Synthesis Section 1: A Dynamic Forest in a Changing Environment

Abstract

Typically, European birches are a characteristic product of primary or human-induced secondary succession (Atkinson 1992). The mountain birch (Betula pubescens ssp. czerepanovii) is, however, an exception to this and forms a stable climax forest in north-western Europe. Mountain birch forests are found in parts of the world with relatively low human populations and long distances to major pollution sources (parts of the Kola peninsula are, however, an exception, Kozlov and Barcan 2000). These characteristics, in combination with the low productivity of this birch forest type (Chaps. 4, 5), may give the impression of a very stable environment where dramatic changes or events are rare. Several chapters in this (and the following) section(s) show that this is not the case. This system is highly dynamic at several different scales and aspects. First, the mountain birch seems to be in a phase where its genome is changing relatively fast. It has repeatedly been suggested that in Iceland and continental Europe that the mountain birch formed through introgression between downy and dwarf birch (B. pubescens and B. nana; see e.g. Vare 2001), but the introgression hypothesis has only recently been confirmed by molecular techniques (Thorsson et al. 2001). The Greenland birch seems to be the result of similar introgression between B. pubescens and B. glandulosa (Sulkinoja 1990). The introgression probably contributes to large variability among individual trees in many characteristics and it seems plausible that introgression is an important factor that improves the fitness of the mountain birch in this particular environment. The dwarf birch successfully grows at considerably higher altitudes and latitudes than the downy birch (for distribution data on these species, see e.g. Chap. 1; Lid 1974; Hulten and Fries 1986 ). In fact, when comparing seedling growth performance under the climatic conditions prevailing in the mountain birch forest zone, the mountain birch outperformed downy and dwarf birch in several measures of growth (Karlsson et al. 2000). Similarly, near the tree line, mountain genotypes grow faster than lowland