In urban environments, newly planted street trees suffer from poor site conditions and limited water availability. It is challenging to provide site conditions that allow the trees to thrive in the long term, particularly under climate change. Knowledge about the hydrological properties of artificial urban planting soils related to the response of tree species-specific growth is crucial, but still lacking. Therefore, we established a three-year experimental field setup to investigate the response of nine tree species (135 individuals) to two common urban planting soils and a loamy silt reference. We determined and measured soil hydrological parameters and monitored tree growth. Our results revealed low plant available water capacities (6% and 10% v/v) and hydraulic conductivity restrictions with the drying of the sandy-textured urban planting soils. Therefore, tree species that are investing in fine root growth to extract water from dry soils might be more successful than trees that are lowering their water potential. Tree growth was overall evidently lower in the urban planting soils compared with the reference and differed between and within the species. We showed that using unfavorable planting soils causes severe, species-specific growth deficits reflecting limited above-ground carbon uptake as a consequence of low water availability.

Urban trees provide important ecosystem services but often experience water stress due to unfavorable soil conditions and decreased atmospheric moisture. While both soil water availability and vapor pressure deficit (VPD) influence plant water status, their combined effects on urban tree water dynamics remain poorly understood. Using high-resolution dendrometer measurements, this study examined how soil water potential (SWP) and VPD influence fluctuations in stem diameter (tree water deficit- TWD) in urban planting substrates and across tree species. During the 2020 growing season, TWD was monitored for Quercus cerris planted in three substrates (sand, FLL- an engineered urban soil, and loess) to assess how soil properties influence tree water dynamics. In 2021, stem radius measurements of eight tree species grown in sand revealed species-specific differences in drought responses. Trees planted in sand spent a significantly higher proportion of the growing season in water deficit (67%) compared to FLL (41%) and loess (23%), and exhibited higher TWDmax. TWD responded more strongly to both decreasing SWP and increasing VPD in sand, indicating greater physiological drought stress in substrates with low plant-available water capacity. Species responses varied, with Liquidambar styraciflua and Quercus palustris showing high sensitivity to both drivers, while Gleditsia triacanthos maintained consistently low TWD and sensitivity primarily to SWP. These results demonstrate that tree water dynamics in urban environments are jointly controlled by substrate hydraulic properties and species-specific water-use strategies. Matching species to substrate conditions, or improving soil water availability, may therefore enhance urban tree resilience under climate change-induced changes in water availability.

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.

The benefit of urban roadside trees to provide ecosystem services and wellbeing of human in expanding and compacted cities gets more and more attention. For northern Germany it is predicted that climate change rises summer temperatures and that precipitation patterns shift to drier vegetation periods. In cities, those impacts will intensify water (soil sealing) and heat problems (urban heat island) even more. Furthermore, roadside trees have to deal with several specific site limitations like extreme soil compaction and soil sealing, low water infiltration rates, sandy and anthropogenic deposited substrates, and soil volume restrictions. The consequences for the trees are drought stress combined with reduced vitality and life expectancy.Our research is based on soil water monitoring at 17 roadside plantation sites across the city of Hamburg.  We focus on the water availability of prepared planting soils and the development of the trees root systems. Sensors for soil water tension and soil temperature were installed in different soil areas of each site: topsoil, root ball, tree pit substrate, lateral space, and subsoil. The general goal of this study was to characterize the soil water availability at roadside planting pits during the first years after plantation (here: 2017, 2018 and 2019). Based on these results the long-term objective is to elaborate recommendations for the soil-related technology of future urban tree planting sites. The Creation of more suitable conditions in the planting site enhances roadside tree vitality and provides ecosystem services by the trees on a higher level.The data analysis focused on two main aspects. First, the effect of weather conditions, especially  the extreme wet and dry vegetation periods, on the soil water availability in the tree pit. Second, the three-year temporal development of soil water distribution in the different soil areas within the planting pitafter plantation.We found that soil water availability in the vegetation period (VP; April-October) at the investigated roadside plantation sites are highly correlated to weather conditions (air temperature (aT) and precipitation (P)). During a cold and wet VP (aT: 14,0 °C, P: 631 mm), soil tensions reached a critical value on average at 24 ± 18 days (11 ± 9 % of VP). In a hot and dry VP (aT: 16,0 °C, P: 222 mm), soil tensions reached a critical value on average at 115 ± 22 days (54 ± 10 % of VP).Furthermore, the results showed that soil water scarcity in the first VP occurred mainly in the root ball, whereas during the second VP water scarcity developed in all soil areas within the planting site, except for the subsoil. Although the amount of precipitation during the last vegetation period was more than doubled compared to the second, the subsoil reached higher water tensions. This finding leads to the conclusion that root development after plantation took place from the root ball over the prepared planting soil into the surrounding soil within depths of up to 1 m.