SOIL AND VEGETATION DYNAMICS IN SECONDARY FOREST REGENERATION IN DEGRADED RUBBER PLANTATION, OROGUN, DELTA STATE

Abstract

In Nigeria, secondary forests of different types are increasing in area, and becoming increasingly important for species diversity conservation and soil fertility regeneration. Previous studies on the dynamics of soil and vegetation have focused on secondary forests regeneration after shifting cultivation. Despite the increasing importance of secondary forest regeneration in degraded rubber plantation, there is hardly any comprehensive study on the dynamics of soil and vegetation in secondary forest regeneration in degraded rubber plantation. This study therefore examined the dynamics of soil and vegetation in secondary forests recovering from degraded rubber plots in Orogun, Delta State. Soil samples were collected from 10 plots (each 30m x 30m) in 1-, 5-, 10- year forest and from adjoining mature forest (control). Soil physico-chemical properties (exchangeable cation (calcium, magnesium, potassium and sodium), available phosphorus, pH, bulk density, water holding capacity (WHC), total porosity, effective cation exchange capacity (ECEC), organic matter (OM) and nitrogen) were analyzed, using standard laboratory procedures. Vegetation above-ground biomass (AGB) and floristic parameters (tree height, density, diameter, basal area, above-ground biomass, number of tree species, number of plant species and species diversity) were measured in each plot. Litterfall was collected monthly in each plot for 12 months using 1m2 litter traps. Analysis of variance was used to ascertain whether the soil and vegetation parameters differ among secondary forest categories and between pairs of secondary forest and mature forest at P < 0.05. Stepwise multiple regression model was used to ascertain the relationships between soil and vegetation parameters. Soil exchangeable cation and available phosphorus concentrations increased significantly during the first five years of fallow but declined by the tenth year and thereafter, increased significantly in the mature forest (control) than all the secondary forest categories. Soil pH and bulk density decreased significantly over time, while soil WHC and total porosity increased significantly. Soil ECEC, OM and nitrogen increased significantly with increasing age of secondary forest. Above-ground biomass decreased significantly from 349 t/ha in the mature forest to 74 t/ha and 5.1 t/ha in the 10-year and 1-year fallows respectively. Species diversity and AGB were significantly higher in the mature forest than in all the secondary forest categories. Litterfall increased significantly from 1114.9kg/ha-1 year-1 in the 1-year fallow to 11324 kg/ha-1 year-1 in the mature forest. Litterfall and the amount of nutrients returned to the soil through litterfall increased significantly in the order of 1-year<5-year<10-year<mature forest. Species diversity, AGB and litterfall jointly accounted for 56%, 84%, 81% and 86% of the increase in ECEC, OM, WHC and total porosity respectively over time. Fifty two percent of the increase in OM was accounted for by above-ground biomass while species diversity and litterfall accounted for 18% and 14% respectively of the increase in OM. Secondary forest regeneration in degraded rubber plantation is capable of minimizing species diversity losses and restoring soil fertility. Therefore, 5- to 10-year fallow cycles should be encouraged as a management strategy in secondary forest regeneration in degraded rubber plots. This would ensure sustainable cropping with or without fertilizer application. Keywords: Litterfall, Soil Dynamics, Above-ground biomass, Species diversity, Degraded rubber plantation Word count: 499