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.