2025: A dry year, a rainy July

In the exceptionally dry year of 2025, hardly any new groundwater was replenished. Only an extremely wet July provided temporary relief. Nevertheless, the water stored in the soil was sufficient to sustain the vegetation, meaning that the trees did not suffer any significant damage from the drought. This is shown by data from our experimental site in Britz.

2025 was a dry year for the forest. Only 533 millimetres of rainfall fell at the intensive monitoring site in Britz. Although this amount of rainfall was below average, it did not yet qualify as extreme. What was unusual was the distribution of rain and snow throughout 2025: in July, 190 millimetres fell, accounting for more than a third of the total annual rainfall. Without the rainy July, 2025 would have been the driest year in the observation period from 1993 to 2025. 2026 also began with particularly low rainfall.

During the growing season, plants can often compensate for the lack of water by drawing on water stored in the soil from the winter months. However, at the end of 2024 and the beginning of 2025, there was so little rain and snow that the soil’s water reserves were only partially replenished. The conditions at the start of the growing season were therefore unfavourable. The rainy July, however, alleviated the water shortage and ensured that no acute Drought stress was observed in the plants on the experimental plot. 

Damage to the forest in 2025 was mainly caused by late night frosts in early May, after the leaves on oaks and beeches had already unfurled. Young stands growing outside established stands without the protection of larger trees are usually particularly affected by this.

In terms of deep infiltration – that is, the water that later becomes new groundwater – 2025 was a poor year. It was notable that, among the young oak stands – which consistently yield the highest values – the researchers measured only 10 millimetres of new groundwater. In the dry year of 2019, the figure was significantly higher at 75 millimetres. No new groundwater was formed beneath either pine or beech trees. Under beech trees, this last occurred during the drought of 2019.

The rain in the wet month of July primarily replenished the water reserves in the soil and directly benefited the vegetation. The water did not seep into deeper soil layers. The groundwater table therefore did not benefit from the rainfall – unlike during the heavy summer downpours of previous years.

Despite the drought, the beech and pine trees in the experimental plot in Britz grew well on average in 2025 compared with the previous six years. This is evident from measurements of trunk circumference. The circumference increased particularly markedly in July 2025 following the heavy rainfall. However, this does not necessarily mean growth, as the bark – particularly on pine trees – can also shrink and swell depending on moisture levels.

When trees are under drought stress, they are unable to build up water and energy reserves. This has a long-term effect on wood formation in subsequent years. The drought can also damage roots and cause crowns to shrink, which likewise affects tree growth in the years that follow.