Tree development in structural soil - an empirical below-ground in-situ study of urban trees in Copenhagen, Denmark

Aims The purpose of this study is to evaluate root and stem growth, nutrient status and soil properties of a tree planting in structural soil. Methods Root and stem growth were measured at an 10-year-old urban planting of Tilia x europaea L. 'Pallida' established in structural soil on a paved square in Copenhagen, Denmark. Root abundance was quantified and soil profiles were established at three different distances from 10 trees (1.0, 1.9, and 2.9 m). Soil and leaf samples were analysed for nutrient concentrations. Results Abundant root presence was measured across the entire profile of structural soil (160 × 60 cm) at all distances, provided that structural soil layers were constructed correctly. Erroneously constructed structural soil layers however, resulted in impaired tree and root growth. Coarse root morphology was affected by the confined growing space in the voids of the stone matrix. The soil was relatively alkaline with an average pH (CaCl₂) of 7.3. Foliar analysis revealed deficiencies of K and Mn. These deficiencies might be due to a combined effect of high pH and low soil concentrations of these nutrients. Conclusions The study documents that correctly constructed structural soils allow and encourage root growth in load bearing layers.