Porous pavement effects on rooting depth and growth of newly planted trees in sidewalk cutouts

Abstract

Resin-bound gravel, a type of porous pavement, is a popular surface treatment around city trees, primarily because it creates a continuous level walking surface. These installations often take place where no mulch has been used, and thus may significantly alter water relations between trees and soils. Although presumed to increase stormwater capture without any detriment to trees, their effect on root distribution, tree growth, soil water distribution, and drought resilience are unknown. In November 2014 12 Platanus ×acerifolia 'Bloodgood' whips were planted in simulated urban pervious pavement profiles, six constructed with pervious resin-bound gravel pavement (PRBGP) and six bare soil (BS), in a clay-loam soil in Blacksburg, Virginia, USA. Root growth was monitored with minirhizotrons, stem diameter was measured throughout the growing season, and soil moisture was recorded at 10, 30 and 60 cm depths. After one growing season, trees in PRBGP were larger (59% greater stem diameter) than those in BS and developed a shallower root system, with 26% of minirhizotron frames having roots in the first 10 cm while 6% had roots below 37 cm, compared to trees in BS that had 12% of frames with roots in the first 10 cm and 21% with roots below 37 cm. These effects are congruent with the increased moisture (23% greater at 10 cm depth) observed under PRBGP during dry periods. A lag phase in root development was also observed in BS compared to trees in PRBGP. It was concluded that the installation of PRBGP increased growth of newly planted trees compared to BS. However, roots appeared to be shallower under PRBGP profiles, which may affect drought resilience of urban trees and potentially increase root/pavement conflicts.