Softwood growth rings

Within a growth ring, softwood interfiber pits are larger and more abundant in earlywood. In latewood they are fewer, smaller, and often appear slitlike in very thick-walled fibers (2). This same type of pit in hardwood fibers varies morphologically with the fiber type, changing from an obviously bordered pit in thin-walled cells to only a slitlike aperture in fibers with thick walls. 

Fast grown softwood from plantation sources is generally characterized by a high proportion of juvenile wood and often poorly developed heartwood. A fast rate of growth results in wide growth rings, producing low-density timber that exhibits inferior mechanical... 

Figure 2. End view of a softwood showing growth rings. Each growth ring consists of a light and dark area. The light portion is called springwood or earlywood, and the dark area is termed summerwood or latewood. 15y ...

Figure 3. Softwood block showing three complete and part of two other growth rings in the cross-sectional plane lX). Individual springwood cells can be detected, whereas the smaller summer-wood cells cannot be seen as individual cells. Also note the absence of vessels and the uniformity of the wood. Two longitudinal surfaces (R—radial T— tangential) are illustrated. Food-storing cells can be easily detected on the radial surface (arrow). 47 X (Courtesy of N. C. Brown Center for Ultrastructural Studies, S.U.N.Y. College of Environmental Science and Forestry)...

Figure 1.3. The softwood Thuja plicata. Tracheids function in both sap conduction and stem support in softwoods. A growth ring is clearly visible with the thicker-walled latewood tracheids to the top left, and the thinner-walled and larger lumened early-wood tracheids to the lower right, x 275.

In softwoods leaning stems and branches are usually characterised by an eccentricity in cross-section, with the wood to the lower side having significantly wider growth rings than normal and consequently the pith is off-centre (Figure... 

As with softwoods, the stems and branches in hardwoods that are subject to asymmetric loading produce wood that is characterized by different anatomical, chemical and physical properties. Thus tension wood is quite common in vertical stems, especially below the point of attachment of lateral branches. However, more usually it is associated with inclined stems and branches, the upper side shows a marked eccentricity with wider growth rings. This is the opposite situation from softwoods where the eccentricity and modified wood lies to the lower side of the inclined stem or branch. The modified wood in the eccentric growth in hardwood is referred to as tension wood on account of its position, and like the compression wood of softwoods, it develops in response to stress and acts to straighten the leaning stem. Tension wood is usually harder and denser than normal wood and is sometimes darker in colour. In sawn timber it shows up as having a woolly appearance. 

Until recently, few have recognized the enormous impact that the microfibril angle has on wood properties, especially in the corewood of plantation softwoods. In particular the microfibril angle strongly determines the stability and stiffhesss of wood within the first 10-15 growth rings of the pith (Barber and Meylan, 1964 Cave, 1968 Preston, 1974). 

Figure 6.20. Hardwoods are favoured for sawlog production in very short rotation plantations (< 10-15 yrs). The high density in the innermost rings may increase the ability of a tree to absorb energy one might speculate that the first 10-15 yrs for second-growth Canadian softwoods might be compressed into only 1-3 yrs for the fastest growing plantation species. See Yang et al. (1994) Evans et al. (2000) Jozsa and Middleton (1994).

In softwoods, the tracheids have considerable pitting in their radial walls. These bordered pits are specialized valves to seal and isolate tracheids if they become damaged or emboli-zed, but remain open for sap flow. At the pit location, the double cell wall takes the shape of the external part of a torus. The external diameter of this torus is in the range 10-20 jm, depending on the position in the annual growth ring (the diameter is smaller in latewood). The torus is suspended by... 

Pitch pocket n. An opening between the growth rings containing resin in certain softwoods. Syn resin pocket, pitch streak. 

Growth increments reach their most advanced form in temperate-zone trees where the norm is a single, generally distinct, and circumferential band of wood production each year (Figure 6). The appearance of these so-called annual rings varies between hardwoods and softwoods with species, tree age, and growing conditions. These factors, together with certain other environmental effects, can also... 

The situation is more complex with hardwoods. The growth rate has little effect on the wood properties of diffuse-porous hardwoods, but has a marked impact on the density of ring-porous hardwoods. Unlike softwoods, these produce denser wood when fast grown. 

Wood has a well-defined cellular structure. In softwoods and ring-porous hardwoods the cells produced in the early part of the growing season, the earlywood, are larger than those produced later in the season, the latewood. In diffuse-porous hardwoods the cells are more uniform in size. These differences in cell structure are responsible for the variation in density observed among wood of different species and even in wood of the same species, depending on local growth conditions." ... 

Wood is structurally composed of fiber shaped cells. The surface texture varies from species to species, hardwoods to softwoods. In addition to grain orientation that results from the fibrous nature of wood, different types of wood occur within a tree. Juvenile wood is formed during the first years of a tree s life and is composed of relatively short fibers. The heartwood of a tree differs chemically from the sapwood. Annual growth consists of alternating rings of springwood and summerwood, each differing in density and porosity. All these factors account for differences in texture as well as permeability, density, and compressibility. 

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