Quantifying Water Stress in Acer pictum subsp. mono and Hovenia dulcis Seedlings Using Thermal Imaging and Sap Flux

Land–atmosphere interactions at the leaf scale play a critical role in regulating surface energy exchange and plant water use under increasing heat and drought, yet quantitative indicators capturing short-term thermal–hydraulic coupling remain limited. This study compared seedlings of Acer pictum subsp. mono, with highly dissected leaves and low boundary-layer resistance, and Hovenia dulcis, with smoother leaves and thicker boundary layers, to test how leaf morphology constrains thermal and hydraulic regulation. Seedlings were exposed to well-watered, control, and severe-drought treatments, creating a clear soil-moisture gradient, while leaf temperature and sap flux were monitored alongside key environmental drivers. This design enabled evaluation of short-term leaf temperature variability (ΔT, 5-min scale) and its coupling with radiation and transpiration across contrasting water conditions. Across both species, ΔT was most strongly coupled with changes in photosynthetically active radiation(PAR). In A. mono, the PAR increase threshold triggering synchronized ΔT responses declined under severe drought (≈103 μmol m⁻² s⁻¹) relative to well-watered conditions (≈135 μmol m⁻² s⁻¹), whereas H. dulcis showed no significant treatment dependence. Under identical PAR reduction levels, higher sap velocity consistently enhanced leaf temperature declines, indicating transpiration-driven amplification of short-term cooling. At high temperatures (30–35 °C), A. mono maintained strong cooling responses, while H. dulcis exhibited flattened sap–ΔT relationships and increased ΔT amplitude under severe drought (≈5.0 °C). These results demonstrate that short-term leaf cooling emerges from the interaction between radiation forcing and transpiration, with species-specific constraints imposed by leaf morphology and hydraulic limitation. Integrating ΔT, PAR, and sap flux provides a quantitative framework for comparing thermal–hydraulic strategies among species and offers a sensitive tool for early diagnosis of drought vulnerability at the seedling stage.