SOIL SEED BANK, FLORA SPECTRUM AND CARBON STOCK IN A COPPICED TEAK (Tectona grandis LINN. F.) PLANTATION IN GAMBARI FOREST RESERVE, IBADAN, NIGERIA

Abstract

Teak, Tectona grandis, is an economic tree widely used for timber production, and its coppice shoot regrowth exploited for pole and firewood. Establishment of Teak Plantation (TP) is important to reduce the growing gap between the demand and supply of timber. Natural and anthropogenic factors like Soil Seed Bank (SSB), flora incursion and grazing could threaten the integrity of TP and its Carbon Stock (CS) capacity. However, there is dearth of information on how SSB, flora spectrum, and CS could help in determining the resilience of coppiced-TP in Gambari Forest Reserve (GFR), Ibadan, Nigeria. Therefore, SSB, flora spectrum and CS were investigated in a coppiced-TP in GFR. Thirty 50m x 50m main-quadrats were randomly located for enumeration of tree species in wet and dry seasons. Within each main-quadrat, five 10m x 10m sub-quadrats and within each sub-quadrat, five 1m x1m sub-sub-quadrats were laid to enumerate shrubs/climbers and herbaceous species. Within each main-quadrat, soil samples were collected from five random spots at 0-5cm (Shallow Topsoil-ST) and 5-10cm (Deep Topsoil-DT); and were each bulked and air-dried in the laboratory. Soil (1kg) was taken from each bulked-soil, and spread in 20cm x 20cm x 3cm perforated plastic trays in three replicates in the screen house to monitor seedling sprouting for five months. In each main-quadrat, height and girth at breast height of teak plants (girth≥30cm) were measured. The Relative Importance Value (RIV) for each plant species in TP and SSB were determined using standard procedures. Community structure indices (Dominance D, Shannon-Wiener-H′ and Jaccard-J) were evaluated. A non-destructive method and allometric equations were used to evaluate teak CS. A total of 106 and 79 species belonging to 37 and 34 families were identified in the coppiced-TP during wet and dry seasons, respectively. The highest RIV in wet and dry seasons were obtained in Tectona grandis (43.5% and 43.8%; trees), Carpolobia alba (17.4% and 26.3%; shrubs), Icacinia trichantha (23.7% and 34.1%; climbers) and Chromolaena odorata (25.1% and 17.0%; herbs), respectively. The D and H′ were: 0.60 and 1.19 (trees), 0.24 and 1.91 (shrubs), 0.15 and 2.17 (climbers) and 0.09 and 2.68 (herbs populations), respectively. The vegetation in both seasons were perfectly similar for trees (J=100.0%), but less similar for shrubs (J=85.7%), climbers (J=55.6%) and herbs (J=46.7%). In SSB, 41 species belonging to 17 families, had 20 and 38 species in wet and dry seasons, respectively. Chromolaena odorata had the highest RIV in wet and iii dry seasons (ST-21.9% and 18.2%; DT-17.1% and 16.3%, respectively). Dominance in ST and DT were 0.10 and 0.13, while H′ was 2.67 and 2.48, respectively. Similarity was least (J=36.5%) between ST (wet season) and DT (dry season) and the highest (J=76.3%) between ST (dry season) and DT (dry season). Jaccard similarity range (SSB/TP) was 0.0-1.8%, 4.2-10.3%, 0.0-3.5% and 25.9-64.7% for SSB/trees, SSB/shrubs, SSB/climbers and SSB/herbs, respectively. Total CS for teak was 877.8 kg/ha. There were incursions of Carpolobia alba, Icacinia trichantha and Chromolaena odorata in the coppiced teak plantation in Gambari Forest Reserve, Ibadan, which caused low teak carbon stock.