Abstract
This study was conducted in 2019 to compare the growth performance and soil quality of two agesequences of Gmelina arborea plantation within the premises of the University of Port Harcourt, Nigeria. Data were collected from two stands of G. arborea established in 2011 and 2015. Growth performance was evaluated based on tree growth variables and above-ground carbon stored. Tree growth variables estimated were total height (TH), diameter at breast height (DBH), crown height (CH), crown diameter (CD) and merchantable height (MH). Topsoil (0 – 30 cm) samples collected from the two sites were analyzed for particle size distribution, organic carbon (OC), total nitrogen (TN), available phosphorus (Av.P), exchangeable bases (Mg, Ca, K and Na), exchangeable acidity (Al+ H+), effective cation exchange capacity (ECEC), base saturation (BS), pH, Manganese (Mn), Iron (Fe), Copper (Cu) and Zinc (Zn). The above-ground biomass (AGB) and carbon stock (CS) were also determined. T-test was used to test for significant difference in the measured parameters between the two age-sequences of G. arborea. Higher values for TH, DBH, CS and MH were recorded for the older stand although the differences between the two age-sequences were not significantly different (p ≥ 0.05). The AGB and CS per hectare were higher for the older than the younger G. arborea stand (302.27 m3 ha-1 and 151.52 m3 ha-1, respectively). Higher values for silt, clay, Ca, Mg, Al+ H+, ECEC, BS, Mn, Fe and Zn were also recorded for the older stand. However, the observed differences were only significant (p < 0.05) for clay, pH, Av.P, Mn and Fe. The study revealed that although soil properties, tree growth as well as carbon sequestration capacity of G. arborea stand improved/increased with age, the differences were mainly not statistically significant (p ≥ 0.05) between the two (eight and four years) age-sequences.
 Keywords: Gmelina arborea, age sequence, soil quality, carbon sequestration
Highlights
The role of tropical tree plantations in carbon sequestration through the capture and storage of carbon in wood and soil, has been underscored
The soil samples were bulked in triplicates for each age-sequence and taken to the laboratory for analysis using standard laboratory procedures described in Agbenin (1995)
The particle size distribution was determined using the hydrometer method; the exchangeable bases were determined using ammonium acetate extraction method; exchangeable acidity was determined by the titrimetric method; available phosphorus was determined by the molybdate blue (Bray No 2 extraction) method, total nitrogen was determined by Kjedahl method; soil pH was measured in 1:1 soil: water ratio; organic carbon was determined by Walkley Black wet oxidation method and organic matter derived there from by multiplying with 1.72 (Agbenin, 1995)
Summary
The role of tropical tree plantations in carbon sequestration through the capture and storage of carbon in wood and soil, has been underscored. The species is native of India and Burma where it reaches its best development but its natural distribution extends from Himalayan in Pakistan to Nepal, Cambodia, Vietnam and southern provinces of China (Onyekwelu and Stimm, 2002) It tolerates a wide range of conditions with annual rainfall from 750 to 500 mm, mean annual temperature of 21-28 °C and deep, well drained, base-rich soil with pH between 5.0 and 8.0 (Onyekwelu, 2002). The species is widely used in land restoration projects, there is paucity of information on changes in growth, soil nutrient and carbon storage capacity as the tree grows. This study evaluated the impact of stand age on tree growth rate, soil quality and carbon storage using two age-sequences (4 and 8 years) of Gmelina arborea plantation.
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