Evaluation of poplar (Populus deltoids Marsh.) stands height growth using a Generalized Additive Model (Case study: Guisoum & Haft-Daghanan region, Guilan province)

Site productivity is a key indicator of forest ecosystem, and is therefore an important criterion for forest managers to estimate the yield, annual exploitation and site growth. In this study, dominant height index was used as the most reliable criterion for evaluating site productivity of even-aged stands. A selective sampling method was used to locate 52 square sample plots of 0.04 ha each in poplar stands. The height and diameter of Populus deltoides Marsh. trees and altitude of each plot was recorded. Also, in each plot, soil samples from 0-20 cm and 20-40 cm depths were taken for analyzing several soil properties. Evaluation of stands productivity by using generalized additive models and AIC showed that altitude, P, saturation, organic carbon , gravel, N from 20-40 cm depth as well as Ca, bulk density and pH from depth 0-20 cm were the significant variables explaining the 90% of variability in productivity of P. deltoides. Using the relative importance criterion showed that altitude, P from 20-40 cm depth and bulk density from 0-20 cm depth were associated with dominant roles, yet the overall contribution of altitude, organic carbon from 20-40 cm depth, bulk density from 0-20 cm depth, together with other variables, were the most significant drivers of poplar site productivity.