Deep compost amendment reduces soil bulk density resulting in improved growth of urban Gymnocladus dioicus trees over time

It is well known that trees provide many benefits in the urban environment, including temperature reduction, carbon sequestration, pollution remediation and storm water reduction. What is less well known are the best methods to use when establishing urban trees to ensure maximized growth and ecosystem services. We examined how amending soil prior to planting by incorporating a compost deep in the soil profile influenced the growth of Kentucky coffeetrees ( Gymnocladus dioicus L Koch ‘Espresso’) planted along a boulevard in the city of Ithaca, NY, USA. We found that amendment reduced soil compaction, as indicated by a soil penetrometer, and bulk density, compared to sites without soil amendment. Amended soils exhibited higher water holding capacities, soil respiration values, as well as organic matter, soil protein, total nitrogen, total carbon and organic carbon contents. Compared to trees growing on sites without amendment, trees planted in amended soils exhibited larger trunk diameters, increased canopy height, longer shoots and leaves, increased leaf areas and leaf greenness, as well as more roots overall and roots growing deeper in the soil profile. Soil amendment prior to tree planting could be a useful technique for improving the vitality of urban trees and their subsequent ecosystem services in cities worldwide. • Urban trees have larger leaves, more leaf area, grow taller, and exhibit a greater DBH when the soil is amended with compost. • Urban trees have more and deeper roots when soil is amended with compost, largely because soil density is decreased. • Ground penetrating radar is a useful tool for non-invasive assessment of tree root characteristics.