HEIGHT-GROWTH AND SITE INDEX EQUATIONS FOR SOCIAL FORESTRY PLANTATIONS OF ACACIA NILOTICA AND EUCALYPTUS HYBRID IN GUJARAT STATE OF INDIA

Abstract

ABSTRACT Acacia nilotica is one of the most important and useful multipurpose tree species for rural population of India; Eucalyptus hybrid is an important tree species adopted for large-scale commercial plantations by the State Forest Departments, non-governmental organizations (NGOs) and farmers in India. Accurate growth models for these species are not yet available. The aim of this study is to calibrate some existing height growth equations for these two species and use them to derive site indices. Five algebraic difference equations were used. Data were obtained from 22 sample plots of A. nilotica and 34 of E. hybrid laid out in six districts of Gujarat; all possible growth intervals was used to fit the equations. A generalized nonlinear least square method was used to take into account the error structure. Autocorrelation was corrected expanding the error term to allow a first-order autoregressive model that was adequate for the data. Different weighting factors were used to satisfy the constant variance assumptions for the error. Bias, root mean square error and Akaike's information criterion were calculated and cross validation residuals were used to evaluate the performance of the equations. The best results were obtained with the algebraic difference equation derived from the base model of Sloboda. The Sloboda equation can be recommended for both the species for site index modelling.