Objective The research on the negative air ion concentration (NAIC) of urban forest in different geographical spaces was to provide scientific support for the theoretical study of negative air ions and the site selection basis of forest therapy base.

Method From September 2019 to January 2020, and from May to August 2020, three forest environments with different geographical locations, forest area and surrounding environment gradients in Guangzhou, Guangdong Province of southern China, i.e. exurban forest, suburban forest, and downtown forest were selected, and an urban control group was set up to compare NAIC differences. Pearson correlation analysis and multiple stepwise regression analysis were used to explore the correlations between NAIC and air oxygen content, temperature, relative humidity as well as wind speed.

Result (1) NAIC of natural forests was distinctly higher than that of planted forests. In plantation, NAIC of high canopy density forests was significantly higher than that of middle canopy density forests. (2) The NAIC of urban forest in different geographical spaces was prominently different, showed as exurban forest > suburban forest > downtown forest > urban control group. (3) The ability of NAIC generation in forest environment was evidently better than that in urban environment without forests. (4) The seasonal variation of NAIC in exurban forest and suburban forest was summer > spring > autumn > winter, while that in downtown forest and urban control group was autumn > winter > spring > summer. (5) NAIC of exurban forest and suburban forest was remarkably positively correlated with air oxygen content, temperature and relative humidity, while the NAIC of downtown forest and urban control group was noticeably negatively correlated with air oxygen content and temperature; NAIC of suburban forest was significantly negatively correlated with wind speed, while that of exurban forest was notably positively correlated. (6) The NAIC of exurban forest was closely related to relative humidity and temperature. The NAIC of suburban forest was mainly affected by air oxygen content, wind speed and temperature. The NAIC of downtown forest and urban control group was mostly affected by temperature.

Conclusion The geographical space has a significant impact on NAIC. Distance from the city center and the forest area reaching a certain scale and concentrated contiguous has a positive impact on NAIC. The relevant results provide a theoretical basis for the location of forest therapy bases. The environmental impact factors of NAIC of urban forest in different geographical spaces are different. It is suggested that future research should comprehensively consider the synergistic effects of different factors in many aspects, such as meteorology, stand, terrain, and surrounding environment, and further explore the impact mechanism of NAIC and various environmental factors.