Tree mortality caused by Gremmeniella abietina in a subalpine afforestation in the central Alps and its relationship with duration of snow cover

Tree survival and causes of mortality were studied in an experimental affor estation in the upper subalpine forest zone in the Swiss Alps. A total of 5 9.8% of Pinus cembra and 45.6% of Pinus mugo were killed by Gremmeniella ab ietina during the first 20 years after planting, compared to 1.5% of Larix decidua trees. The mortality rates caused by G. abietina were highly correl ated with the duration of snow cover in spring. Tree losses were lowest at sites where the snow melted early and highest at sires where the snow ablat ion was delayed in spring. Tree mortality varied greatly between years. In the year after the coldest summer of the observation period mortality due t o G. abietina infections was highest, suggesting high susceptibility of tre es in poor condition, Phacidium infestans, which was the second most import ant factor for mortality in P. cembra, killed trees irrespective of their c ondition. Other biotic and abiotic causes of tree mortality had negligible influence compared with the impact of G. abietina and P. infestans. Excludi ng anthropogenic impacts, the diverse spatial pattern of forested and treel ess sites close to the subalpine timberline may predominantly result from t he action of parasitic fungi, depending on the ablation pattern of the snow cover in spring.