Seasonal contrasting effects of PM2.5 on forest productivity in peri‑urban region of Seoul Metropolitan Area, Republic of Korea

Abstract

Urban forests are recognized as eco-friendly filters to reduce PM2.5 concentrations. However, there remains a lack of consensus on how PM2.5 affects forests’ productivity in highly polluted urban areas by altering solar radiation components. Here, we tested the hypothesis that the effects of PM2.5 on the solar radiation components and forest productivity differ by season because of seasonal variations in solar elevation angle (β) and PM2.5 concentrations. The PM2.5 concentrations and CO2 fluxes were measured using optical particle counters and an eddy covariance system, respectively, from April 2018 to December 2019 at a natural deciduous oak stand and an evergreen pine plantation in the peri‑urban region of the Seoul Metropolitan Area, Republic of Korea. Complex relationships among PM2.5 concentration, direct radiation (Sdir), diffuse radiation (Sdf), air temperature (Ta), and net ecosystem production (NEP) were disentangled by season and β using structural equation modeling. We found that the Sdf was the controlling factor of NEP in both the stands, showing a more substantial effect than that of Sdir, Ta, and PM2.5. In the oak stand, when oaks hold their leaves (May–October), the increase in PM2.5 concentrations enhanced Sdf and NEP, and it was more significant when β increased from 10 to 20° to > 60° However, in the pine plantation, the increase in PM2.5 concentrations enhanced Sdf and NEP when β > 40° and PM2.5 < 10,100 cm–3. In contrast, when β < 40° and PM2.5 > 10,100 cm–3, PM2.5 increase resulted in a greater Sdir reduction than Sdf enhancement, which consequently led to a NEP reduction. Our study results demonstrated the contrasting seasonal effects of PM2.5 on solar radiation components and forest productivity and highlighted the importance of integrating them into ecosystem modeling and environment management plans in urban or peri‑urban regions.