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

The effect of permeable pavements with an underlying base layer on the ecophysiological status of urban trees

Impervious surfaces can adversely alter the soil conditions encountered by tree roots in urban environments. However, these conditions may be rendered more suitable for tree growth by the use of permeable surfaces. This study assessed whether permeable pavements with varying depths (0, 100 or 300mm) of underlying base layer affected the ecophysiological status of broad-leaf paperbark (Melaleuca quinquenervia) trees planted in sandy or clay soils. This study measured instantaneous leaf gas exchange, including photosynthesis (A1400), CO2 concentration at the carboxylation site (Ci), stomatal conductance (gs) and intrinsic water use efficiency (iWUE), and assessed soil and leaf total nitrogen (TN) concentrations. This study also determined longer-term nitrogen cycling and water use efficiency by measuring nitrogen and carbon isotope compositions (δ15N and δ13C) of the soil and leaves. Each of these variables was then related to tree growth over the 18 months of the study. The study found that the different permeable pavement treatments often did not affect A1400, Ci, gs or iWUE, and no significant correlation was found between these four variables and tree growth during the initial tree establishment phase when growth was slow. However, tree height and DBH growth during this phase did correlate with leaf δ15N in both soil types, suggesting that rapid nitrogen cycling was beneficial for initial growth. In contrast, trunk-diameter growth increments during the subsequent period of rapid growth were positively correlated with A1400, Ci and gs, and negatively correlated with leaf δ13C, for trees in clay soil. Trees in clay soil were prone to waterlogging. However, installation of a base layer below the permeable pavement surface was found to reduce waterlogging, decrease leaf δ13C and increase tree growth. These results demonstrate that inclusion of a base layer is important for promoting tree growth when permeable pavements are installed over poorly draining soils such as clay. © 2015 Elsevier GmbH