In the years from 2007 to 2014 a fast progressing decline in foliation and health was observed in middle-aged oak tree stands, mainly in Southeastern Denmark. Foresters reported the death of individual trees (up to 10%, and locally more) and the decline syndrome was also observed in two long-term oak thinning experiments, which had previously been healthy. It was hypothesized that precipitation increase and pattern change was involved in the decline through water stress caused by occasionally rising water tables killing fine roots. Stressed trees would be less resistant to attacks from biotic pests and pathogens. Insect defoliations would reduce interception in tree crowns and cause water saturation in poorly drained soils. The possible presence of Phytophthora, including P. ramorum, was investigated in stem bleeds from unhealthy trees, but was not found. Similarly, the presence of oak buprestid beetle, Agrilus biguttatus, was investigated, but only A. sulcicollis was discovered, and this bark beetle is considered to be a secondary colonizer of dead or dying oak trees. Honey fungus (Armillaria sp.) was present in weakened trees and undoubtedly contributed to individual tree death.
An investigation focused on site factors in healthy and unhealthy (22 altogether) stands at four forest estates in Southeastern Denmark. Soils in the area were typically developed as Cambisols from calcareous lodgement till and melt-out till, mostly with signs of poor drainage.
Healthy and unhealthy stands had different site properties. As expected, unhealthy stands had a relatively low position in the landscape, and the parent materials were more often lodgement till with clay loam texture or lacustrine clay loam in contrast to healthy stands located on melt-out tills in slightly more sloping terrains. The water table in unhealthy stands was fluctuating in response to rain events within one meter depth in contrast to healthy stands with a steadily declining water table throughout the growing season. Water table increases in the wet summers 2007, 2010 and 2011 were not directly monitored. Water table rise during those years could be confirmed by a long term monitoring of water tables in a ditching experiment in the study area. The data and simulations indicated that water saturation and lack of aeration was the main cause of the oak health decline in Southeastern Denmark. A few unmanaged, semi-natural oak stands on apparently well-drained sandy soils in sloping terrain in Western Jutland were also affected. The cause on those areas is unknown, and therefore, the presence of biotic pests along with soil drainage conditions should be further monitored in the years to come.