Comparison of Smartphone App and digital hemispherical photography for Estimating Leaf Area Index

Leaf area index (LAI) is a structural parameter that describes distribution characteristics of vegetation leaves in canopy．LAI can affect redistribution of solar radiation, photosynthesis capacity of vegetation and microclimate in vegetation growth environment. Therefore, obtaining LAI in ground measurements is of great significance to understand growth status of vegetation．Digital hemispherical photography (DHP) has been widely adopted due to its simplicity and low cost．In recent years, smartphone-based LAI measurement methods have been extensively used．However, in-depth comparison and verification between smartphone and DHP methods have not been done．Performance of smartphone for measuring LAI needs to be validated and its potential to replace DHP needs to be explored．In this paper we measured coniferous-broadleaved mixed forest, broadleaved forest and crop by self-developed smartphone APP (LAISmart)．These DHP measurements were used as reference for comparative studies．LAI values derived from LAISmart were found highly consistent with DHP (R²=0.95, RMSE=0.68), but performance of LAISmart was affected by density of vegetation leaves．When LAI was > 3.9 , LAISmart significantly underestimated measurements of DHP．Auto-exposure mode of smartphone imaging sensor was an important factor for LAISmart underestimations in LAI in high-value areas．When exposure of LAISmart images in LAI high-value areas are reduced, deviation between LAISmart and DHP was reduced, accuracy of LAISmart was improved by 49%．Narrow sensor field of view (FOV) of LAISmart has little effect on measurement results．These data suggest that LAISmart, with higher efficiency and lower costs, can be an alternative to DHP．