Sap is a fluid that moves continuously through the tree’s internal vascular tissues, which act as the plant’s circulatory network. Its primary purpose is to carry the water, minerals, and sugars necessary for the tree’s health and growth. Nearly all vascular plants contain this fluid, but only certain species produce a noticeable flow when tapped or injured.
The Biological Role of Sap in Trees
Sap is divided into two chemical types, each with a distinct function in the tree’s transport system. The first is xylem sap, composed primarily of water and dissolved mineral nutrients absorbed from the soil by the roots. Xylem sap moves in a mostly upward, unidirectional flow, hydrating the canopy and providing raw materials for photosynthesis in the leaves.
The movement of xylem sap is driven by root pressure and the evaporative pull from the leaves, which lifts water against gravity, even in the tallest trees. The second type is phloem sap, which is rich in sugars—mostly sucrose—produced during photosynthesis. This fluid is the tree’s food supply, moving bidirectionally to nourish growing tissues, roots, and storage organs.
Phloem sap transports stored organic molecules, including proteins and hormones, to all non-photosynthesizing parts of the tree. This ensures energy is distributed for cellular respiration, growth, and the storage of reserves for the dormant season. The circulation of these two saps is fundamental to the tree’s ability to survive, grow, and respond to environmental changes.
Common Trees Known for Sap Production
While all trees contain sap, species in the maple, birch, and walnut families are known for producing a substantial flow that can be commercially harvested. Sugar maples (Acer saccharum) are the most famous, prized because their xylem sap boasts a high sugar content, typically ranging between two and three percent. This concentration makes the sugar maple the most efficient tree for syrup production, requiring roughly 40 gallons of sap to yield one gallon of syrup.
Other maple varieties, such as Red and Black maples, also produce sweet sap, though their sugar content or flow duration can be slightly less than the Sugar maple. The sap from these trees flows best during the late winter and early spring when freezing nights and thawing days create the pressure necessary to push the xylem fluid out of a tap hole. Box Elder (Acer negundo) is another maple that can be tapped, with producers in the Canadian prairies relying on it for its sweet yield.
Birch trees, including Yellow, Sweet, and Paper Birch, are tapped for their sap, which is often consumed as a beverage or boiled down into syrup. Birch sap has a much lower sugar content, often less than one percent, requiring significantly more boiling than maple sap to reach a syrup consistency. The resulting birch syrup is distinctively different from maple, often described as having spicy, smoky, or earthy undertones.
Trees in the walnut family, such as Black Walnut and Butternut, yield a sweet sap that can be processed into syrup. Butternut sap (Juglans cinerea) has a sugar content similar to the Sugar maple, though the total volume collected is often less. Sycamore (Platanus occidentalis) sap is also occasionally tapped, yielding a syrup described as having a subtle butterscotch flavor.
Distinguishing Sap from Other Tree Secretions
Sap is sometimes confused with other viscous fluids that trees exude, but these substances are chemically and functionally distinct. True sap is an internal, water-based transport fluid, whereas other secretions are external, defensive compounds. Resin, often called pitch, is a thick, sticky, hydrocarbon-rich substance produced primarily by coniferous trees like pines, firs, and spruces.
Resin’s function is to serve as a defense mechanism, sealing wounds and acting as a barrier against insects and fungal pathogens. Unlike sap, which is mostly water and sugar, resin is composed of terpenes and other volatile organic compounds. When a conifer is injured, resin flows out to harden over the damage like a scab, protecting the inner wood.
Another distinct secretion is latex, a milky fluid produced by certain trees, such as the rubber tree. Latex is a complex emulsion of organic compounds that serves as a defense against herbivores and pathogens. Finally, some hardwood trees, like cherry or acacia, exude gum, a water-soluble polysaccharide that acts as a wound response.