Thermal Dynamics of Xylem and Soil–Root Temperatures in Olive and Almond Trees and Their Relationship with Air Temperature

Air temperature is commonly used to represent plant thermal conditions, although temperatures within woody tissues and the soil–root zone can differ substantially under field conditions. This study characterized the thermal dynamics of xylem tissue and the soil–root interface in almond and olive orchards under Mediterranean field conditions in Southern Spain. Using long-term in-field measurements, temperatures were monitored in branch and trunk xylem tissues and at the soil–root interface, and regression models were developed to provide empirical correction relationships between air and internal temperatures across seasons and sensor position. Branch xylem temperatures closely matched air temperature for both minima and maxima. In contrast, trunk xylem and the soil–root interface showed pronounced thermal buffering. Trunk xylem maximum temperature was significantly (3.4 to 5.4 °C) lower than air temperature during summer. Shaded soil–root interface temperatures were 5.2 to 9.0 °C lower than air temperature in spring and summer but 5.9 to 11.7 °C higher than air temperature in autumn and winter. These patterns indicate a strong capacity of woody tissues and the soil–root system to moderate external thermal conditions. By quantifying air-to-tissue and air-to-soil relationships under field conditions, this study provides microclimatic data that can improve agronomic models and temperature-driven disease risk frameworks for vascular pathogens infecting woody crops.