Influence of disturbances and climate on high-mountain Norway spruce forests in the Low Tatra Mts., Slovakia

Survival and growth of trees in high-mountain regions is limited by harsh environmental conditions, and is therefore sensitive to climate change. The main aims of this study were to develop the regional boundary line for high-mountain spruce forests of the Low Tatra Mts., to reconstruct and compare the disturbance regime of investigated localities, and to present the impact of recent climate change on release potential of Norway spruce. The study area included three localities with forest stands dominated by Norway spruce that were selected to represent the altitudinal zone 1200-1500 m a.s.l., excluding the tree line and extreme sites. In order to determine the most important climate characteristics driving tree growth, site chronologies were computed from the sample cores of dominant and co-dominant trees, and correlations between chronologies and monthly climate variables from the previous May to the current September were computed for the period 1901-2008. Analysis of release potential and disturbance histories was performed using the boundary line method. Uniform macro-climate and site conditions in the selected region enabled to construct the regional boundary line for Norway spruce in the Low Tatra. The comparison between boundary lines developed for Norway spruce in the Low Tatra and the other regions showed considerable differences. Therefore, in the high-mountain conditions, the transfer of boundary lines from other regions cannot be recommended. In investigated localities, we identified different disturbance regimes that resulted in variable stand structures and modified the growth responsiveness of Norway spruce to climate. A higher sensitivity to previous October temperature was observed in the most disturbed locality, whereas the growth of Norway spruce on the site with the lowest disturbance rates showed the highest correlation with current June-July temperature. After 1990, we registered enhanced release potential of Norway spruce accompanied by the increase of disturbance rates. Increased release potential was detected even for the trees older than 150 years.