The effect of permeable pavements with an underlying base layer on the growth and nutrient status of urban trees

Trees in urban areas increase the liveability of towns and cities but street tree growth can be constrained by the limited soil volume available for root growth and by reduced water and nutrient availability under impervious urban surfaces. An approach to improve the growth and health of street trees is the installation of permeable pavements. This study assessed whether permeable pavements with varying depths (0, 100 or 300 mm) of an underlying base layer affected soil moisture, soil temperature, tree growth and leaf nutrient status when broad-leaf paperbark (Melaleuca quinquenervia) trees were planted in two soil types, sand and clay. Permeable pavements increased moisture levels in drier sandy soil but decreased moisture levels in wetter clay soil after rainfall, with the stabilising effect of the permeable pavement on moisture levels being related to the depth of the underlying base layer. Permeable pavements and their base layers also had moderating effects on soil temperature. Permeable pavement with a deep drainage layer (300 mm) increased trunk diameter growth by 55%, although growth was only increased when trees were planted in clay. Permeable pavements with a shallower drainage layer (0 or 100 mm) did not affect tree growth compared with trees in control asphalt pavements. However, permeable pavements without a drainage layer reduced tree height growth in clay soil by 37 to 38% compared with pavements with a 100 mm or 300 mm drainage layer. In contrast, permeable pavements without a drainage layer increased trunk diameter growth in sandy soil by 65% compared with permeable pavement with a 100 mm drainage layer. Leaf nutrient concentrations were often unaffected by pavement design but differences in growth between treatments were associated with differences in the concentrations of some nutrients including K, Mg and S. This study demonstrated that inclusion of underlying base layers is important for tree growth when permeable pavements are installed on poorly-draining soils. Inclusion of a base layer may not be optimal for tree growth when permeable pavements are installed over freely-draining soils