positive allelopathic influence on Zea mays except Enterolobium cyclocarpum.