The growth and health of street trees planted in permeable pavements

The growth and health of street trees can be limited by the amount of water available under impervious urban surfaces. Two approaches for improving the growth of street trees are the installation of permeable pavements, which allow water to infiltrate through the pavement surface and into the soil, and the integration of underlying base layers to increase the water storage capacity of the pavement system. Permeable pavements with varying depths (0, 100 or 300 mm) of underlying base layer were tested to determine whether they affected the trunk diameter, height and canopy area of broad-leaf paperbark (Melaleuca quinquenervia) trees in two soil types, sand and clay. Control trees were planted within conventional asphaltic concrete pavements without a base layer. We found that the pavement treatment effects depended on the soil type. In sandy soil, trunk diameter growth was greatest when permeable pavements were installed without a base layer. However, in clay soil, trunk diameter growth, height growth and canopy area were greatest when permeable pavements were installed with a 300-mm deep base layer. This study demonstrates that inclusion of a deep base layer may be important for maximising street tree growth when permeable pavements are installed over poorly draining soils.