Tree survival and causes of mortality were studied in an experimental afforestation in the upper subalpine forest zone in the Swiss Alps. A total of 59.8% of Pinus cembra and 45.6% of Pinus mugo were killed by Gremmeniella abietina during the first 20 years after planting, compared to 1.5% of Larix deddua trees. The mortality rates caused by G. abietina were highly correlated with the duration of snow cover in spring. Tree losses were lowest at sites where the snow melted early and highest at sites where the snow ablation was delayed in spring. Tree mortality varied greatly between years. In the year after the coldest summer of the observation period mortality due to G. abietina infections was highest, suggesting high susceptibility of trees in poor condition. Phaddium infestans, which was the second most important factor for mortality in P. cembra, killed trees irrespective of their condition. Other biotic and abiotic causes of tree mortality had negligible influence compared with the impact of G abietina and P. infestans. Excluding anthropogenic impacts, the diverse spatial pattern of forested and treeless sites close to the subalpine timberline may predominantly result from the action of parasitic fungi, depending on the ablation pattern of the snow cover in spring.