As the late winter sun climbs in the southern sky, sap is rising in sugar maples and in some other deciduous tree species. Tapped for maple syrup production, the xylem sap of the sugar maple is generally obtained during February and early March in the northeastern U.S.
Since this sap rises before buds open and leaves unfurl, its movement cannot be explained by the "pull" of transpiration on fluid in the xylem tubules. Neither is there evidence that the flow is triggered by a pressure gradient between the roots and the xylem in the upper trunk and limbs. Rather, current scientific evidence suggests that the freeze-thaw cycle of late winter alters the pressure of gas (primarily carbon dioxide) in the xylem tubules, drawing in fluid from the adjacent xylem cells when gas is absorbed and inducing upward osmotic flow through the cells.
In trees and other plants that do not manifest a late winter sap rise, the xylem flow commences in spring when buds open and transpiration begins. While the xylem sap contains a varied percentage of sugars and minerals (depending on the plant species), it is primarily a water transport mechanism; nutrients for leaf growth reside within the buds and, once the leaves initiate photosynthesis, they manufacture sugars for the entire plant, distributed via the phloem sap. Sugars not utilized during the growing season are stored as starch in the roots, stems and buds.
