Evaluation of oriental beech (Fagus orientalis L.) site productivity using generalized additive model (Case study: Tarbiat Modares University Forest Research Station)

Site productivity is a key indicator of forest ecosystem; therefore it is an important criterion for forest manager for estimating the yield, annual exploitation and growth, and choosing the most suitable tree species for sites as well. In this study site form index which is the most reliable criterion for evaluating site productivity of mixed and uneven stands was used. A random-systematic sampling method was used to locate 43 0.1 ha circular sample plots in beech dominated forests. The height and diameter of Fagus orientalis Lipsky trees with DBH ≥ 7.5 cm within each plot was recorded along with elevation, azimuth and slope of the ground. Also, at the center of plot, soil samples from first layer (0-10 cm) were taken for analyzing several soil variables. Evaluation of site productivity by using generalized additive models in GRASP package in R 2.9.2 software showed that 53% of variability in oriental beech productivity could be justified using environmental variables.  Bulk density, altitude, percent of slope, percent of silt, carbon to nitrogen ratio and saturation are among the significant variables affecting the site form. By using relative importance criterion, the alone contribution showed altitude had the dominant role, but the overall contribution of the slope percent was the highest in relation to other variables. Using AIC, RMSE, R squared and adjusted R squared showed GAM performs better than GLM.