Using the Gini Index to quantify urban green inequality: A systematic review and recommended reporting standards

Access to parks, ecosystem services, and urban trees support healthy people and communities. Unfortunately, access is often unequally distributed, leading to differential outcomes. Measuring the within-city distributional equality and comparing between cities can be facilitated by the Gini Index, a measure originally developed for economic disparities. To examine its applications in urban forestry and urban greening, a systematic review was conducted across 5 databases and 10 journals. Forty-one English, peer-reviewed articles were identified that used the Gini Index to measure urban green inequality, increasing exponentially since the first urban greening-related use of the Gini Index in 2011. Most studies were from China (n = 22, 54 %) and the United States (n = 10, 24 %). A Gini Index equation was reported in 27 studies (66 %) with 10 different variations used. Lorenz curves were included in 18 papers (44 %). The Gini Index was used to assess the distribution of parks and greenspaces (n = 28, 68 %), ecosystem disservices and services (n = 8, 20 %), and trees and street greenery (n = 7, 17 %). Fifteen papers (37 %) used multiple points in time to measure changes in inequality, including modeling future inequalities under different management scenarios. The Gini Index provides a quantitative measure of distributional inequality that facilitates comparisons between cities. The application of the Gini Index can help improve global comparative analyses, but only with consistent reporting of methods and findings. We provide recommended reporting procedures for researchers using the Gini Index, including 1) report the Gini Index equation, 2) visualize the Gini Index using a Lorenz curve, and 3) report the variable inputs. Greenspace research should also clearly define the inclusion/exclusion criteria of greenspace, differentiating parks versus green cover.