Drought Assessment in Swiss forests (DRAINS)

The hot, dry summer of 2018 left a lasting mark on Swiss forests: trees turned brown before the end of the summer season, and in the following years, many of these stressed trees died or had increased crown die-back. The hot and dry summer 2026 appears to be stressing Switzerland's forests even further.

To help identifiy hotspots of tree stress in 2026, we provide a web viewer showing the development of drought stress and browning across Switzerland's European beech (Fagus Sylvatica L.) dominated forests.

What you'll find in the viewer

The tool offers two complementary views of forest condition, both updated every 5 days using gap-filled satellite data (Sentinel-2) focused specifically on forest stands dominated by European beech and targeting drought stress within the year 2026. We intentionally try to exclude the effects of forest managment and previous drought like crown-dieback:

Browning as seen on the ground (GRVI)

This layer, an annually normalized green-red vegetation index (GRVI), shows visible discoloration of the canopy. It is essentially, what you'd see for yourself if you were standing in the forest expressed as a leaf color-change from green to brown (and completely defoliated).

Drought stress probability (DiscoCH)

This layer comes from a machine-learning model trained on seven years of ground observations (2017–2023), which uses five satellite-based vegetation signals to estimate the probability that a stand is dioscolored and under drought stress. In some cases this index may show drought stress before it becomes fully visible to the naked eye. The underlying machine learning model was developed using field observations of beech trees and Sentinel-2 data from 2017 to 2023. Normalizing the satellite time series is intended to separate the discoloration occurring in the current year from longer-term damage and other disturbances in the forest.

A note to users: The data viewer and both map products are under active development and may change without warning. Additionally, the provided maps are only a rough approximation of Switzerland's European beech forests in 2026. Species mixture and recent forest managment alongside extended data gaps from cloud cover and topographic shaddow prevent us from acurately estimating browning and drought stress in some areas. In each case, the data is excluded from the map viewer. In previous versions of the DiscoCH tool and in the publications below, the green-red vegetation index (GRVI) was incorrectly described as the chlorophyll/carotenoid index (CCI), following the FORCE documentation. The band calculation remains unchanged: (Green − Red)/(Green + Red) but we have updated the index name to reflect its more commonly used designation.

Learn more

For the full scientific methodology behind these products, see our publications or contact Andri Baltensweiler or Colin Bloom:

• Ecological Indicators (2025)
• International Journal of Applied Earth Observation and Geoinformation (2026)