<論文>間伐とシカ排除柵設置がスギの幹成長に及ぼす直接的・間接的影響 --芦生の未間伐スギ人工林における事例--

Abstract

間伐やシカ排除柵の設置を, 立地環境が悪く, 間伐が遅れているスギ人工林において検討した事例は見当たらない. このようなスギ人工林では間伐の効果がほとんどない可能性があり, また, シカ排除柵の設置は下層植生の増加を通じてスギの直径成長を間接的に抑制するかもしれない. そこで本研究では, 間伐とシカ排除柵がスギの定期直径成長量および下層植生の植被率・群落高に及ぼす効果を芦生研究林にある未間伐の40年生スギ人工林で調べた. 間伐試験では, 定期直径成長量は間伐区で有意に大きくなっており, 40年生の未間伐スギ人工林でも間伐は残存木の直径成長を促進することが示された. 下層植生の植被率と群落高は間伐の有無で違いはほとんどなく, 間伐が下層植生の変化を通じてスギの成長に及ぼす影響はよくわからなかった. シカ排除試験では植被率と群落高は柵内で有意に大きくなっていたが, 柵内外の定期直径成長量の違いは認められなかった. 下層植生の植彼率・群落高が大きくなるにもかかわらずスギの直径成長量が抑制されない原因はよくわからなかったが, 少なくとも間伐後3 年目の時点では, シカ排除柵の設置はスギの直径成長を抑制しないことが示唆された.

Effect of thinning and deer exclosure on periodic annual increments in diameter of Cryptomeria japonica and vegetation coverage and height were investigated in a 40 years cedar plantation in Ashiu, Japan. We examined whether thinning promotes stem growth of cedars in plantation with delayed thinning on bad growing condition and whether the deer exclosure restrict the stem growth through competition between cedars and understories. In thinning experiment, periodic annual increments in diameter was significantly larger for individuals in thinning plot than those in control plot, indicating that the thinning was effective for a plantation with delayed thinning on bad growing condition to promote diameter growth of cedars. Effect of thinning on vegetation cover and height was not significant in the thinning experiment, and hence, the indirect effect of thinning on the stem growth through understory vegetation was obscure. Deer exclosure significantly increased the vegetation cover and height, whereas the diameter growth of cedars inside fence was not different from that of cedars outside fence. These results show that the deer exclosure does not reduce diameter growth of cedars within three years. However, how the effect of understory on the growth of cedars was reduced remains unsolved.