The question of where tree sap goes in winter assumes the liquid vanishes, but the truth involves a sophisticated chemical and physical transformation. Tree sap, the fluid transporting water, nutrients, and sugars, does not drain away or entirely freeze. Instead, its composition is dramatically altered, and its bulk water content is strategically relocated. The tree enters a state of dormancy, a metabolic hibernation signaled by environmental cues, to protect its living cells from catastrophic damage caused by ice crystal formation. This process primarily concerns the water-transport system (xylem) and the living cells surrounding it.
The Anatomy of Sap Flow
During the spring and summer growing season, the tree’s water-transport system is highly active, driven largely by transpiration. As leaves release water vapor through tiny pores called stomata, a continuous negative pressure (tension) is created. This tension pulls water upward through the xylem vessels from the roots to the canopy. This movement is rapid, widespread, and involves a large volume of relatively dilute, water-based fluid.
The sap flowing through the xylem is mainly composed of water and dissolved minerals. A separate, thicker fluid runs through the phloem, transporting sugars produced during photosynthesis from the leaves to the rest of the tree. This phloem sap moves through a source-to-sink gradient, supplying energy to growing tissue and storing excess carbohydrates in the roots and ray cells. This high-volume flow is only sustainable when temperatures are warm and water is readily available.
Initiating Dormancy and Water Withdrawal
The signal for winter preparation is not a single frost but the predictable shortening of daylight hours in late summer and early autumn. This photoperiod cue initiates cold acclimation, which triggers a profound slowdown in the tree’s metabolism and growth. The tree begins the active process of cellular dehydration, which is the physical answer to where the water goes.
The most vulnerable component is the free water residing inside the cells and within the non-living xylem conduits. To prevent ice crystals from forming and rupturing living wood cells, the tree actively pumps this free water out of the cells and into the intercellular spaces (the small gaps between the cells). Much of this bulk liquid is sequestered in specialized storage cells, such as parenchyma and ray cells, or drawn down into the roots and the base of the trunk.
This withdrawal of water is a self-protective measure against cavitation (embolism), where air bubbles form in the xylem vessels. When water freezes and thaws, dissolved gases can come out of solution, creating blockages that prevent the upward flow of water when spring arrives. By reducing the overall water volume in the upper trunk and branches, the tree minimizes the risk of widespread damage to its transport system. The cell membranes also become more flexible to withstand the pressure of surrounding ice that forms in the empty intercellular spaces.
Winter Survival: Antifreeze and Storage
The final stage of winter preparation involves a massive chemical change in the remaining cell fluid. Stored starches, the tree’s primary energy reserve accumulated in the roots and inner wood cells, are broken down into soluble sugars. This hydrolysis reaction converts large, insoluble starch molecules into smaller, highly concentrated sugar molecules (e.g., glucose, sucrose, and raffinose).
These concentrated sugars act as natural cryoprotectants, effectively lowering the freezing point of the remaining fluid (protoplasm) inside the living cells. This biological “antifreeze” allows the living portions of the tree to survive temperatures that would freeze pure water solid, protecting the cell structures from ice damage. The sap does not disappear; it becomes a thick, non-circulating, sugary syrup held within the deepest cells of the trunk and root system.
The tree maintains this high-sugar, low-volume state throughout the winter, with protected living cells ready to resume growth when temperatures rise. When spring returns, the concentrated sugar is converted back into starch for long-term storage. The bulk water is then drawn up from the roots, rehydrating the system and initiating the high-volume sap flow characteristic of the growing season.