Long-Term Influence of Herbivores on Northern Birch Forests

摘要: Species of many insects, some mammals and birds feed on buds leaves mountain birch, Betula pubescens ssp czerepanovii (Haukioja Heino 1974; Brittas 1988; Neuvonen et al. 2001). Of these species, two geometrid moths, Epirrita autumnata (autumnal moth) Operophtera brumata (winter moth), are the most important because their large-scale long-term influence birch ecosystems. The earliest report an outbreak is by Ruge in 1700s from Nord-Valdres southern Norway (see references Tenow 1972), next a mass occurrence 1852 at Alta northern Norway. Since then, 15 periods have been documented sixteenth currently (2004) going (Tenow 1972; Ruohomaki 2000; Helena Bylund, pers. observ). Conclusively, outbreaks indigenous ancient phenomena birch/insect system. population fluctuations more or less synchronized, caterpillars causing defoliating about every 9–11 years Haukioja but cf. Chap. 9). consequences depend several circumstances, primarily what type forest attacked, i.e. heath forests mainly with polycormic (multi-stemmed) trees which propagate particular basal sprouts (cf. 1), meadow richer sites monocormic (single-stemmed) propagating seeds (Sandberg 1963; Holm 1994). Circumstances influencing severity age previous abiotic biotic stresses. In case severe defoliations, die, as do stems trees. surviving mortality shoots heavy. Stems that survive benefit reduced competition will soon produce new leaves. However, long time it takes for seedlings to grow replace leaf-carrying canopy was lost tree individuals died Bylund