Each year the ancient woodland at Heartwood Trees is engulfed by a beautiful carpet of blue. Bluebells are protected by law but sadly trampling occurs and they can be easily damaged.
When a cross-section of an old tree is cut it can be seen that there are two different sections of growth rings – the alive lighter color outer rings are Sapwood and the dead darker color inner rings make up the Heartwood.
A tree’s heartwood is the dead, dark inner wood that eventually makes up most of a stem cross-section. It carries no physiological functions other than mechanical support but is very strong and resistant to decay. Heartwood cells die and chemical compounds, including resins, terpenes, and phenols, accumulate in their place.
As a result, the cells lose their ability to conduct water or store energy reserves. The resulting pores fill with organic matter and chemicals, generically called extractives, which change the color and other properties of the wood.
The transition from sapwood to heartwood can range in intensity from gradual (as in the case of catalpa) to very sharp, as in eastern white cedar. One study found that the proportion of heartwood is significantly correlated with tree diameter at breast height (DBH), suggesting that this characteristic can be used in forest inventory to select trees with good durability and disease resistance. In addition, this study showed a positive correlation between the number of heartwood rings and tree age.
Some species of wood form a wide band of sapwood with small central bands of heartwood. The color of the sapwood varies but is not clearly demarcated from the heartwood, which can be a straw-like or yew-like brown color. The heartwood is also characterized by narrow streaks of resin ducts that darken with age.
On the other hand, some species have a distinct difference in color between the heartwood and the sapwood, which provides a visual effect for furnituremakers. Examples include walnut, cherry, and ziricote.
The color of the heartwood varies from species to species, but it can range from light yellow or pale brown to a yew-like brown. The grain patterning of the heartwood varies and may be straight or figured in some species. Tamo ash, for example, has a unique peanut figure that is anecdotally believed to be caused by the tree being surrounded by vines when it was young. This restricted the flow of nutrients and caused the tree to grow in an uneven stop-and-go fashion.
During heartwood formation, parenchyma cells in the wood die, leaving dead cell walls that are darker in color. As a result, the heartwood is darker in color than the sapwood. In addition, the dead cell walls become coated with minerals and chemicals called extractives. These chemicals make the heartwood more durable against fungus and insects and give it its rich character.
Because sapwood is alive it contains a higher amount of moisture than heartwood. This can be advantageous for the tree, but it is not good for woodworkers. High moisture content can lead to shrinkage and warping of lumber and can also inhibit chemical treatments.
As a result, most hardwood species with heartwood include sapwood to help with their durability. Maple and birch are exceptions because they begin forming their heartwood before their sapwood. This results in a thinner band of live sapwood than other species.
Heartwood is very durable and resists rot and insect attack. This makes it perfect for woodworking projects like furniture, cabinetry, and flooring. It can also be used to make shingles and other roofing materials. In fact, a piece of heartwood 12″ long and 1″ by 2″ in the cross section set vertically can support twenty tons!
However, it is not as durable against termite attacks and fungal decay. Laboratory tests with basidiomycete fungi in artificial, sterile conditions can’t adequately recreate the natural interactions between a tree’s microorganisms.
We still don’t know how or why heartwood forms, but theories suggest it may be due to lack of oxygen, excess sugars or food production, physical blockage of passage ways, and even bacterial infection. Many Maple and Birch species develop “false” heartwood around any wounds the trees may have had. This can darken the wood giving it unique, beautiful characteristics like zebra stripes or cloudy patterns. In addition, it can make the wood stronger and denser than sapwood.