Impacts of technically modified plant pits on water balance dynamics and tree vitality in urban environments

Trees as essential components of green urban structures are of crucial importance for the regulation of the urban climate and human wellbeing. Despite this, the currently rising demand for living space and infrastructure causes an increase in the share of sealed and compacted soils. These trends directly affect soil-plant interactions in urban environments. The synergy of the increasing land use pressure and changing climatic conditions worsen the site and growth conditions and thus the vitality for young and mature trees. A possible adaptation strategy is the transformation of plant pits into water reservoirs combining the discharge of excess water with impermeable sole materials and substrates that optimise the water conductivity and storage capacity. The corresponding aim of this study is the quantification of the effects of the water balance dynamic in the rooting zone on the vitality of young trees at highly sealed sites in the city of Hamburg. The two main questions are 1) Do technically modified plant pits reduce summerly drought stress inside the rooting zone and thus improve the root water uptake and tree vitality?, and 2) Does excess water after high rainfall limit the gas exchange and thus the root growth? To answer these questions, we selected two different sites, one residential area and one pedestrian zone. A total of 13 tree planting pits, including 5 technically modified and 8 generally constructed ones, with two types of substrates and water discharge, are equipped with TDR- and water tension sensors for a continuous monitoring of the soil water balance and O₂ and CO₂ sensors and tubes for monitoring the gas household. Stomatal resistance, chlorophyll content and fluorescence as well as Δ13C isotope measurements are combined with branch and trunk growth measurements and a tree appraisal to investigate the tree vitality. The comparative data analysis will be used for evaluating the different planting pit variants to give development as well as dimensioning recommendations for prospective planting pit constructions, improving the soil-plant interaction.