Growth pattern analysis and stemwood production in an unmanaged old plantation of hiba, Thujopsis dolabrata, in northern Japan

Abstract

We analyzed the growth patterns of Thujopsis dolabrata var. hondai trees in an old plantation (161 years old), where no silvicultural treatments (e.g., thinning) have been conducted since the initial planting. The analysis focused on understanding individual growth under a long-term self-thinning process, and the stand-level stemwood production at the mature stage was evaluated. Nine canopy-layer trees and one suppressed tree were used for the analysis of annual increments in stem diameters, heights, and stemwood volumes for a given past year using the ring-width data. Both the diameter (at basal portion) and height of all the canopy-layer trees increased at similar rates during the early stage (i.e., 60–70 years after planting); however, after this period, only the height growth rates declined sharply. The annual growth rates of stemwood volume also simultaneously leveled off at the stand age of 40–60 years. Subsequently, the patterns diverged conspicuously, e.g., the growth rates were maintained or increased in some individuals, while it gradually decreased in the case of others until the present year. The divergence of growth pattern was likely to be triggered by intertree competition at several decades after the onset of canopy closure. The current stemwood production of the sample trees, including the suppressed one, was positively correlated with certain size parameters such as stem diameter at breast height and sapwood area at a height of 4m. Based on the diameter-base allometry, the total stand stemwood production was estimated to be about 12.8m³ha⁻¹ year⁻¹. This estimate was higher than those of some old natural T. dolabrata forests (2.0–8.6m³ha⁻¹ year⁻¹) that have been well managed by repetitive selection thinning. Furthermore, individual mean stemwood production of the study plantation (0.03m³ tree⁻¹ year⁻¹) was within the range of these natural stands (0.01–0.05m³ tree⁻¹ year⁻¹). These comparisons suggested that the old T. dolabrata plantation still maintained a relatively high stemwood production potential despite the absence of artificial controls of tree density in the past. In terms of timber production, this fact implied that a rather long rotation (>100 years) can be applicable in the management of T. dolabrata plantations.

Key words:

• Allometry
• Growth analysis
• Plantation management
• Stemwood production
• Thujopsis dolabrata