POPULATION TRENDS OF FOREST BIRDS IN A FINNISH LAPLAND LANDSCAPE OF LARGE HABITAT BLOCKS - CONSEQUENCES OF STOCHASTIC ENVIRONMENTAL VARIATION OR REGIONAL HABITAT ALTERATION

Birds were counted in Northern Finnish Lapland in an area situated 200 km north of the Arctic Circle in virgin forests in the 1940-50s, and both in virgin and managed forests in 1982-89. The area consisted of very large habitat patches: blocks of virgin and managed forests were separated and covered hundreds of square kilometers. The most abundant species fluctuated similarly both in virgin and managed forests in the 1980s. The yearly total density of tropical migrants (insectivores) was negatively correlated with the number of cold days in spring. Short-distance migrants were probably more affected by weather conditions during migration and wintering area. Species which increased and those which decreased in Northern Finland due to habitat alteration had no such trends in large virgin forests, either from the 1940-50s to the 1980s or during the 1980s. The brambling Fringilla montifringilla was the only species having a significant within-habitat trend: it increased from 1940 to the 1950s but decreased in the 1980s. The brambling is a forest-habitat generalist, and therefore its trends are not likely to be explained by habitat alterations caused by forestry. Year-to-year variations in densities of species seem to reflect stochastic environmental changes, such as in weather conditions and seed crop. Regional population changes caused by habitat alteration are not reflected in population densities in large virgin forests. The northern taiga species preferring virgin forests (Tetrao urogallus, Picoides tridactylus, Parus cinctus, Perisoreus infaustus and Pinicola enucleator) have drastically declined in the whole of Northern Finland due to habitat loss caused by forest management. These decreased species seem to survive in large, virgin forests, and therefore the protection of these old-aged coniferous forests is essential.