Thursday, April 7, 2016

Why we call it sapwood

   Last week, we removed a couple of large native pecan trees that were shading the growth of some young trees in an adjacent planting. After cutting the trees, we left the stumps in place for a couple of days while we hauled off the brush. One day after cutting the trees down, you could see large amounts of tree sap flowing from the outer growth rings of the trunk (photo above). In every case, the sap flow was limited to the outer 4 to 5 growth rings. The vast majority of the wood in the tree stump remained relatively dry.
    This high flow of sap from the outer-most layers of wood is the origin of the term "sapwood". However, the term sapwood means something entirely different to those that cut pecan trees for lumber. The sapwood refers to the lighter, almost white colored wood on the outer portion of the trunk. The heartwood of pecan trees is found in the center of the tree and is typically brown in color. You can see darker colored heartwood at the far right in the photo above.

     Next time you cut down a pecan tree, look to see if you can identify the different tissues that make up a tree's trunk. In the photo above, I've labeled the important tissues.  Starting on the left you can see several annual rings of wood tissue. This photo was taken just moments after the tree was felled and you can already see wet spots developing around the largest pores in the outer-most growth rings. Botanically, the wood of a pecan tree is xylem tissue which is responsible for conducting water from the roots upwards to the leaves. As wood tissue ages, becoming increasingly buried under new layers of wood (annual growth rings), the pores in the wood becomes clogged with lignin. Lignin blocks water flow but increases wood strength and rot resistance.
    Outside the wood is a narrow band of cells called the cambium (red arrow above). Every spring these cells become active creating new wood cells on one side and new bark cells on the other. The activity of the cambium layer is responsible for the tree's annual increase in diameter.
   On a mature pecan tree, you will find two distinct layers of bark. The inner bark or phloem functions to transport carbohydrates from the leaves downward to all other portions of the tree (branches, trunk, roots). The outer bark provides protection for the living tissues underneath. The outer bark acts as a vapor barrier to prevent moisture loss and as insulation against heat and cold.