Stand and individual growth in an old <i>Cryptomeria japonica</i> plantation after intensive thinning

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  • 強度間伐が行われたスギ高齢人工林における林分および個体の成長
  • キョウド カンバツ ガ オコナワレタ スギ コウレイ ジンコウリン ニ オケル ハヤシ ブン オヨビ コタイ ノ セイチョウ

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Abstract

Stand- and individual-level growth responses after intensive thinning were investigated in a 114-year-old Cryptomeria japonica plantation in Iwate Prefecture, focusing on the effects of relatively late thinning (when the stand was 90-year-old) on growth, and on factors causing variation in individual diameter growth. Before this late thinning, stem density, the relative yield index, and the crown length ratio (crown/height) were 458 trees ha-1, 0.55, and 0.45, respectively, indicating that the stand was not crowded. The thinning removed 64％ and 53％ of trees in terms of　number and volume, respectively, and reduced stem density and the relative yield index to 167 trees ha-1 and 0.27. The mean annual height growth rate after the thinning was almost the same as the value before the thinning. The mean annual diameter growth rate was 0.21 cm year-1 before the thinning, and increased to 0.43 cm year-1 after the thinning. The stand-level stem volume growth rate after the thinning was 8.20 m3 ha-1 year-1, which was not greatly lower than the pre-thinning value, 8.55 m3 ha-1 year-1. The diameter growth rate at the individual level before the thinning was found to correlate significantly with the initial diameter size at the beginning of each census period. However, this correlation became unclear after the thinning, and then re-emerged at 20 years after the thinning. Analysis using a neighborhood inter-tree competition model recognized effects of one-sided competition on diameter growth after the thinning, while the effects of two-sided competition were not observed. These results suggest that, in a well-managed old C. japonica plantation, intensive thinning at a relatively late growth stage (ca. 100 years) to keep the stand at extremely low density may be a good option for producing large-sized and high quality timber in the future when a hyper-long rotation is applied (e.g., >150 years).

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