Cost-effective method for the estimation of tree crown density in urban settings using a smartphone

Urban trees provide vital ecosystem services, and assessing their health is crucial for managing urban infrastructure. Traditional methods of assessing crown density, an indicator of tree vitality, involve horizontal perspectives of unobstructed canopies. This study presents a novel method for estimating crown density in urban street trees that are surrounded by obstructing objects like buildings. The approach is based on photographs of the tree crown from defined positions using a smartphone. The method was validated on eight small-leaved lime trees in Leipzig during the 2021 vegetation period, demonstrating that crown density can be estimated by analyzing smartphone-photographs from various perspectives. The method provides data to quantify crown development and can be used to compare the vitality status of individual trees. The different perspectives are consistent in their estimates of crown density throughout the annual plateau phase of crown development. During the initial greening phase, crown photographs taken from angularly oriented positions showed a higher slope value than those taken from other positions. The method can also estimate the effect of blue-green infrastructures on tree vitality compared to regular urban tree planting methods. The approach is a practical and cost-effective tool for assessing tree vitality in spatially confined urban areas.