Total Wood Volume Equations and their Validation for Tecomella undulata Plantations in Hot Arid Region of India

Tecomella undulata is one of the most important indigenous timber tree species found in hot arid areas of India but no volume equations are yet available for this species. Accurate estimation of tree volume is of fundamental importance in forest ecosystem modeling and regional carbon accounting. The aim of this study is to develop equations for predicting individual tree total volume for T. undulata. A total of 75 trees with ages between 15 and 19 years and breast height diameter (dbh) between 4.2 and 22.9 cm were felled for model fitting while an independent data set of 35 trees (age between 14 and 18 years and dbh between 5 and 19.2 cm) was used for model validation. Linear and non-linear equations were used to model the relationship of total volume with dbh, and with dbh and total height of the trees. Most of the equations tested fitted the data well. Based on the fit and validation statistics, equation of the type V = aD^bH^c showed, in general, the best results. An equation that fits very well to a data set may not necessarily be the best when applied to another independent data set collected from the same population. The contrasting results obtained between model fitting and validation emphasize the need for model validation as an important step in the model construction process. Model validation with an independent data set indicated that for greater accuracy models should be calibrated with local data.