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

Background Understanding of the ratio of photosynthetic photon flux density（Qp） to global solar radiation（Rs）(Qp/Rs) is crucial for applying Rs to ecology-related studies.Previous studies reported Qp/Rs 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 Qp/Rs 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 Qp and Rs 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 Qp/Rs.Results The results identified significant differences in Qp/Rs of the three towers at both daily and half-hour scales,with larger differences on clear days than on overcast days.Qp/Rs was positively correlated with SVFs of T1 and T3,while this correlation was negative with that of T2.The effect of SVFs on Qp/Rs interacted with clearness index,water vapor pressure and solar zenith angle.Random forest-based importance assessment demonstrated that explanation（R2） on Qp/Rs was improved when SVFs was included in the predictor variable set,indicating that incorporating terrain effects enhances the prediction accuracy of Qp/Rs.The improvement in the R2 values was more pronounced on clear days than on overcast days,suggesting that the effect of terrain on Qp/Rs depended on sky conditions.Conclusions All findings suggested that Qp/Rs is affected by terrain,and integrating terrain information into existing Qp/Rs models is a feasible solution to improve Qp/Rs estimates in mountainous areas.