Street‐side trees control pavement wetness in a moist‐temperate region with cold winters

Abstract

Urban tree canopies intercept rain and snowfall, potentially extending the service life of asphalt pavement below them. However, the process of pavement wetting has not been quantified and controlling factors are poorly understood. In this study, we document four forms of pavement moisture and experimentally test the role of trees in regulating wetness. Study plots were established on asphalt pavement under and adjacent to broad‐leaved street trees in a residential neighbourhood typical of cities in the Northeastern Unites States. Pavement wetting by rainfall, drying after experimental wetting, and accumulation and persistence of snow and ice were recorded. Pavement wetness under a tree canopy lagged 25 to 35 minutes behind pavement under an open sky, a delay that would prevent or reduce wetting in 28% of rain events in the study region. Pavement drying was 70% slower under a canopy than in open plots. Partially covered plots showed intermediate wetting and drying behaviour consistent with direct‐beam solar exposure. Significantly less snow was observed under the leafless canopy than under the open sky, and snow melted faster under the canopy. Ice cover and persistence were unrelated to canopy openness but strongly influenced by traffic and drainage. Pavement microclimate did not differ significantly with tree size or canopy porosity. We conclude that street trees control pavement moisture within ranges that potentially affect pavement structure and longevity. Tree‐scale arrangement appears to be more important than fine‐scale canopy structure.