Oskars Krišāns, a researcher at the Silava Forestry Research Institute, told ReTV News that sap is present in trees all year round; the key is what conditions encourage it to flow. In birch trees, sap is produced in the roots, while in maple trees, it originates in the trunk as temperatures rise above freezing. "Unlike other hardwood trees, a maple tree does not have sap in its heartwood, but gas. As this gas warms up, the pressure increases, pushing the sap through via internal pressure."

The researcher explains that to obtain sap from maple trees, the tree must experience temperatures falling below freezing; for instance, this season, the substantial sub-zero temperatures in early December encouraged sap formation. However, the current thaw does not necessarily mean that the flow of maple sap will end soon. Researchers in North American countries have also confirmed this.
Video: https://www.youtube.com/watch?v=MqXpL5a10rQ