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Tree height-diameter relationship can be used as an important input component in forest growth and yield models, and description of stand dynamics. Five nonlinear growth functions were fitted to tree height-diameter data of a 21-year-old Gmelina arborea plantation in Ibadan, Nigeria. The data consisted of three sets obtained in 2004, 2008 and 2011 for total tree height and diameter at breast height (DBH). According to the model statistics, the five growth functions fitted the data equally well, but resulted in different asymptote estimates. Modified exponential fit was observed to give the best fit for the three data sets based on least square error, coefficient of determination and significance. The predicted values follow the same nonlinear pattern and formed close to the line of best fit without much outlier. The result of this study revealed that the ability of DBH in determining height is not strong enough based on the model's goodness of fit and the model's ability for predictive purposes. Hence, more variables such as age, crown area and soil fertility were recommended to be incorporated in future prediction of the tree height in the study area.

Forest carbon is identified in three major pools that is, above and below ground living vegetation, dead organic matter and soil organic carbon whose quantities have been identified for few forest types. The study investigated the carbon stock in above and below ground biomass in three age series of Pinus (1991, 1992 and 1996) and Nauclea (1974, 1975 and 1976) plantations at area J4, Ogun state Nigeria. Five plots of 20m x 20m dimension were randomly laid making a total of 30 plots. Diameter at breast height and height were measured using diameter tape and Spiegel Relaskop respectively. Soil samples were collected in each plantation at 0–15cm and 15–30cm depth with the aid of soil auger and laboratory analysis carried out. Data were analyzed using the General Linear Model of SAS software. Analysis of Variance (ANOVA) and Duncan Multiple Range Test was used to separate means and correlation analysis was carried out. Above ground was estimated using Brown et al. (1989) equation {Y= 34.4703- 8.0671 (DBH) + 0.6589(DBH2)} and below ground biomass was also estimated as 20% of above ground biomass. Result showed that Organic carbon stored up in the soil was highest (3.05±0.52g/kg) in Nauclea plantation (1975). Pinus plantation (1996) had a higher total carbon stock (1166.35ton/ha) than Nauclea plantation (1976) with (380.41ton/ha). Pinus plantation (1996) had the highest above ground biomass, below ground biomass and carbon dioxide concentration (728966.19, 160372.56 and 4198845.28/ha) lowest (227644.66, 50081.83 and 1311233.26/ha), Pinus plantation (1992) respectively. Variation in the above ground carbon stock among these plantations could be result of tree stand density, hence, higher number of trees in a plantation will eventually result to high above ground carbon stock of that plantation.

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