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

Abstract

tImpervious surfaces can adversely alter the soil conditions encountered by tree roots in urban environ-ments. However, these conditions may be rendered more suitable for tree growth by the use of permeablesurfaces. This study assessed whether permeable pavements with varying depths (0, 100 or 300 mm) ofunderlying base layer affected the ecophysiological status of broad-leaf paperbark (Melaleuca quinquen-ervia) trees planted in sandy or clay soils. This study measured instantaneous leaf gas exchange, includingphotosynthesis (A1400), CO2concentration at the carboxylation site (Ci), stomatal conductance (gs) andintrinsic water use efficiency (iWUE), and assessed soil and leaf total nitrogen (TN) concentrations. Thisstudy also determined longer-term nitrogen cycling and water use efficiency by measuring nitrogen andcarbon isotope compositions ( 15N and 13C) of the soil and leaves. Each of these variables was thenrelated to tree growth over the 18 months of the study. The study found that the different permeablepavement treatments often did not affect A1400, Ci, gsor iWUE, and no significant correlation was foundbetween these four variables and tree growth during the initial tree establishment phase when growthwas slow. However, tree height and DBH growth during this phase did correlate with leaf 15N in bothsoil 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 correlatedwith A1400, Ciand gs, and negatively correlated with leaf 13C, for trees in clay soil. Trees in clay soil wereprone to waterlogging. However, installation of a base layer below the permeable pavement surface wasfound to reduce waterlogging, decrease leaf 13C and increase tree growth. These results demonstratethat inclusion of a base layer is important for promoting tree growth when permeable pavements areinstalled over poorly draining soils such as clay.