Long time risk assessment of soil water shortage in planting pits of young urban roadside trees in the city of Hamburg, Germany

Urban trees as main part of urban green infrastructure provide manifold ecosystem services and contribute to the wellbeing of humans. Unfortunately, urban trees, especially roadside trees, are severely challenged by both, political conflicts of interests in terms of city development and a variety of physically stressors. Contrary to the known benefits of urban green, its proportion in most cities is still decreasing. Furthermore, climate change exacerbates the already challenging preconditions.

For northern Germany, climate change is predicted to shift temperature- and precipitation patterns. Simultaneously the frequency of “summer days” and “hot days” are likely to increase, leading to elevated risk of soil drying during the vegetation period.

The city of Hamburg is home to almost 220.000 roadside trees. Especially trees planted nowadays are exposed to harsh roadside conditions. In the event of drought, young-trees compared to well-established trees, are not in touch with deep- or distant water reservoirs and the risk of vitality loss or death increases.

Our research aims to characterize the soil hydrological conditions in the rooting zone of roadside young-trees during the first years after plantation. Further it aims to identify spatio-temporal dynamics of soil water response during phases of extreme meteorological drought. Our findings are based on a long-term soil water monitoring across the city of Hamburg, which was started in 2016. The monitoring covers 20 trees from 7 species, planted between 2007 and 2019 with large, medium and low soil sealing. Soil water tension and soil temperature were measured hourly with sensors in the root ball, in the tree pit filled with structural soil and the surrounding soil (16 sensors per site).

Our data provides a broad characterization of soil water conditions for young-tree sites in urban areas, and show that water supply in years of moderate meteorological drought is not only extremely heterogeneous on large scales, but can also vary greatly on a small scale. The water tension in the root ball, which should provide the highest amount of water per unit, was highly variable and exceeded thresholds even in the first year after plantation and in almost every vegetation period across all sites. In years of high meteorological drought like in 2018, the soil water tensions exceeded the thresholds in almost all compartments, which leads to a risk of vitality losses and mortality.

Our data show the need for adaption of general tree site concepts for future plantations. This unique dataset will be further completed with the aim to include future sites and plantation strategies e.g. the underground connection of planting pits, to increase the diversity of site characteristics and to develop reliable modelling and recommendations.