Inter-relationships between canopy features and fruit yield in citrus as detected by airborne multispectral imagery.

摘要

The objective of this research was to examine the interrelationships between canopy features and the fruit yield of citrus crops. Satsuma Mandarin (Citrus unshiu), a native citrus variety in southeastern Asia, grown in an orchard located at Nebukawa Agricultural Research Station, Kanagawa prefecture, Japan, was used for this preliminary analysis. Airborne multispectral images in the red, green, blue, and near-infrared (NIR) bands with a high spatial resolution of 0.2x0.2 m were acquired over the experimental site at four time periods in 2002 and 2003. Images based on normalized difference vegetation index (NDVI) were generated with ERDAS Imagine 8.6 software. From these images, thresholded pixel counts (TPCs), indicators of the relative leaf areas of several leaf types in each canopy, for 48 selected tree samples were extracted using a program developed in MATLAB R12. Pearson's correlation analysis was employed to examine the relationships between each of the TPCs and the fruit yields of citrus in 2002, 2003, and 2004. Results indicated that some TPCs showed a higher correlation with citrus fruit yield than those corresponding to the entire canopy size, particularly the TPCs extracted from the visible red, green, and blue wavelengths. The TPCs corresponding to the mature leaves before the fast vegetative growth (May) were found to be significantly correlated with the fruit yield of the same growing season, while those corresponding to the younger leaves during this period were more significantly correlated with the fruit yields of the previous and the following growing seasons. These results confirmed the inter-relationships between canopy features and the fruit yield of citrus crops. This information also implies an unmatched energy allocation dynamic between different leaf types within the canopy, which may lead to an unsynchronized leaf energy contribution, direct (mature leaves) or delayed (younger leaves), to the fruiting of citrus crops. In addition, the models based on the TPCs extracted from early season's images demonstrated the potential of airborne multispectral imagery to forecast the fruit yield of citrus trees. The obtained yield estimates can provide valuable information for planning fruit harvest schedules and generating prescription maps for tree-specific management practices on an individual tree basis. However, further investigations are necessary before these models can be applied in a practical situation.