How Sap Flow Works

​The study of cyclic changes in the natural world based largely on climate’s effects on plants and animals is called “phenology.” In northern Michigan, many of us welcome the cyclic change we observe this time of year as the days get longer and warmer. And perhaps nothing is more a harbinger of spring than sap flow in our sugar and red maples, when cold nights are followed by warm days. But how does the process of sap flow work and what can a private landowner do to promote vibrant trees from which sap can be collected and syrup made? “Physiology” refers to the mechanical and chemical workings of plants and animals. Sap flow in maple trees is a physiological process driven by phenological change. To understand how and why sap flows in a tree one must understand how a tree produces and stores its energy and how it transports water and other materials from its roots to its leaves. Most green plants, including trees, produce energy through “photosynthesis.” Plants are called “phototrophs” because they use light energy (photons) to combine the gas carbon dioxide and water to produce sugars. In this process, oxygen is released. Without green plants, animals would not have oxygen they need to live. And without the sun, plants would not be able to produce energy. While all green plants need sunlight, some tree species need more sunlight than others. We refer to tree species that require less sunlight as “shade tolerant” species, and those requiring more sunlight as “shade intolerant” species. All native maples, as well as American beech, ironwood, and a few other of our deciduous tree species, are shade tolerants. These species are able to produce energy with relatively little sunlight. This energy (in the form of sugars) produced by photosynthesis is used to meet the demands of tree growth and maintenance. When excess sugars are produced, they are stored in the tree roots. Conversely, other tree species, such as aspen and oaks, are shade intolerant and need more sunlight. These tree species rarely produce extra sugars and do not typically store much energy. Tree sap is primarily a mix of water and sugars. Tree sap move in tubes in a manner slightly similar to the way blood moves in our arteries and veins. The tubes in which maple sap typically flows is called “xylem” tissue. Dead xylem is the “heartwood” of a tree and living xylem is the “sapwood”. Most of the material used in making lumber from a tree is xylem tissue. In other tree species, sap can also flow in the other tubes (phloem). Syrup made from other tree species has a distinctive taste quite different than maple syrup. During the spring of each year, sap flows from the roots, where sugars are stored in the winter, to the crown of the tree. Sap flows because of the pressure in the roots is greater than the pressure in the crown. What can a landowner do to promote vigorous maple trees form which syrup can be made? Tree vigor is the primary consideration for syrup production. Vigorous trees with large, healthy crowns tend to make and store more energy and produce more sap. Site (soil type) and location, as well as tree genetics, are factors that influence tree vigor. On sites with well drained, upland soils and plenty of sun (often south facing slopes), maple trees can thrive and be vigorous, especially if competition with other trees is reduced. When provided more sunlight, a maple tree will respond by growing more leaves and a bigger crown. This is the type of tree that if not impacted by disease or physical harm can produce sap for decades. Thinning a forest to produce trees of 10-25 inches in diameter separated by 20-25 feet is a good starting point. Removing diseased trees or trees of low vigor can promote more diameter growth on the retained trees. Do not, however, reduce overall stand tree diversity. Forests with more tree diversity tend to be healthier overall, and a vibrant maple stand (sugarbush) produces more maple sap. To capture sap, the general recommendation is that trees less than 20 inches in diameter should have one tap, while trees over 25 inches may have up to 3 taps in a given year. Just like all types of forest management, managing a sugarbush depends on many factors and involves a great deal of art guided by science. In the end, an understanding of how a tree, and a forest, forms and functions provides a solid foundation for planning and management. Greg Corace is the forester for the Alpena-Montmorency Conservation District. For more information, including sources used in this article, Greg can be contacted via email (greg.corace@macd.org) or phone (989.356.3596 x102)