Implications of Urban Land Management on the Cooling Properties of Urban Trees: Citizen Science and Laboratory Analysis

Jérôme Ngao,Macarena L Cárdenas,Marc Saudreau,Thierry Améglio,Jérôme Colin

doi:10.3390/su132413656

Abstract

Trees participate in mitigating the urban heat island phenomenon thanks to their transpiration and shading. This cooling potential is highly dependent on leaf area. Nevertheless, leaf traits potentially vary across different land management practices in urban settings, thereby challenging the models used to estimate thermal budgets. The present study aims to investigate the variability of leaf area traits of linden (Tilia spp.) urban trees, and their effect on simulated tree transpiration. Reconstruction of the leaf area was undertaken at the tree scale at three different urban land management sites from three cities: London and Birmingham (UK) and Chantilly (France). The reconstruction combined allometric measurements at shoot and leaf scales, and a tree-scale 3D digitization with laboratory analysis using field data collected by citizen scientists. The management practices had a significant impact on leaf area, and on tree allometric relationships, which were propagated through the reconstruction process. Relative differences between the management practices ranged between 12% and 48% according to the city where the variable was considered (e.g., leaf area index, total leaf area, or tree transpiration). Trees in managed sites (i.e., individualized leaf crowns, frequent leaf litter removal, and standard thinning/pruning operations) develop a higher leaf area, thus promoting cooling potential. This study shows that the variability of leaf traits, and their responses to different land management, can be studied by comprehensive data collection through citizen science and lab-based modelling. It also highlights the importance of appropriate, well-designed urban planning, where landscaping using urban trees can play an even better role in climate proofing cities.

Highlights

The leaves of the UM trees and the MA trees were smaller than the other management practices in these two sites
This study provides a novel methodology that engages the public and researchers to collaborate towards understanding and improving the best implementation of naturebased solutions to ameliorate the urban heat island (UHI) effect in urban areas
This methodology facilitates collecting a robust database of empirical data to be used for models for understanding of the properties of trees

Summary

Introduction

The phenomenon known as the urban heat island (UHI) effect is dramatically affecting cities and quality of life, and is directly related to poor vegetation and landscaping practices that include the usage of materials with low reflectivity to solar radiation in pavements [4,5,6]. Urban green spaces, such as parks, are proven to be cooler than the built environment and rich in grey infrastructure, especially when nature-based solutions, such as trees, are planted [7,8]. Some benefits of trees include environmental (e.g., habitat, cooling and carbon sink, and storage), health (e.g., lower obesity, faster recovery, and reducing stress), social (e.g., higher social cohesion, leisure spaces, and enhanced life quality), and economic-related (e.g., higher land and property values, reduced expenditure on air pollution removal, and lowered heating and cooling costs) fields [9,10]. The diversity of tree species, their growth, and their functioning, and that of land management practices, is overwhelming, complicating the role of urban planners to choose the most suitable tree species for a particular desired ecosystem service

Results

Discussion

Conclusion