STRUCTURAL-CHANGE IN A SUB-ALPINE BIRCH WOODLAND IN NORTH SWEDEN DURING THE PAST CENTURY

An isolated subalpine woodland with predominant mountain birch (Betula pubescens Ehrh. subsp. tortuosa (Ledeb.) Nyman) was studied with respect to structural change during the past century. Present-day age and size data of tree populations and the field-layer flora were analysed against the perspective of an early twentieth century account of the forest structure and flora. Air temperature recordings during some periods of the 1980s provided information on the local thermal characteristics in relation to the physiological tolerances of the tree species. Extensive invasion of spruce (Picea abies (L.) Karst.), and to a lesser extent pine (Pinus sylvestris L.), peaked in the 1930s, coinciding with the general summer warming (about 1-degrees-C) of the first half of the twentieth century. The subalpine birch forest changed mainly by increased height and proliferation of basal shoots. The spacing structure of birch individuals, however, was largely unaffected by climatic change. Presumably, increased vegetative growth related to breakage of main stems from rime ice, snow and wind in combination (loss of apical dominance), and increased nutrient mineralization. Conceivably, mechanical snow impact has selected for mountain birch predominance and restricted the abundance of coniferous tree species. Tree invasion into an open forest occurred despite the increased vigour of the pre-existing tree dominant, although it was possibly preceded and facilitated by disturbance in the form of transient defoliation from insect attack (Epirrita autumnata Bkh.) or some other agent.