Ratio of photosynthetically active radiation to global solar radiation above forest canopy in complex terrain: measurements and analyses based on Qingyuan Ker Towers

Abstract

Abstract Background Understanding of the ratio of photosynthetic photon flux density (Q p ) to global solar radiation (R s ) (Q p /R s ) is crucial for applying R s to ecology-related studies. Previous studies reported Q p /R s and its variations based on measurements from a single observatory tower, instead of multi-site-based measurements over complex terrains. This may neglect spatial heterogeneity in the terrain, creating a gap in an understanding of how terrain affects Q p /R s and how this effect interacts with meteorological factors. Methods Here the Qingyuan Ker Towers (three towers in a valley with different terrains: T1, T2, and T3) were utilized to measure Q p and R s over mountainous forests of Northeast China. An airborne LiDAR system was used to generate a digital elevation model, and sky view factor of sectors (SVFs) divided from the field of view of tower’s pyranometer was calculated as a topographic factor to explain the variations of Q p /R s . Results The results identified significant differences in Q p /R s of the three towers at both daily and half-hour scales, with larger differences on clear days than on overcast days. Q p /R s was positively correlated with SVFs of T1 and T3, while this correlation was negative with that of T2. The effect of SVFs on Q p /R s interacted with clearness index, water vapor pressure and solar zenith angle. Random forest-based importance assessment demonstrated that explanation (R 2) on Q p /R s was improved when SVFs was included in the predictor variable set, indicating that incorporating terrain effects enhances the prediction accuracy of Q p /R s . The improvement in the R 2 values was more pronounced on clear days than on overcast days, suggesting that the effect of terrain on Q p /R s depended on sky conditions. Conclusions All findings suggested that Q p /R s is affected by terrain, and integrating terrain information into existing Q p /R s models is a feasible solution to improve Q p /R s estimates in mountainous areas.

Keywords

• Photosynthetic photon flux density
• Clearness index
• Water vapor pressure
• Solar zenith angle
• Terrain
• Sky view factor