Death of Oaks in Sweden

Kornik

1991

S. 59

Unselbständiges Werk

200075056

Oaks, Quercus robur, mainly within stands, but also some solitary trees, started to die in Sweden in the early autumn 1987. Damaged sites contain pure oak stands or oak mixed with beech Fagus sylvatica and/or Norway spruce Picea abies. Most oaks seem to have been killed in 1988. However, the dying continued in 1989 and some trees will probably die also this year, 1990. The region of damage of oaks within stands is in the very south and southwest of Sweden. Further north only solitary oaks are reported to have been killed. In Sweden the northern border of natural regeneration of Quercus robur is close to the N 60Grad. This is about 500km north of the main area where oaks in stands are damaged. Five stands, out of 15 reported, were briefly investigated during 1984 /85, 1985/86 and 1986/87. More over the summer 1987 was extremely cold and rainy. It seems as if the winter ending in 1987 was exceptional. At the same time, as January was the coldest month in hundred years all over the country the layer of snow in the south of Sweden was very thin. These circumstances might have predisposed the trees to a bark necrosis agent. This gives us the hope that the dying of oaks will be occasional 1988, 1989 and 1990. The trees, which were chosen for close examination, were still alive but severely damaged. Some dead trees and some healthy ones were also examined. Growth of the annual ring had suddenly stopped in 1987 in all trees studied. A characteristic bark necrosis was found in the trunk in the damaged and dead trees. The necrosis developed from places in the trunk and in thick branches. The necrosis might be caused by a fungus, but so far fruit- bodies have not been found on the trunks. The bark seems to be killed without host reactions e.g. callus trees affected, but it is not clesr whether they are related to the place where the necroses started or not. The symptoms of the solitary oaks investigated are different from those in stands. They have not shown the typical bark necrosis. In Sweden we had three unusually cold winters