Oak decline in Helsinki portrayed by tree-rings, climate and soil data

Oak decline has recently been observed in and around Helsinki. Tree-ring widths of pedunculate oak were used to assess pre-mortem growth patterns, their dependence on climatic factors and linkages to soil thickness. Tree-ring chronologies were constructed in three tree vigour classes (healthy, declining and dying oaks). Characteristic "summer response" was found as a positive influence of summer precipitation was the most important climatic factor limiting the radial growth in all vigour classes. On the other hand, a differing "winter response" was found for tree-rings of dying and other classes of oaks. The growth of dying oaks was more sensitive to variations in mid-winter temperatures, due presumably to higher risk of frost damage to their roots. Recent summer droughts, which may have increased the potential for bark necrosis due to reinforcing effects from defoliation in decreasing the capability of oaks to acclimatize to winter frost, may thus have played a role in this decline. Amplified water stress was indicated by dendrochronological parameters on sites with shallow soils.