Stand structure and environment jointly determine negative air ion concentrations in forests: Evidence from concurrent on-site monitoring in four typical subtropical forests during the growing season

Negative air ions (NAIs) have been extensively studied by scientists and administrators due to their crucial role in assessing the benefits of forest health to humans and improving air quality. This paper presents the results of a study conducted in four typical subtropical forests, where 24-hour on-site monitoring of NAIs, air temperature, relative humidity, wind speed, direct radiation, and particulate matter was conducted during the growing season. The vegetation characteristics of these forests were also investigated. Correlation and random forest analyses were employed to jointly evidence the effects of stand structure and environmental factors on NAI concentrations using mean data for the monitoring period. Additionally, regression analyses were employed to discuss the response of NAIs to continuous changes in meteorological factors using screened nighttime data. Our analysis revealed that NAI diurnal dynamics followed a unimodal pattern, with peaks observed in the afternoon (13:30–16:30) and troughs in the early morning (4:00–7:00). Significant variations in NAI concentrations were observed among the forest types, with mixed broadleaved forest showing the highest concentrations, followed by phoebe forest, Moso bamboo plantation, and Chinese fir plantation. The differences in NAI concentrations among forest types were primarily influenced by plant density at monitor heights, plant diversity, leaf area index, and tree height diversity. Overall, NAI concentrations were positively influenced by temperature and relative humidity and negatively influenced by particulate matter, while the influence of wind speed varied with forest type. Furthermore, our findings indicate that changes in particulate matter had a greater contribution to NAI concentrations compared to meteorological factors. By quantifying the spatial and temporal characteristics of NAIs in typical subtropical forests and their relationships with influencing factors from the perspectives of both stand structure and environment, our study provides new insights into scheduling human forest therapy activities and managing therapeutic forests.