Cooling effects of urban green spaces on residential neighbourhoods: a review and empirical study

Abstract

With increasing human populations and rising temperatures, cities are growing warmer, and there is a dire need for local climate regulation. Urban green spaces (UGS) provide an important ecosystem service, namely local climate regulation: a cooling effect, which extends into the surroundings of UGS. Vegetation cover in UGS is being increased in many cities globally for the purpose of lowering the temperatures. Thus, UGS play a vital role in mitigating urban heat problems. To understand the relationship between the design aspect (size, shape) and tree diversity of UGS, and local climate regulation in urban areas, the quantification of the cooling effects of UGS is the main focus of this study. The research presented in this dissertation was conducted in two steps: a comprehensive literature review on studies that calculated cooling effects of UGS over a temperature gradient and an empirical study to quantify the indicators of cooling and the influence of variables that were found to be missing in other studies. Therefore, the main research questions were to i. review the current studies that quantify cooling effects calculated over temperature gradients of UGS globally (Chapter 2); ii. analyse the influence of different UGS characteristics on the cooling effect, between different types of UGS (parks and forests) based on data collected for a case study (Chapter 4, 5); iii. assess the diversity of trees in the UGS and its influence on cooling effects to understand the role that UGS design, biodiversity, and characteristics of residential surroundings play using statistical models for the same case study (Chapter 4, 5). To substantiate the choice of indicators and variables for the empirical study, and to see the various methods involved in the quantification of cooling effects calculated over temperature gradients, a review study was done focusing on the research question i. From the review (including 23 publications) it was evident that the common indicators of cooling were difference in temperature (21 indicators) and the extent of cooling (26 indicators). The literature review also provided an overview of the variables that have an influence on the cooling effects. As biodiversity as a variable was not considered in any of the studies that were reviewed, I included this variable in the statistical analysis for the empirical study. Also, it was observed that the literature lacked studies specifically for urban ecosystems. The empirical study was conducted in the city of Leipzig in Germany in two parts. The first part (temperature analysis) mainly aimed at quantifying and comparing the strength of the cooling effects of 62 UGS (parks and forests), to determine how far the cooling effects extended into the surrounding residential area, and to better understand how these indicators of cooling effect are affected by the physical characteristics, vegetation cover and the surroundings of the green space. This temperature analysis of the empirical study provides answers for the research question ii. For this, the change in temperature (ΔT) at the park-width distance (PWD), the fitted maximum ΔT and the cooling distance were the three indicators of cooling that were calculated from daytime air temperature measurements. Multiple regression models were used to analyse the relationships of the indicators of cooling on to the physical characteristics of the UGS and the surroundings. Cooling effects were found to be larger in urban forests than in parks. The characteristics of the green spaces were found to be more important than the characteristics of the residential surroundings in explaining the cooling effects. The influence of the area and shape of the park was found to be complex, hinting at a trade-off between maximizing temperature differences and the distance at which cooling is still noticeable. Since it was found that the percentage of tree/shrub cover did not aid in cooling, detailed investigations of vegetation cover were performed. In the second part (biodiversity analysis) of the empirical study that focusses on the research question iii, tree sampling was carried out in a subsample of 54 UGS that were accessible; and results showed that specific aspects of tree diversity play a stronger role in temperature mitigation, such as functional diversity and mean traits rather than taxonomic diversity. Therefore, it was important to look at the influence of various diversity variables such as taxonomic diversity, functional diversity and mean traits (height and diameter at breast height - DBH) of tree vegetation on cooling effects in different types of UGS. The explanatory power of the model increased with the inclusion of diversity variables. The main result of this dissertation from the methodological point is that transect measurements of temperature data are found to be more reliable than point measurements in assessing the cooling effects. It is to be noted that not all UGS provide cooling effects, and the cooling effects differ based on the type of UGS as forests were found to be better than parks for providing a cooling effect. The intensity of temperature difference and the spatial extent of cooling cannot be both achieved together. Larger and more structurally diverse UGS provide better cooling distance, with the size of the UGS being most important. The effects of tree diversity were found to be less important compared to that of physical characteristics of the UGS (size and type of UGS). Diverse vegetation structure in terms of tree height, rather than species diversity, improved the cooling effects of parks. Large irregularly shaped green spaces and especially forests in urban areas have a stronger cooling effect than small green spaces and parks. Thus, this study also provides insights regarding the importance of species diversity vs. functional diversity and mean traits of tree vegetation on the cooling effect in UGS, which may guide effective management and conservation strategies in urban environments. Therefore, to improve local climate regulation in cities, it can be said that it is important to recognize the need to either have higher intensity cooling or a longer distance along which cooling is noticeable. The indicators that are quantified over a transect are better in assessing the cooling effects. In terms of the design of the UGS, the bigger the area, the better is the cooling, with tree diversity being diverse in terms of structural variation in tree heights rather than species diversity.