The concept of sap moving “down” in a tree refers to a seasonal biological process fundamental to survival through colder periods. Sap flow is dynamic, changing direction and composition based on the plant’s needs and environment. During the active growing season, fluid transport supports photosynthesis and growth. As seasons shift, the flow redirects to store energy, preparing the tree for dormancy. Understanding this downward movement requires distinguishing between the two primary types of sap and the environmental signals that trigger the change.
The Two Types of Tree Sap
The fluid transport system in trees relies on two distinct tissues, the xylem and the phloem, each carrying a unique type of sap. Xylem sap consists primarily of water and dissolved mineral nutrients absorbed from the soil by the roots. This sap moves almost exclusively upward, driven by the negative pressure created by water evaporating from the leaves, a process known as transpiration.
Phloem sap, in contrast, is a concentrated solution of sugars, mainly sucrose, along with hormones and amino acids. This sugar-rich sap is the product of photosynthesis and travels from “source” areas, like the leaves, to “sink” areas where energy is needed or stored. The movement within the phloem is bidirectional, but the distinct downward flow is the transport of sugars to the roots and stem tissues for winter storage.
Environmental Cues for Seasonal Change
The trigger for the sap’s downward movement and storage is a predictable sequence of environmental signals. The primary cue is the shortening of the photoperiod, or the reduction in daylight hours, which begins in late summer. This decline in light signals the tree that the growing season is nearing its end and initiates hormonal changes.
These cues cause the tree to produce hormones like abscisic acid, which signals the leaves to begin senescence and prepare for abscission. This hormonal shift directs the tree to halt outward growth and redirect its energy focus inward toward survival. While decreasing temperatures accelerate the process and solidify dormancy, the change in day length is the initial signal that begins the shut-down process.
The Process and Timing of Storage
The critical downward movement of sap—the storage phase—begins in late summer and continues through the early fall, typically from August to October in temperate climates. During this period, the leaves, which are the sugar “sources,” actively load newly created sugars into the phloem vessels. This action creates high pressure that drives the phloem sap downward into the “sinks.”
These storage sinks are the roots, the trunk’s woody tissues, and larger branches. The concentrated sucrose solution traveling downward is then converted into starch, a complex carbohydrate that is more stable for long-term storage throughout the dormant season. This process is when the sap “goes down,” as the tree systematically drains mobile energy from the crown into permanent woody structures. Once deciduous leaves fall and the tree enters deep dormancy, the active flow of phloem sap largely ceases until the following spring.
Variation Across Tree Species
The timing and nature of sap movement vary significantly depending on the tree’s life strategy. Deciduous trees, such as maples, oaks, and elms, exhibit a rapid downward flow of phloem sap in the fall, linked to leaf senescence and the onset of dormancy. Their storage phase is completed before severe cold sets in, allowing them to rely on stored starch reserves throughout the winter.
Evergreen trees, like pines and spruces, maintain their needles year-round. While they reduce metabolic activity, their sap flow is slower and more continuous throughout the year, with a less pronounced downward storage phase. A notable exception is the unique, pressure-driven flow of xylem sap in maple trees during the late winter. This flow, used for maple syrup production, is an upward movement of water and stored sugars caused by alternating freeze-thaw cycles and is distinct from the phloem’s autumn descent for storage.