What Is Tree Sap Made Of?

Tree sap is mostly water, but dissolved in that water is a surprisingly diverse cocktail of sugars, amino acids, minerals, hormones, and organic acids whose exact recipe depends on which of a tree’s internal transport systems you’re tapping into. A sugar maple’s spring xylem sap, for instance, is roughly 97 percent water with about 2–3 percent sucrose and traces of minerals, while the phloem sap flowing through the inner bark of a citrus tree is a much more concentrated sugar solution. Things get even more varied when you include resins, latex, and gums, which are technically different substances that people often lump together under “sap.” The chemistry behind each of these fluids reveals how trees feed themselves, defend against attack, and survive freezing winters.

Two Pipelines, Two Very Different Fluids

Trees run two parallel plumbing systems, and each carries sap with a distinct composition. Xylem sap travels upward from roots to leaves through dead, hollow cells in the wood. It’s the more dilute of the two, consisting mainly of water pulled upward by evaporation from leaves. Dissolved in that water are mineral ions and nitrogen-carrying compounds the roots have absorbed from soil. In kiwifruit vines, for example, the dominant charged species in xylem sap are calcium, potassium, magnesium, malate, and phosphate, and the main nitrogen carrier is the amino acid glutamine, which accounts for about 58 percent of total nitrogen in the sap.¹ A survey across more than 50 tree species found that seven amino acids and related compounds dominate xylem sap nitrogen, with glutamine being the single most abundant in pines, where it represents 73 to 88 percent of organic nitrogen.²

Phloem sap, by contrast, flows through living cells in the inner bark, carrying the sugars that leaves produce during photosynthesis down to roots, fruits, and growing tips. It’s far richer than xylem sap. In sweet orange trees, total sugar concentration in phloem sap runs around 103 millimolar, with sucrose making up roughly 64 percent, glucose about 20 percent, and fructose about 10 percent, plus a small fraction of sugar alcohols.³ But sugars and amino acids are just the beginning. Metabolomics studies have shown that phloem sap contains compounds representing many metabolic pathways beyond simple sugars and amino acids, including hormones, defense molecules, and signaling compounds.⁴

So when someone asks “what is tree sap made of,” the honest answer depends on which pipe you’re looking at. Xylem sap is a dilute mineral-and-amino-acid solution. Phloem sap is a concentrated sugar highway loaded with signaling molecules. Most of what people encounter in everyday life, whether it’s the sticky stuff on a pine tree or the liquid dripping from a tapped maple, comes from one system or the other, or from a defensive secretion that isn’t really part of the transport system at all.

Why Maple Sap Flows Only in Spring

Maple syrup season exists because of an unusual physical trick that maple trees pull off during the transition from winter to spring. On cold nights, temperatures drop below freezing and water inside the wood’s tiny fiber cells turns to ice. On warmer days, that ice melts. In most trees, this freeze-thaw cycle actually causes sap to be absorbed during thawing. Maples do the opposite: they push sap out of the wood during the thaw, but only because their vessel sap contains sucrose. Without sucrose present in the vessels, maple stems absorb sap on thawing just like other species.⁵

The mechanism hinges on the distinctive cellular architecture of maple sapwood, which consists of gas-filled fiber cells connected to sap-filled vessels through walls that allow water through but hold back dissolved sugar. During freezing, pure water is drawn into the fibers through osmotic suction and freezes there, while the sugary sap in neighboring vessels stays liquid because dissolved sugar lowers the freezing point by a small but critical amount. When the temperature rises, the accumulated ice in the fibers melts and the compressed gas expands, generating positive pressure that pushes sap out of any tap hole drilled into the trunk.⁶ Mathematical modeling of this process produces stem pressures consistent with what’s measured in real trees, supporting the idea that the exudation is driven entirely by physics rather than any active biological pumping.⁷

What comes out of the tap is xylem sap, and at the start of maple season it’s about 2–3 percent sucrose with small amounts of amino acids and minerals. As the season progresses, microbial activity in the tap holes and warming temperatures change the chemistry considerably. Sucrose drops sharply in late-season sap, while amino acids like leucine, isoleucine, lysine, and glutamine spike. Mannitol, a sugar alcohol, can climb from under 500 micrograms per milliliter early on to over 2,000 by late season.⁸ These changes are why early-season syrup tends to be light and delicately flavored, while late-season syrup is darker and more complex. Commercially, the sap is concentrated into syrup through thermal evaporation or membrane separation.⁹

Sugars as Antifreeze

The sucrose in maple xylem sap isn’t just fuel; it plays a critical role in helping the tree survive winter. During autumn and into the cold months, trees convert stored starch in their wood into soluble sugars. In sugar maples that have experienced drought stress, the cold-season sugar content in wood tissue is significantly higher than can be explained by starch breakdown alone, suggesting the tree actively imports sugars from other storage sites. Elevated levels of sucrose, glucose, and fructose in these trees during winter function as osmoregulators, lowering the freezing point of cellular fluids and protecting living cells from ice damage.¹⁰

Different tree species use different sugars for this purpose. Japanese pear trees, for instance, rely heavily on sorbitol, a sugar alcohol. Their xylem sap sorbitol concentrations surge in mid- to late December in response to both cold temperatures and the completion of dormancy. Trees exposed to near-freezing conditions accumulated far more sorbitol than those kept slightly warmer.¹¹ In walnut trees, the process involves converting starch in living wood cells into sucrose, which then leaks into the xylem vessels at low temperatures. At cold temperatures, the cells are inefficient at reabsorbing the sugar, so it builds up in the sap, but when temperatures warm to around 15°C, the cells efficiently take the sugar back up.¹² This sugar loading also helps repair air bubbles that form in xylem vessels during freeze events, restoring the tree’s ability to move water come spring.

Resin Is Not Sap, but People Call It That Anyway

The sticky golden substance that oozes from a cut pine tree is resin, not sap in the transport sense. Unlike xylem or phloem fluid, resin isn’t part of the tree’s circulatory system. It’s a defensive secretion produced in specialized canals and stored under pressure. When bark beetles bore through the wood, or a branch snaps, resin floods the wound, physically trapping insects and sealing the opening against fungal invasion.

Chemically, resin is a mix of volatile terpenes (which give pines and firs their sharp smell) and heavier, non-volatile compounds called diterpenes that form the solid residue left behind once the volatile fraction evaporates. The specific terpene profile varies by species and even between individual trees. In slash pine, for example, the ratio of alpha-pinene to beta-pinene differs between high-resin-yielding and low-yielding trees, and high producers carry more limonene in their total terpene mix.¹³ A study of resins from Agathis and Araucaria conifers found that the monoterpene composition is species-specific and barely changes with season, suggesting it’s tightly controlled genetically.¹⁴

This distinction matters practically. If you’ve got resin on your hands after handling firewood, you’re dealing with a hydrocarbon-rich substance that’s essentially a natural varnish. Soap and water won’t cut it the way they would for watery xylem sap. Rubbing alcohol or a cooking oil works better for dissolving the terpenes.

Tree Gums and Other Exudates

Beyond xylem sap, phloem sap, and resin, some trees produce gums, which are yet another class of exudate with a completely different chemistry. Gum arabic, one of the most commercially important, comes from Senegalia senegal (formerly classified under Acacia). It’s rich in arabinogalactan proteins and forms through a process where cell walls separate along the middle lamella without the cells themselves breaking apart.¹⁵ Unlike resin, which repels water, tree gums dissolve in water and form thick, sticky solutions. Gum arabic is used as an emulsifier in soft drinks, a binder in watercolor paints, and a coating for pharmaceutical tablets.

Latex is yet another exudate often confused with sap. Rubber tree latex, from Hevea brasiliensis, is a milky white fluid that contains rubber particles suspended in water along with proteins, lipids, and sugars. It flows through a network of specialized latex vessels called laticifers, which are separate from both the xylem and phloem systems. The rubber particles themselves are made of polyisoprene, a long-chain polymer that gives natural rubber its elastic properties. Like resin, latex serves primarily as a wound-sealing defense, but its chemical identity is entirely different from either sap or resin.

Sap as Chemical Signal Under Stress

Trees can’t run from drought or move toward better conditions, so they rely on chemical signals dissolved in their sap to coordinate responses across their entire body. When roots sense dry soil, they send chemical messages upward through the xylem to tell the leaves to close their stomata and conserve water. Abscisic acid is the best-known of these long-distance chemical messengers, but researchers have implicated several other xylem-borne signals in drought response, including changes in sap pH, cytokinins, a precursor of ethylene, and malate.¹⁶ The sap, in other words, isn’t just plumbing. It functions as a chemical communication network, with the composition of the fluid carrying real-time information about conditions in distant parts of the tree.

Who Else Drinks the Sap

Phloem sap is an attractive food source. It’s loaded with sugar, generally lacks toxins, and sits in a pressurized tube that will push itself into any mouth small enough to tap it. Yet the number of animals that survive on phloem sap alone is remarkably narrow: essentially just sap-feeding insects in the order Hemiptera, including aphids, whiteflies, and scale insects. These insects face two big nutritional challenges. First, phloem sap has plenty of non-essential amino acids but is low in essential ones. Aphids solve this by hosting symbiotic bacteria, such as Buchnera, which can supply up to 90 percent of the essential amino acids the pea aphid needs. Second, the sugar concentration is so high it would cause osmotic problems if absorbed directly. So these insects carry gut enzymes that convert excess sugar into long-chain oligosaccharides, which are excreted as honeydew, the sticky residue you find on cars parked under infested trees.¹⁷

Xylem sap feeders face the opposite problem: the fluid is so dilute that getting enough nutrition requires processing enormous volumes. Xylem-feeding insects like sharpshooter leafhoppers and spittlebugs essentially drink nonstop and excrete nearly all of what they consume, filtering out the tiny amounts of amino acids and minerals. Woodpeckers, sapsuckers, and some squirrels also drink sap by drilling or gnawing into the phloem, though they supplement this with other foods.

A Trillion Bacteria Inside the Wood

One reason late-season maple sap changes so dramatically is microbial activity, but the microbes colonizing tap holes are just the tip of the iceberg. Recent research has revealed that healthy, living trees host an astonishingly dense internal microbiome. A single tree can harbor roughly one trillion bacteria in its woody tissues, with microbial communities sharply divided between heartwood and sapwood, each maintaining distinct populations with little overlap to microbes found on leaves, roots, or the surrounding soil.¹⁸ These internal bacteria are bathed in sap and likely interact with sap chemistry in ways that are still poorly understood. Some may help break down complex molecules, others may compete with pathogenic fungi for resources, and some may be passengers with no clear role. The finding overturns the long-held assumption that the interior of a healthy tree trunk is essentially sterile.

Birch Sap and Other Tapping Traditions

Maple gets most of the attention in North America, but birch sap tapping has a much longer continuous tradition across northern and eastern Europe. The species most commonly tapped are silver birch and downy birch. Russia, Ukraine, Belarus, and the Baltic states are the only countries where sap gathering remained a significant economic activity into the modern era, driven by vast birch forests, low population density, and the incorporation of sap collection into the former Soviet planned economy.¹⁹

Birch sap is more dilute than maple sap, typically containing only about 0.5–1 percent sugar, which means you need to collect far more of it to make syrup. Its sugar profile also differs: birch sap contains glucose and fructose rather than sucrose, giving it a slightly different flavor. Because of the lower sugar content, birch sap has historically been consumed fresh as a spring tonic beverage or fermented into mildly alcoholic drinks, rather than boiled down into syrup. In recent years, birch water has been marketed as a health drink in western Europe and North America, though the claimed benefits beyond basic hydration and a small mineral boost remain largely unproven.

From Resin to Amber Over Millions of Years

The terpene-rich resin that oozes from conifers and some flowering trees can, under the right geological conditions, become amber. The process takes millions of years and involves the volatile compounds evaporating and the remaining diterpene molecules linking together into an increasingly rigid polymer network. Experimental maturation of pine resin in sediment shows a decrease in the carbonyl and carboxyl chemical groups associated with free diterpene monomers, as those monomers get incorporated into the growing macromolecular structure.²⁰ The intermediate stage between fresh resin and true amber is called copal, which is harder than resin but hasn’t fully polymerized. The chemical differences between amber from different geographic regions trace back to differences in the original resin chemistry of the source trees.²¹

Amber’s fame as a fossil preservation medium comes from exactly the properties that make resin good at defense: it flows into wounds quickly, hardens, is antimicrobial, and seals out oxygen. Insects, spiders, feathers, and even small lizards trapped in ancient resin flows have been preserved in extraordinary detail for tens of millions of years. The chemistry of the resin effectively pickles whatever it engulfs while the surrounding matrix slowly transforms from a sticky liquid into a gemstone.

How Old Are Sap Transport Systems

The machinery trees use to move sap is ancient. Fossil evidence places the earliest specialized transporting tissues in land plants at least 440 million years ago, during the early Silurian period. These earliest systems consisted of simple fluid-conducting cells similar to those found in modern mosses and liverworts, and lacked the more advanced water-conducting cells that characterize today’s trees.²² The evolution of xylem, with its dead, rigid, lignin-reinforced cells capable of carrying water under tension, was a major innovation that allowed plants to grow tall. The gene responsible for making lignin, the tough polymer that gives wood its strength and waterproofs xylem vessels, appears to have entered land plants through horizontal gene transfer from a soil microbe rather than evolving from scratch.²³

The phloem system for transporting sugars evolved separately from xylem, and early Devonian fossils show little evidence of it. How the earliest vascular plants moved their photosynthetic products without phloem remains an open question. The idea that sugar transport through phloem is driven by osmotically generated pressure differences between source and sink tissues was proposed by the German forestry scientist Ernst Münch in the 1920s and remains the dominant framework, though both components of his hypothesis, mass flow and osmotic pressure drive, were already established ideas at the time that Münch synthesized into a single model.²⁴ Nearly a century later, the basic physics of the Münch model holds up well, though the molecular details of how living phloem cells load and unload sugars continue to be refined.