Unraveling water pathways and sources in leafless maples during early spring sap ascent.

Maple trees repair cold-induced embolism by generating positive stem pressure during their leafless state, altering sap transport in ways that remain poorly understood. This xylem pressure also drives sap exudation, enabling maple sap harvest for syrup production. This study investigates water source dynamics in leafless maples in early spring and its impact on sap yields. We used heavy water (DO) as a tracer, injected into the soil for root uptake or directly into the stem at various heights of 55 tapped maple trees. Sap isotopic composition was monitored over time and analyzed in relation to weather, sap volume, and sugar content. Our findings show that maple trees absorb soil water gradually in early spring, even when leafless and still under snow cover, with limited sap transport and significant water mixing within the stem. Root-derived water first appeared around the sixth freeze-thaw event, marking a key rehydration phase coinciding with peak sap yields. Optimal daily maple sap yields also depended on cold nights to enhance sugar concentration, followed by extended thaws at moderate temperatures (3-5°C) to increase sap volume. These results highlight the combined influence of xylem physiology and weather conditions on spring stem rehydration and maple sap yields.