The percentage of water in a living, actively growing tree is highly variable, often ranging between 50% and 75% of its total weight. In wood science, this amount is expressed as moisture content (MC). MC is calculated by dividing the weight of the water in a sample by the weight of the same wood after it has been fully dried. Because fresh wood can hold more water than the dry wood weighs, this calculation can result in a moisture content percentage that exceeds 100%, particularly in the outer layers of the trunk.
Factors Causing Water Content Variation
The water content within a tree changes based on biological and environmental factors. The most significant variation occurs between sapwood and heartwood. Sapwood, the lighter-colored, outer layer, is the tree’s active transport system, resulting in very high moisture levels that sometimes exceed 100%.
Heartwood, the dense, darker core, is composed of dead, non-functional cells filled with resins and extractives, which reduces its capacity to hold water. Heartwood typically maintains a much lower moisture content. The overall water content is influenced by the ratio of high-moisture sapwood to lower-moisture heartwood, a ratio that increases as the tree matures.
Seasonal changes also play a role, with water content peaking in the spring during the active growth period when sap flow is high. Conversely, water content drops during the dormant winter months or in response to dry summer conditions. Different tree species also have different moisture profiles; for example, many softwoods (conifers) tend to have a higher overall water content than many hardwoods (deciduous species) when first harvested.
Essential Functions of Water in Tree Life
Water is maintained at high levels because it is the medium for all internal functions necessary for growth. One primary role is nutrient transport, achieved through the transpiration stream. As water evaporates from the leaves through tiny pores called stomata, it creates a negative pressure that pulls an unbroken column of water up the xylem from the roots.
This continuous upward current, known as the cohesion-tension theory, carries dissolved mineral nutrients from the soil to all parts of the tree. Water also acts as a necessary reactant in photosynthesis, where it is split in the presence of sunlight. This reaction converts carbon dioxide and water into glucose (sugar) for energy and releases oxygen as a byproduct.
Water provides structural support through hydrostatic pressure, known as turgor. The influx of water into plant cells creates an outward pressure against the rigid cell walls. This internal pressure keeps non-woody tissues like leaves and small branches firm and upright, and drives the cellular expansion required for growth.
Why Moisture Level Matters for Wood
Once a tree is harvested, the wood’s moisture level affects its processing and durability. Wood with high moisture content, often called “green wood,” is significantly heavier, increasing the cost and difficulty of transportation and handling. The presence of water also directly affects the wood’s density and strength properties.
As wood dries, it begins to shrink, which can lead to warping, checking, and cracking if the drying process is not carefully controlled. Seasoning, or controlled drying, is necessary to reduce the moisture content to a level in equilibrium with the surrounding environment. This typically involves bringing the wood down to a range of 6% to 12% moisture content for interior applications.
The moisture level is also directly related to the wood’s susceptibility to biological decay. Fungi and insects that cause rot and deterioration require a moisture content above 20% to thrive. Drying the wood below this threshold is a primary method of preservation. Furthermore, the high water content of green wood makes it highly resistant to ignition, which is why dried wood is far more effective as a fuel source.