Confessions of solitary oaks: We grow fast but we fear the drought

Competition effects and management related disturbances can contribute to strong non-climatic signals in ring-width trends of trees from closed canopy forests. Removing this noise comes at the price of losing a considerable amount of (climatic) information on decadal and centennial time scales. Alternatively, open grown solitary trees, which never competed for light or other resources could function as less biased climate proxies. To explore this potential we analysed individual growth trends and climate-growth relationships of solitary oaks, potentially representative of former, open forests, and compared it to those of trees growing in adjacent closed canopy forests. Solitary oaks show differing tree allometry with bigger crowns and lower absolute heights. Their radial growth was significantly higher compared to forest trees. Contrary to our expectations, these solitary oaks show highly individualistic decadal growth trends. Their inter-annual variations are nevertheless in good agreement between individuals and stronger compared to trees from closed canopy forests. Climate –growth analysis revealed a significantly higher drought sensitivity of solitary oaks. Given the typical open medieval forest structure of large trees with extensive crowns growing in a mosaic of grazed woodlands and grasslands, it is likely that these trees contribute to the historical part of tree-ring collections. When trees from today’s denser forests forming the recent parts of the collections are used to calibrate reconstruction models, the differing drought sensitivity might therefore lead to an overestimation of historical droughts.