Lower Stem Freezing Point Tied to Greater Frost Embolism Vulnerability: A Comparison Between Ring- and Diffuse-Porous...

Temperate ring-porous tree species are more vulnerable to freezing-induced embolism than their diffuse-porous counterparts due to their large earlywood vessels. Although ring-porous species can restore hydraulic function by forming earlywood vessels in early spring, they may face high risk of embolism during spring frosts. Here, we tested the hypothesis that ring-porous temperate trees employ a strategy of enhanced freezing avoidance to protect their vulnerable earlywood vessels from the risk of freezing-induced embolism. We quantified the degree of embolism in overwintering stems, ice nucleation temperature (INT_xylem), water content (WC_xylem), and non-structural carbohydrate (NSC) concentrations in stem xylems of 20 temperate tree species (nine diffuse-porous and 11 ring-porous) grown in a common garden. Consistent with our hypothesis, ring-porous species suffered severe embolism over the winter and exhibited stronger freezing avoidance during spring. While diffuse-porous species rely on smaller vessels to resist freezing-induced embolism, ring-porous species showed stronger ability of freezing avoidance by lowering INT_xylem via reduced WC_xylem and higher NSC concentrations. This strategy likely protects their vulnerable, newly developed earlywood vessels from freezing-induced hydraulic dysfunction. These findings reveal distinct strategies between temperate diffuse-porous and ring-porous species for coping with spring frost stress, providing insights into forest responses to climate change.