Rejuvenation of a mountain birch forest by an Epirrita autumnata (Lepidoptera : Geometridae) outbreak

摘要

In 1955, the mountain birch forest in the Abisko valley of northern Sweden was defoliated by caterpillars of the moth Epirrita autumnata with death of stems of the polycormic trees as a result. Since then, some stands have recovered from surviving trees, producing new shoots while other stands became rejuvenated from basal sprouts. Usually, basal sprouting occurs after the death of whole trees or stems. In this case, extensive basal sprouting occurred in spite of only minor stem mortality. Two associated explanations to these differences were explored: (i) the abundant basal sprouting was caused by more top damage (and, hence, a reduced apical dominance) than in stands with less sprouting, (ii) this top damage was caused by a higher caterpillar density in the upper part of trees. Studies made during the outbreak and reported here confirm (i) and reject (ii) by showing that caterpillar densities were lower in the upper part of crowns than in the lower. The more extensive damage in the upper part of crowns may have several explanations. However, a specific sequence of events seemed to produce the rejuvenated stand. The sequence included a severe stress existing before the outbreak (as indicated by a decreased radial stem growth) which was increased by defoliations in two successive years. We, therefore, hypothesize that top dieback was caused by between - and within - stem competition for depleted carbohydrate resources at the expense of tree tops. The contributions of different types of sprouts to the rejuvenation and prerequisites for an outbreak to rejuvenate stands are discussed. (C) 2003 Elsevier SAS. All rights reserved.