Core samples from 72 dead and 72 living oaks were analysed using dendrochronological methods. A method called cross dating, in which the growth pattern of living oaks is compared to the growth pattern of dead oaks, was used to determine the specific year of death for the dead trees. As seen in Figure 1, most trees died during the last decade, with two pronounced peaks in 2004 and 2006. Studying each site separately revealed that the peak in 2004 was represented mainly by data from Tinnerö, while the peak in 2006 could be devoted foremost to data from Sturefors. No distinct pattern in the frequency of dead oaks was observed for Norrköping.

In order to determine whether the oak decline in this study could be attributed to a specific environmental factor, the cores were analyzed for a possible common year for which the majority of the cores of the trees destined to die started to express reduced growth. The average growth chronologies of dead and living oaks are shown in Figure 1. The two curves had an excellent match up until around 1992, after which the dead trees started to express reduced growth. Climatic data showed that the precipitation for 1992 were well below normal for most part of the year, and in combination with an extremely high summer temperature, the oaks were most likely under a severe drought stress during most part of the growing season. Both curves also showed a particularly strong growth reduction in 2003-2004, after which the living trees seemed to recovery over the following years. This is most likely the result of a combination of a defoliation caused by the European oak leaf roller (Tortrix viridana) and a mildew infection of the replacement shoots during the following year.

The length of a growth depression prior to death gives valuable information about the character of the death inducing event. Extensive growth depressions occurring over several decades before the death event are thought to be caused by long-term stresses, such as competition or changes in climate, while shorter growth depressions are more likely to be caused by short-duration stress factors, such as insect defoliations or extreme droughts. In this study, several more or less pronounced growth depressions were detected over time when studying averaged dead tree ring series on a time-prior-to-death scale, representing each site separately. As seen in Figure 3, a growth depression prior to death was evident for at least 1- 5 years prior to death for Tinnerö and Sturefors, and at least 25 years prior to death for Norrköping. This gives support to the proposal of the insect outbreak and mildew infection as the ultimate mortality factor causing a relatively rapid death of the oaks growing in Tinnerö, and most likely also in Sturefors. The more extended mortality process illustrated by trees growing in Norrköping suggests a rather long-term stress, such as climate, as the triggering factor of the oak decline. Furthermore, Figure 2 shows that the general growth trend differentiated among the sites, with trees from Tinnerö expressing an overall higher growth rate.