2. Study Site

The experiment was carried out in the SE of Berlin, Germany (Fig. 1). Berlin is located on the flat North European Plain where the topography and geology are dominated by deposits from the Pleistocene glaciation . The climate is continental temperate with long-term (1981–2010) mean annual rainfall of 577-602 mm ranging between stations and mean annual air temperatures of 9.4–10.2 °C . Berlin covers 891 km², with a population of 3.66 million (, 2020; Fig. 1B). The majority of the city is covered by residential areas and streets (~59%), but there are large amounts of green and blue spaces: vegetation covers ~34% (forests, parks, agriculture) plus ~7% surface waters .
Our study site is located at the grounds of the Leibniz-Institute of Freshwater Ecology and Inland Fisheries (IGB), roughly 220 m north of Lake Müggelsee (Berlin’s largest lake) (Fig. 1B). The geology is characterized by sand and gravel deposits of the Berlin-Warsaw glacial spillway, (, SenUVK online, 2007). The surrounding district (Fig. 1B) is characterized by residential areas and roads (38 %), forest (40 %), water bodies (12 %) and public green space (0.06 %; ). The study site is a park-like space with older trees (~30-100 years old) surrounded by brick buildings of former 19thcentury water works and extensive rough grassland above subsurface slow sand filter systems, which were used for drinking water treatment until the beginning of the 1990s (, online). Within a 100 m radius of the study site center (Fig 1C), premises are covered by buildings (10 %), different types of non-irrigated urban green spaces, including grassland (49 %), shrubs (8 %) and trees (17 %); and streets, semi-permeable or sealed pathways and parking spaces (16 %).
The experiment focused on two small areas; one tree dominated, the other grassland dominated and 16 m apart (Fig 1C). The grassland site was covered by grass (e.g. Lolium perenne , Arrhenatherum elatius ) and herbs (e.g. Trifolium pratense, Achillea millefolium ) of 30-50 cm height, mowed twice a year and can be referred to as an urban meadow . The tree site was dominated by black locust, lime, oak, birch and maple trees. We selected one dominant maple tree (Acer platanoides ) with a stem diameter of 550 mm (August 2021) and height of ~16 m. In other studies, Acer platanoides has been shown to have a high drought tolerance and can maintain low leaf gas exchange rates .
The soils reflect anthropogenic impacts, such as partly backfilled ground after construction work. They are classified as Anthrosols (SenUVK, , 2017, online), which consist of debris, sandy materials and a shallow humus layer from extensive gardening.