The Impact of Extreme Weather conditions on the forest dieback in the Polish Mountains

In the last decade (2011-2020), forests in the Sudetes Mts. (Poland) have shown a marked increase in the release of deadwood and an acceleration of the process of tree dieback. One of main tree species in the Sudetes is spruce, which quickly disappears from this area. The aim of the work was to determine the impact of climatic conditions on this process. The research used two measures characterizing the tree dieback process: the tree dieback intensity index and the tree species dieback intensity index. The following bioclimatic variables were used to estimate the impact of climatic conditions: air temperature, precipitation, hydrothermal Sielianinow index (HTC), forest aridity index (FAI), and the frequency of hurricane winds. Since 2015, there has been a very significant acceleration (10 times on average for the area) of tree dieback; the intensity coefficient of this process in 2015-2020 was several times higher than in 2011-2014. The greatest intensity of stand decay occurred 300-600 m above sea level, mainly coniferous species die, especially spruce, and to a lesser extent deciduous species. The intensification of the dieback process began after the extreme drought in August 2015. Another extreme spring–summer drought in 2018 and summer heat waves intensified this process. An additional factor weakening tree stands was the more frequent occurrence of strong winds (e.g., the hurricane ‘Grzegorz’ in 2018). The high volume of deadwood in 2017 and 2018 was augmented by quite a high volume of wind-broken and wind-fallen trees. These weather conditions constituted an important factor in the development of secondary pests. The weakened spruces were attacked by the bark beetles, the outbreak of which caused a massive dieback of spruce stands. In the last two years, mistletoe (Viscum album ssp. album) has become an increasing threat to forests in the Sudetes.