Monthly changes in the water content of Fagus crenata (Japanese beech) monitored by near infrared spectroscopy

Leaf water content is a key physiological parameter in deciduous trees, and its accurate nondestructive measurement is essential for monitoring tree health and detecting early signs of disease and drought stress. Previous near infrared spectroscopy studies have rarely investigated seasonal variation in leaf water content systematically across multiple months. In this study, near infrared spectroscopy combined with partial least squares regression was applied to systematically investigate monthly changes in the leaf water content of Fagus crenata (Japanese beech) from April to November. Each month, 200 leaf samples were measured and water content was determined on a dry basis. Prediction accuracy varied substantially with sampling month; the April model yielded the best performance (R 2 p = 0.94), while the July model yielded the worst (R 2 p = 0.80). Loading plot analysis identified the region near 1400 nm — corresponding to the inflection point of the first-overtone O–H stretching absorption band of water centered around 1450 nm — as particularly important for water content prediction in young leaves. These results provide a seasonal baseline for Fagus crenata that could serve as a reference for future field-based real-time monitoring of tree health using portable near infrared spectrometers.