Good things take time: findings on forest health from long-term observations

The condition of the tree crown is an important indicator of the vitality of a tree. In Swiss forests, the crown condition has been surveyed since 1985 as part of the Sanasilva inventories and since 1994 as part of the research programme “Long-term Forest Ecosystem Research”. During this period, the vitality of the forest has declined significantly. Pronounced and widespread deterioration was particularly evident after dry summers. The condition of the tree crowns is also an indicator of the probability of a tree dying within a few years. The main reason for the loss of vitality in the forest is the increased drought stress, which particularly affects forests at lower altitudes and in dry regions of Switzerland. An extreme example of this is the development of the Scots pine (Pinus sylvestris L.) in Valais. In several waves, a large proportion of the Scots pines there died off regionally. Over the years, the findings on causes and processes have become increasingly clear. The increasingly longer and more valuable time series of a large number of observed parameters also allow us to place and interpret individual events and developments in a wider context. Continuous long-term observations are therefore indispensable for recognising and understanding changes in the forest at an early stage and for making appropriate recommendations to policymakers.