Prediction of Stem Volume, Biomass and Carbon Using Age for Eucalyptus grandis in Nuwara Eliya and Badulla Districts

Abstract

Forest trees act as carbon sinks and the main stem of the trees contribute most to the carbonstorage. Since Eucalyptus grandis has been extensively planted by both government andprivate sectors in the upcountry wet zone of Sri Lanka, this study attempted to predict thevolume, biomass and carbon content of the main stem of this species with age. In order toobtain a wider geographical coverage in data collection, 30 even-aged Eucalyptus grandismonoculture plantations were selected from Nuwara Eliya and Badulla Forest Divisions. Ageof those plantations varied from 7 to 40 years. Radom sampling of 0.02 ha plot size was usedfor data collection. Diameter at breast height and total height were measured from thesampled trees and a core sample was obtained from the stem for the determination of biomassand carbon amounts. Stem volume was estimated by using the model v = 0.3648(g * h) builtby Subasinghe in 2001 for the same species. Stem biomass and carbon amounts of the stemwere estimated by converting biomass and carbon contents of the extracted core samples ofknown volumes. Walkley-Black method was used for carbon content determination and thetree age was obtained from the plantation records available in the Sri Lanka ForestDepartment. Non-linear regression models were then built by using GENSTAT software topredict the selected variables using tree age. Both exponential and logistic curves were fittedto the data and model evaluation was done by the R2 values and the fitted line plots.

According to the results, the stem biomass of the average tree varied from 110.8 kg at age 7to 1,095.4 at age 40. Stem carbon content varied from 68.7 kg at age 7 to 679.4 kg at age 40.When compared with the exponential models, logistic models showed poor performances andthe finally selected models for predicting dbh, height and stem volume with age weredbh = 17.83×1.028age, h = 20.09×1.019age, v = 0.264×1.066age. The R2 values for the selectedmodels were 79.1%, 84.0% and 80.1% respectively. The selected stem biomass and stemcarbon prediction models were BMstem = 108.3×1.059age, Cstem = 69.9×1.059age and those twomodels had R2 of 78.7% and 82.1% respectively. The fitted line plots too indicate very goodfits of all the selected models.