Comparing the performance of generalized diameter-height equations for Maritime pine in Northwestern Spain

The objective of this study was to develop and compare generalized diameter -height equations for pure stands of maritime pine (Pinus pinaster Air.) in Galicia, Northwestern Spain. The data used to fit the equations consisted of 621 diameter-height observations sampled from 67 plots installed in pure , uniformly stocked stands covering a wide range of forest stand and site c onditions across the province of Pontevedra. In a first step, nonlinear lea st squares regression was used to test the performance of two published equ ations, one derived from the RICHARDS function (equation 1) and the other o ne from the SCHUMACHER function (equation 2). In both cases, the residuals indicated no severe violation of the fundamental statistical assumptions, w ith root mean square errors of 1.44 m for equation (1) and 1.49 m for equat ion (2). In a second step, equation (2) was modified resulting in two new e quations which showed only a marginal improvement. Finally, the precision w as evaluated for all equations using an independent data set. The strongest correlation between observed and predicted values was obtained with equati on (1). However, differences between the four equations were rather small.