The effects of thinning on within-stand light conditions and the composition and structure of saplings in a mixed stand of Sugi (Cryptomeria japonica) and deciduous hardwood species in Iwate Prefecture, northern Japan

本研究の目的は,スギ－落葉広葉樹複層混交林への除間伐が林内光環境と林床木本の組成と構造に及ぼす影響を明らかにすることである｡除間伐区と対照区に0.04ha(20mX20m)のプロットを4つずつ設定し,林分構造,林床木本,林内光環境を調査した｡除間伐後4年を経過した除間伐区の林内rPFDの平均は12.1%であり,間伐直後には林内rPFDが25%に達したと推定された｡スギに対する本数間伐率30%前後の下層間伐に加え,林冠上部を構成する広葉樹優勢木が除伐されたことで,顕著に林内光環境が改善されたと考えられる｡林床木本の種数と優勢椎樹高は,林内rPFDの高い除間伐区で高かった｡また,.林内rPFDが高いプロットほど,林床木本の種数と優勢椎樹高も高くなった｡これらのことから,除間伐による光環境の改善が,林床木本の侵入･生存,そして樹高成長を促進したと考えられる。The effects of thinning on within-stand light conditions and the composition and structure of saplings in a mixed stand of Cryptomeria japonica and deciduous hardwood species, located in Iwate Prefecture, northern Japan, were investigated. In 2002, thinning was carried out in part of the mixed stand. In 2006, four 0.04-ha plots each were established in thinned and control blocks of the mixed stand, and stand structure, sapling characteristics and within-stand light condition were studied. The relative photon flux density (rPFD) was 12.1% in the thinned block 4 years after thinning and was estimated to be 25% immediately after thinning. The mean rPFD in the thinned block was significantly higher than in the control block. Within-stand light conditions were found to be improved by low thinning of Cryptomeria japonica and selected cutting of dominant hardwoods. The number of species and the upper height of saplings positively correlated with rPFD. In conclusion, thinning of mixed stands effectively promotes colonization and the growth of saplings