Special issue: Urban green infrastructure and the ecological functions

Sustainable cities in the future will depend on urban green infrastructure and its ecological functions (Mell 2009;Breuste et al. 2015).Green infrastructure is broadly used to describe the multifunctional approach to ecological planning.Various ecological functions, such as regulation of water flows and water purification (Lee et al. 2018), local climate regulation and adaptation (Lehmann et al. 2014), carbon storage and sequestration (Mensah et al. 2016), biodiversity conservation (Watts et al. 2010), accessibility for amenity and recreation, environmental education (Wolsink 2016), and other ecosystem services, can be arranged simultaneously in one green space.This concept of green infrastructure is especially relevant for the urban area, because only limited green spaces are allowed to be conserved or additionally restored in cities. Controversy on whether or not to harmonize the benefits from traditional gray infrastructure and those multipleecological functions from green infrastructure remain in the planning literature (Tiwary and Kumar 2014).Therefore, further studies are needed on the structure and functions of the urban green infrastructure, specifically on how to optimize the solutions in a limited urban area.We can divide the challenges in urban green infrastructure into monitoring and assessment issues.