Canopy Light Interception Conversion in Upright Fruiting Offshoot (UFO) Sweet Cherry Orchard

Abstract

<abstract> The sun‘s position varies throughout the day. The changes in time and/or location cause different angles between the sunbeam and tree canopy (sunbeam-canopy angle, θ). Canopy light interception, the fractional interception of photosynthetically active radiation (PAR), varies as θ changes, resulting in a different measurement basis of light interception if light interception is measured at different θ. This situation is more noticeable in planar tree architecture, such as upright fruiting offshoot (UFO) architecture. This study aimed to develop a conversion method to provide a common basis for determining canopy light interception at different sun positions for such architectures. In addition to sun azimuth and zenith angles (α and β, respectively), conversion inputs included canopy dimensions, orchard configuration, canopy light interception, and canopy ground occupation porosity (the fraction of non-shaded area). Field data collection was conducted at 18 different times within a time window of ±2.50 h from solar noon (no mutual shadows were cast between adjacent rows) with 0.25 h intervals for eight sample blocks. The conversion method was able to convert canopy light interception with an overall root mean square error (RMSE) of 0.03 and mean absolute percentage error (MAPE) of 7.85%. Based on the overall accuracies when using a different basis and using different measured values in this study, conversion is recommended when θ is greater than 8°. In addition, we recommend conducting conversion with canopy projection from the same side to avoid the influence of asymmetrical canopies.