Disturbances in variation of the annual ring width of Norway spruce in the Polish Western Beskids Mountains

Changes in radial increment of Norway spruce in 25 stands more than 100 years old were analysed. Stands were growing on sites of different exposure situated at 450-1,200 m above sea level in the Polish Western Beskids Mountains. In the mid-1990s a considerable increase in annual ring width was observed. The greatest increase occurred in trees growing at the highest altitudes. This phenomenon occurred after a long period of increment decrease at the turn of the 1970s. After 1990 as the annual ring width was increasing, the variation and amplitude of ring width also increased. The cause of a long-term increase in radial increment was the improvement of thermal and pluvial conditions during late winter and early spring having a substantial effect on Norway spruce growth. In the 1990s the period of rapid changes in thermal and pluvial conditions of summer began. After moist and cold summers there were hot and dry summers. This resulted in increased variations and amplitude of the growth responses of Norway spruce and contributed to the weakening of trees above 100 years of age. The increasing break-up of Norway spruce stands growing in the lower mountain zone of the Polish Western Beskids Mountains, observed in recent years, is the final effect of this process.