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beetle outbreaks. Windstorms followed by bark beetle outbreaks lasting for several decades, so called the spruce phenomenon, are well known in conditions of spruce dominated forests (Okland; Bjornstad 2006; Lausch et al 2011).
Based on the analysis of growth sequences we can state that moderate to major wind events affected the surveyed subalpine spruce forest almost constantly over the last two centuries. Reconstruction of disturbance regime revealed three periods of growth releases during the last 150 years. This suggests that moderate to high intensity disturbance events occur rather periodically every 70 years and the growth pulses spanned for several decades. The first period of release events spanned from 1860 to 1890. When we considered the maximum growth response as the date when the event occurs, we can confirm the temporal position of disturbance events by examination of written historical sources. Our results are comparable with disturbance chronology constructed by Zielonka et al. (2011) for the High Tatras, because of the vicinity of these two sites (approx. 50 km). The first major event (1860) was also recorded by Zielonka et al. (2011) but authors were not able to find any historical record confirming the windstorm in this locality. However, the study of Zúbrik (2013) presents the high severity windstorm in the Low Tatras in 1870. Author notes that more than 6 million m3 of wood were downed by the wind during this event. The second period of disturbances in 1920−1940 has been documented by Zielonka et al. (2011). Author revealed the set of disturbance events in 1915, 1919 and 1941. Our results refer to the same sequence but we recorded a high severity event in 1931, what is consequently confirmed by Zúbrik (2013). Author presents the series of windstorms in 1921, 1925 and 1930. The sequence of wind disturbances, especially the event in 1930 was proven by historical record in forest management plan for the years 1920−1930. This series of windstorms are well documented by exact volumes of uprooted trees and reforestation goals. The volume of downed wood in affected management units increased during the windstorm period almost threefold. The third period shows the tight linkage with the broadly documented dieback of subalpine spruce forests due to emissions and acid rains during the 1990s.
The analysis of growth changes and tree recruitment clearly showed that the wind disturbances occurred periodically in the last 150 years. The recruitment evaluation confirmed that the disturbance regime of subalpine spruce forest is driven by the combination of gap and patch dynamics. For this reason, the recent state of studied spruce subalpine forest is probably only the natural stage of the spruce forest development. However, the lack of silvicultural intervention connected with global climate change contributes significantly to the current condition of subalpine forests. That’s why the increased attention of forest ecologists and foresters should result in ecologically based, systematic management of mentioned ecosystems. The silvicultural management should secure the sustainable presence and functioning of diversified forest stand. Special attention has to be paid to preservation of non-productive functions (Saniga 1997; Gubka 1998; 2000; Kucbel 2000a).
According to analysis of tree ages, type of recruitment as well as the reaction of sampled trees on the canopy opening we can state that researched subalpine spruce forest was affected by combination of large scale and small scale disturbances. The occurrence of large scale high severity events was proved almost periodically in the analyzed period while the intensive growth reaction of survived trees spanned for more than one decade.
This study was supported by the Slovak Research and Development Agency, project APVV-0286-10.
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