Effects of insect defoliation and radial growth on decline of Betula maximowicziana

为阐明次生林中美洲桦（Betulamaximowicziana Regel）在食草性昆虫（日本木虱幼虫）发生后的衰退过程，选取46株进行观测，研究了发生前美洲桦的食草性昆虫落叶量、树冠状况和径向生长的年变化。在连续三年发生的第二年，有15%的观察树发生了严重的冠顶枯（即衰退）。广义线性模型表明，随着落叶程度的增加和径向生长速率的降低，冠顶枯病发生的可能性较大。然后，80%的严重衰退的树木在两年内死亡。这些结果表明，B.具有长期低生长速率的Maximowicziana将易受严重的昆虫落叶的影响。

To clarify a decline process of Betula maximowicziana Regel in a secondary forest after the outbreaks of herbivorous insects (larvae of Caligula japonica), we selected 46 trees for the observation and investigated the annual variation in degree of insect defoliation and crown condition, and radial growth before the outbreaks. Severe crown dieback (i.e. decline) occurred in 15% of the observed tree in the year following the outbreak of the three consecutive years. Generalized liner model revealed that the severe crown dieback is likely to occur with increasing degree of defoliation and with decreasing the radial growth rate before the outbreaks. Then, 80% of the severe decline trees died within two years. These results indicate that B. maximowicziana that have a long-lasting low growth rate would be susceptible to severe insect defoliation.