SOIL SEED BANK, FLORISTIC COMPOSITION AND CARBON STOCK IN THE SECONDARY FOREST AT COCOA RESEARCH INSTITUTE OF NIGERIA, IBADAN

Abstract

Exploration of Soil Seed Bank (SSB), Floristic Composition (FC) and Carbon Stock (CS) represents a standard approach to determining the health of a forest ecosystem. Anthropogenic disturbances change the composition, physiognomy and health of the forest. The SSB diversities are key components of ecosystem resilience for restoration of many plants after forest disturbances. However, there is dearth of information on effect of season and soil depth on SSB as it relates to vegetation, and CS in Secondary Forests (SF) in Nigeria. Therefore, SSB, FC and CS in the SF atCocoa Research Institute of Nigeria (CRIN), Ibadan, were investigated. Twenty five 50 m x 50 m main plots were randomly mapped out in the SF of CRIN to enumerate tree species in Wet Season (WS) and Dry Season (DS). Within each plot, five 10 m x 10 m sub-plots were randomly mapped out to enumerate shrubs. Three quadrats (1 m x 1 m) were laid in the sub-plots to enumerate herbaceous species. Soil samples were collected at 0-5 cm (Shallow Topsoil-ST) and 5-15 cm (Deep Topsoil DT) from five points within each main plot and each bulked in both seasons following standard procedures to determine SSB composition and diversity. Bulked soils (1kg) were each spread in 150 germination trays (1200 cm3 ) in completely randomised design to monitor seedling emergence for 24 weeks in the screen house. Relative Importance Value (RIV) and community structure indices (Dominance, Shannon-Wiener, Equitability and Jaccard) of the vegetation and SSB at ST (SSB-ST) and DT (SSB-DT) were determined. Estimation of CS (ton/ha) of trees (with girth ≥30 cm) was carried out using a non-destructive method and allometric equations. A total of 219 plant species from 59 families which included 63 trees, 34 shrubs and 86 herbaceous species were identified. In the WS, Triplochiton scleroxylon (5.6%), Lonchocarpus griffonianus (8.8%) and Chromolaena odorata (25.2%) had the highest RIV while in the DS, Terminalia superba (5.0%) Lonchocarpus cyanescens (8.6%) and C. odorata (12.3%) were highest among trees, shrubs and herbs, respectively. In the SSB, C. odorata had highest RIV in WS (13.9%-ST and 16.8%-DT) and Peperomia pellucida in the DS (10.4%-ST and 13.4%-DT). Dominance was 0.03, 0.03, 0.11, 0.08 and 0.11; Shannon-Wiener was 3.81, 3.60, 3.54, 3.10 and 2.61; while Equitability was 0.92, 0.93, 0.75, 0.80 and 0.74 for trees, shrubs, herbs, SSB-ST and SSB-DT, respectively. In both seasons, vegetation were perfectly similar (C=100%) for trees and shrubs, but less similar (76.4%) for herbaceous species. Jaccard was 19.0-28.1%, 0% and 0.0-1.7% for SSB/herbs, SSB/shrubs and SSB/trees, respectively. Similarity iii (Jaccard) was the least (29.6%) between DT-WS and ST-DS and the highest (75.0%) between DT-WS and ST-WS in SSB. The total CS of SF was 19.46 and the highest value of 2.84 for Albizia coriaria alone. There were high diversity and random distribution of species with low restoration potential and resilience for trees and shrubs, but high potential for herbs in secondary forest of Cocoa Research Institute of Nigeria, Ibadan. The trees of the secondary forest had potential for accumulating carbon.