An assessment of height–diameter growth variation in an unmanaged Fagus orientalis-dominated forest

Abstract

Allometric variations in tree height and stem diameter are a genetically controlled trait that reflects the ability of trees to adjust to different environmental conditions. This paper examines the ecological control of abiotic and forest-state variables on height- and stem diameter-growth in oriental beech (Fagus orientalis Lipsky) in an unmanaged, high-elevation forest of northern Iran. Spatially explicit abiotic variables of growing-season mean potential solar radiation, mean air temperature, topographic wetness index (TWI, as a proxy of soil water content) and wind velocity essential to the analysis were generated numerically by computer models. Forest-state variables of total tree height (H) and diameter at breast height (D) and stand basal area (BA) were assessed at individual sample plots. Degree of explanation of observed variation in individual-tree H and plot-level changes in stem D (i.e. dD/dt) by the assemblage of modelled abiotic and observed tree variables was 87.4%, with a root mean squared error (RMSE) and mean absolute error (MAE) of 3.60 and 2.84 m and 67.3%, RMSE = 0.13 cm and MAE = 0.10 cm, respectively. Wind velocity, TWI and mean air temperature provided the greatest overall influence on the calculation of static H and annual D-increment, with relative impact of 38.3, 37.3 and 9.6 and 7.7, 6.4 and 6.7, respectively. The other variables, including BA, had weak to no control on tree-growth response. Methods used here are sufficiently general to address tree-growth response in many other tree species around the world, with or without changes to site conditions.