CAUSES AND PRINCIPLES OF THE CATASTROPHIC, LARGE-SCALE DEMISE OF HEMLOCK (TSUGA CANADENSIS) AND ELM (ULMUS SPEC.) ON THE NORTHERN HEMISPHERE AROUND 3700 BC

Holocene forest ecosystem changes happen normally on a long-term scale. Therefore, the sudden, catastrophic and famous decline of hemlock (Tsuga canadensis) in north-eastern North America and of Elm (Ulmus spec.) in (northern) Europe during the first half of the 4th Millennium BC is very intriguing. Both tree species more or less disappeared within a few years (or decades) from the Northern Hemisphere after having been important forest trees for centuries before. Today, both declines are commonly attributed to pathogen attacks and in case of the elm-decline also to human impact. However, in view of these nearly synchronous events on both sides of the Atlantic Ocean the possibility of a common triggering mechanism of these major tree declines has to be assessed. Several proxies point to the possibility that global climatic factors may have been involved in this rapid and unprecedented arboreal declines, which finally lead to major forest composition changes within the dense American and European forest after 3500 BC.