Abstract Decline of green alder ( Alnus viridis spp. viridis [Chaix] D.C.) has been reported since the 1990s in the Alps. In recent years, this disease has spread all over the Alps and it is now recorded over all Italian alpine regions, with several secondary green alder stands heavily affected. Old damaged stands show dramatic changes both in tree species composition and coverage. Investigations were carried out in Trentino province (northern Italy) to describe the pathological and ecological aspects of this phenomenon. Various fungi and insects were detected on declining trees, but no single agent appeared to be a primary cause; the most common coloniser of declining stems, Cryptodiaporthe oxystoma (Rehm) Urb., had an endophytic behaviour in green healthy tissues but failed to produce symptoms in artificial inoculations. There was a negative relation between altitude and alder decline. Furthermore, reduction in snow cover and trends of increase in winter temperature are possible influencing factors. The spread of the syndrome may be related to climate change, reducing green alder vigour and allowing opportunistic parasites to cause host decline. The disappearance of green alder stands will likely affect soil protection, biodiversity and stand evolution in treeline forests of the Alps. More research is needed to define future management options.