Impervious Surfaces Do Not Impact Urban Tree Crown Growth

Tree canopies in urban areas have many important ecosystem functions, and councils have targets to increase urban tree canopy area, which has proved challenging. Urban centres have large areas of impervious surfaces, and there is a perception that impervious surfaces are harmful for urban tree growth as water cannot penetrate the soil in which trees are growing. We investigated tree crown growth of young trees of eight common urban tree species in a suburb of Melbourne, Australia, that either grew in streets that were impacted by impervious surfaces or in parks. Four tree species had shown sensitivity to rainfall by lower crown growth in a low-rainfall suburb before, while the other four species had similar crown growth in high- and low-rainfall suburbs. We identified 40 trees per species and location (street vs. park) that were planted between 2009 and 2011 and measured their tree crown area in 2014 and again in 2018 using remote-sensed images. Trees that grew in streets that were impacted by impervious surfaces had larger crowns in 2014 than trees in parks, but both showed similar crown growth rates of 2.3 m2 yr−1 in the four-year period. Only one species (Eucalyptus sideroxylon A.Cunn ex Woolls) had a statistically significant greater relative crown growth rate in parks compared to streets. There was no statistically significant difference in the relative or absolute crown growth rates in tree species that have shown a previous sensitivity to rainfall compared to those that were insensitive to rainfall. Our data indicate that impervious surfaces had no detrimental impact on tree crown growth. It is possible that trees grown in streets have sufficient water resources and may benefit from the lack of competition for water, whereas trees in parks must compete for water resources with other plants.