Growth patterns of plantation-grown Taiwania cryptomerioides following thinning

Abstract

Summary Taiwania (Taiwania cryptomerioides Hayata) is an important tree species in Taiwan; its rapid growth makes it a choice species for intensive silviculture. To assist silviculturalists in planning operations, we developed growth and yield models for taiwania stands that had undergone four levels of thinning: none, light, medium, and heavy. The variables that we modelled are average diameter at breast height, average height, basal area per hectare and volume per hectare. We used data from a thinning experiment with the four levels of thinning undertaken at ages 17 and 25 y. The Chapman-Richards function provided the basis for our model. We modified this function to account for both the immediate effects of the thinnings and their longer-term effects on growth. The model was fitted using nonlinear mixed modelling with plot as the subject for the random effect. Since the thinnings were from below, they tended to increase the average diameter and height but decreased the basal area and volume of the residual stands. However, the total volume available (volume removed+volume standing) increased as the thinning intensity increased. Taiwania is found mainly at higher elevations and consequently the sites it grows on can be sensitive to erosion and landslides. Managing for wood production must therefore consider site protection and other aspects of resource management. We discuss the practical applications of these models that will be useful to managers trying to efficiently manage taiwania plantations on sites that are limited by biophysical and ecophysiological constraints.