Читать книгу Draw Manga - Tom Carpenter - Страница 19

At the very center of a tree is a small area of softer tissue called pith. Surrounding the pith are numerous annual rings of growth, already dead, that provide support and structure to the tree. This is the heartwood, the area most treasured by woodworkers because of its even density and grain pattern. Beyond the heartwood is a thinner section of still-living rings, called sapwood, that provide a conduit from the roots to the leaves for transporting soluble mineral salts. The outermost sapwood ring—the cambium—is the growth region in a living tree. Cambium contributes girth to the trunk over time, adding another new layer of sapwood each year. Between the cambium and the protective layer of bark is yet another thin region called the phloem. This is the conduit that brings food (made in the leaves through photosynthesis) back down to the root system.

Most mills remove all the exterior layers (bark, phloem, cambium and sapwood) from logs before milling them into boards or dimensional stock such as 2 x 4s. Today, most of the bark and sapwood is ground up and used as mulch in gardens, sold to paper mills, burned as fuel or even used as animal bedding. Occasionally you’ll run across a board at the lumberyard that contains sapwood. In darker-grained varieties, like walnut or cherry, sapwood appears as a band of lightly colored, softer wood that runs lengthwise near one long edge of the board. If incorporated into a project, sapwood will become more prominent when you apply a finish unless you stain it to match the rest of the board. For this reason, sapwood is seldom used for furniture.

This Chinese Elm sample exhibits all the major anatomical areas of a tree: bark, phloem, cambium, sapwood, heartwood and pith. The cross-section shown here, with two pith regions, likely came from a tree whose trunk split into two major branches. Notice also the darker area, a sign that the tree experienced a period of injury or disease.

FOREST MANAGEMENT

Softwoods (also called conifers) nearly always grow at a faster rate than hardwoods, and this fact helps explain how supply and demand influence prices of both softwood and hardwood lumber. The rapid growth rate of softwoods allows for frequent replanting and harvesting—sometimes in as little as 15 years—compared to a minimum of 75 years for most common hardwoods. Shorter harvest time helps to keep softwood quantities stable and costs below that of hardwoods—a benefit to both the construction industry and to softwood supplies for woodworking.

Because softwood trees reproduce with heavy cones rather than flowers or nuts, the seeds often fall close to the parent tree. This natural adaptation enables softwoods to grow close together—a fact that can be a boon to a lumber mill. The mill’s forester can plant more trees per acre. Each plant, seeking light above the canopy created by its siblings, will tend to grow straight and true. Loggers have long taken advantage of this growth pattern, replanting conifers in tightly spaced rows that yield easily milled, straight logs.

Softwood seedlings (right photo) are planted in tightly-spaced rows, then transplated in wider rows (top photo) until they mature into trees suitable for lumber harvest. Note the tall, straight trunks—a characteristic trait of softwood trees.