Tree condition in Swiss forests has been monitored since 1985 in the framework of the Sanasilva inventory. We use specific indicators, which allows us to asses the development over time. The Sanasilva surveys are carried out annually on a systematic 16x16 km grid as well as on the LWF plots.
We use international standards for defoliation, defined as the percentage of trees displaying more than 25% leaf or needle loss as compared to a reference tree with full foliage.
We also assess the proportion of trees with an abnormal amount of yellow leaves or needles, and we monitor the development of all indicators over time.