When the forest looks like autumn in summer: What we can learn from the Hardwald’s unique history
This summer has been a tough one for nature. Prolonged drought and high temperatures have placed severe stress on forests across Switzerland. The European beech (Fagus sylvatica), in particular, has suffered greatly. By July, many forests already looked as though autumn had arrived early, with leaves turning brown, while branches were snapping and trees were dying under the severe water and heat stress.
This was particularly evident in the Hardwald in Birsfelden, close to Basel. This forest has repeatedly appeared as an outlier in previous genetic studies of beech populations across Switzerland. This makes it of particular interest to the Spatial Genetics of Ecosystems research group. The findings to date raise questions about the origin and history of its trees, as well as whether and how they can persist in its exceptionally dry topsoils.
A living laboratory: Genetic mysteries and climate resilience
According to records from the community of Muttenz, approximately 1.5 million beech trees were planted in the Hardwald between 1872 and 1935. While the exact origin of the seedlings remains unclear, initial genetic analyses indicate an origin outside of Switzerland and that younger trees may have intermixed with local Swiss genotypes. This unusual history makes the forest a kind of long-term living experiment and a valuable research site for studying how genetic variation interacts with environmental conditions.
Now, one major question remains: will the trees recover? Or has the combination of prolonged drought and extreme heat pushed some – or even all – of them past a tipping point?
By combining LiDAR, hyperspectral and thermal airborne measurements with detailed ground-based data, the ARES campaign provides a high-resolution snapshot of the Hardwald during an exceptional climatic event. These data may help us understand which trees can withstand extreme drought – and which cannot.
For the Hardwald, this summer could therefore mark another important chapter in its unusual history, while providing valuable insights into how forests may respond to a changing climate.
Researcher Ting Tang with an ASD FieldSpec taking white reference measurements, photo by Alison Muster-Bender
| How we conducted our measurements Details of the forest state were documented in 2026 flight campaigns by ARES, the platform for Airborne Research of the Earth System, run by the UZH Remote Sensing Laboratories Remote Sensing Spectroscopy group. Airborne data were collected about the forest structure using a Riegel full-waveform LiDAR, and about the state of the canopy using the AVIRIS-4 imaging spectrometer, built by NASA-JPL and owned and operated by UZH, and Telops HyperCam thermal imaging camera, owned by GFZ Potsdam and operated by Freie Universität Berlin. Data were acquired during the peak of the summer drought and again after August rains, to measure the extent and severity of forest stress, and look for evidence – however slight – of possible recovery events. To calibrate the airborne measurements, a team of three researchers collected spectral reference data on the ground using an ASD FieldSpec spectroradiometer and reference targets: light, intermediate, and dark tarps, and large yoga balls, as easily identifiable objects in the airborne acquisitions. These ground measurements provide important reference points for interpreting and calibrating the data collected from above, over time. For the projects in Hardwald, we collaborate with the Institute for Applied Plant Biology and forestry offices in Basel Land and Muttenz. |
Toja Guerra, Meredith Schuman, Spatial Genetics of Ecosystems