The benefits of access to local green space are well documented, including, among many others, those related to physical health (Maas et al., 2006), stress recovery (Van den Berg etal., 2007), mental well-being (Fuller et al., 2007), social cohesion (Coley et al., 1997), provision of ecosystem services (Bolund & Hunhammar, 1999), and biodiversity conservation. Urban green spaces (UGSs) are becoming increasingly important in developing countries (Thaiutsa et al., 2008). Trees, especially those of maturity, are considered “key elements” within urban areas (Lindenmayer et al., 2012; Stagoll et al., 2012); trees contribute to various ecosystem functions, such as habitat function (maintenance of biological and genetic diversity) and information function (spiritual enrichment, mental development and leisure) (McKinney, 2002; Dobbs et al., 2011).). Urban trees can also have a stronger effect on carbon budgets than non-urban trees (Dearborn & Kark, 2010), in addition to playing a high impact role in reducing the intensity of urban heat island effects (UHI) (Chow & Roth, 2006).
Current studies of urban trees largely focus on the following aspects: (1) The ecological service of urban trees: mitigating urban heat island effects (Nowak et al., 2017; Zhao et al., 2018; Fan et al., 2019; Ow et al., 2019), aerodynamic characterization of urban trees (Rafael et al., 2018; Yuan et al., 2017; Zeng et al., 2020), and the impact of urban trees on air pollutants (Sicard et al., 2018; Yli-Pelkonen et al., 2018; Chen et al., 2019; Lai & Kontokosta, 2019; ; Xing et al., 2019; Mukherjee et al., 2020; Sgrigna et al., 2020) are still research hotspots, although some negative ecological services related to human health have also attracted much attention, such as pollen (Sicard et al., 2018; Cariñanos et al., 2020; Aerts et al., 2021) and plant (Lara et al., 2019) allergies; (2) Urban tree management: maintenance and assessment practices, including the health (North et al., 2017; Stravinskienė et al., 2018; Abbas et al., 2020; Großmann et al., 2020; Schollaert et al., 2020), survival (Ossola & Hopton, 2018; chollaert et al., 2020), and protection (Morgenroth et al., 2017; Guo et al., 2018; Lin et al., 2020) of urban trees and urban forestry policy (Breger et al., 2019; Namin et al., 2020; Quinton et al., 2020) have been researched to establish a healthy and rational urban forest structure; (3) Physiological and morphological responses of urban trees to human disturbance and other environmental stresses were studied (Cobley & Pataki, 2019; Moser-Reischl et al., 2019; Wang et al., 2019; Kuzmin et al., 2020) to guide urban tree planting; and (4) Research on urban forest structure and diversity, with tree canopy, species composition (Chimaimba et al., 2020; Dangulla et al., 2020) and diversity (Kendal et al., 2012; Bentsen et al., 2010 Sjöman et al., 2016; e Silva et al., 2020) being studied in detail.
Few studies, however, have assessed urban tree DBH and its influencing factors – in fact, little is known about the magnitude and dynamics of urban tree DBH. This measure is considered to reflect the age structure of an urban forest, and also infers urban vegetation dynamics. The DBH structure is the most basic stand structure for a forest community, and could reflect the relationship between trees and their habitat. Typically, the greater the DBH, the greater the ecological benefits associated with trees (e.g., carbon storage, carbon sequestration, and air purification), but the maintenance costs for trees with large diameters is often higher than for smaller trees. Both the size and the age of a tree affect characteristics such as the tree mortality. Typically, tree-level data are needed for maintaining the urban tree reserve. To maximize the ecological benefits of urban green space, the DBH structure of trees should be reasonably configured. Therefore, we ask two key questions: (1) What is the magnitude of tree DBH in Zhanjiang UGS? (2) Which factors could contribute to changes in urban tree DBH? We expect that trees with smaller DBHs would be more abundant than trees with larger DBHs, due to rapid urbanization and increased attention towards urban tree planting in recent decades.