Stress markers in the leaves were measured for four tree species (London plane, Norway maple, small-leaved lime, and English oak) on KIT’s South Campus. A total of 16 trees from each species—half located on sealed sites and half on open sites—were studied during the two growing seasons of 2025 and 2026. The partners in Freiburg used dendrochronology to measure annual growth.

Across all tree species, proline accumulation in the leaves indicated that stress levels were significantly higher at sealed sites. Regardless of location, London plane trees exhibited the lowest stress levels, small-leaved linden trees the highest, with Norway maple and English oak falling in between. High proline levels were generally associated with low growth; this correlation was most evident in London plane trees.

In collaboration with KIT Facility Management and the maintenance company Merkle, the research findings are currently being integrated into tree care practices under scientific supervision. To this end, grass clippings from KIT areas that have been developed as wild meadows—and are therefore being left to grow naturally—are being applied as mulch under selected trees to investigate the impact on stress levels. It is hypothesized that improved soil moisture and stimulation of the microbiome may contribute to greater tree resilience. Soil probes developed by IMTEK Freiburg are also being integrated into this “real-world laboratory,” enabling real-time monitoring of the environmental conditions critical to the trees’ well-being.

For the past two years, the rare orchid Anacamptis pyramidalis has been appearing repeatedly in the wild meadows near the Audimax. As part of a “real-world laboratory” experiment, grass clippings from trees selected for thinning are currently being applied as mulch, and the effect on the trees’ stress levels is being studied.

To determine which tree species will persist in the urban landscape over the long term, Norway maple (Acer platanoides), London plane (Platanus x acerifolia), English oak (Quercus robur), and small-leaved lime (Tilia cordata) were studied using dendroecological methods. To this end, core samples were taken from selected trees and later prepared in the laboratory to measure the width of the growth rings and subsequently examine the wood anatomy.

In selecting the trees, not only was tree species taken into account, but a distinction was also made between street and park locations, with the question of whether the presence or absence of green space beneath the tree influences the trees’ growth patterns.

However, no such effect could be demonstrated with regard to annual ring width in relation to summer drought. Summer drought during the months of June, July, and August did not have a significantly different effect on the annual ring width of the trees, whether they were located along streets with little to no open green space beneath the canopy or in parks with ample green space beneath the canopy.

When analyzing by tree species, a significant influence of summer drought in June, July, and August on annual ring width was demonstrated only for English oaks and London plane trees.

For the wood anatomical analyses, the focus was on cell count and lumen area. The different strategies of the common linden and Norway maple tree species became apparent. While the common linden increases its lumen area as the number of cells increases, the Norway maple reduces its lumen area as the number of cells increases. No significant effect of summer drought on lumen area could be demonstrated. A significant effect of summer drought was detected only on the cell count in small-leaved linden at roadside sites.

In conclusion, it can be stated that summer drought is only one of many factors influencing annual ring width. At the wood anatomical level, small-leaved linden and Norway maple employ different strategies to respond to environmental influences.