Historical contextualization of tree damages caused by the drought 2018 ¶

Meteorologists have shown that the summer drought of 2018 was the longest and most severe period with no precipitation since the start of systematic weather records in 1864. It is evident that this year's tree damages are directly related to this event. However, surprisingly little information is at hand on the immediate and mid-term aftermath of earlier comparable weather extremes (e.g. 1911, 1921, 1947, 1976 and even 2003). In order to properly contextualize the effects on trees and forests summer drought 2018 in the last 155 years (since 1864), the following questions are posed:

Q1: Which damages to trees and tree stands have been reported since 1864 after prolonged drought?

Q2: Which tree species have been most damaged by drought periods during the last 155 years?

Q3: Which regions in Central Europe have been most affected by drought-triggered tree damage?

Q4: Which interactions of drought and other disturbance agents have been mentioned regarding tree damages?

A literature search has been conducted on drought-triggered damages to trees and tree stands. A first focus has been put on grey literature in Switzerland by means of e-periodica, a service of the ETH-library that provides PDF-texts of most of the cantonal and national monthly or annual periodicas. In addition, literature from Germany (collaboration Peter Meyer and Christian Ammer, both Göttingen) has been reviewed in order to broaden the scope of the search to Central Europe.

The drought of 2018 between April and September is comparable to periods in 2003 and 1947. This raises the question of whether damage to trees or stands after those and other past drought events have been reported. An analysis from 1988 of reports on such damages by weather extremes since 1850 shows that dry wood after drought occurred particularly in the years 1911–12 and in the years 1943–50. Repeatedly, spruce and fir (bark beetle infestations) were affected in these years, but also beech and oak. In the 19th and early 20th centuries, the annual forest reports of the cantons often mentioned damage to plantations and tree nurseries. According to the first results, the dry year 2018 can best be compared with 1947. As a result of repeated drought events during the post-war years, for example, the situation in 1949 was described as "close to catastrophic for spruce in the Wein- and Unterland [of the canton of Zurich]".