Chapter 11 Variability of wood within species 木材种内变异性 Wood produced by trees of the same species is often mistakenly assumed to be identical in all structural and physical characteristics. In fact, different pieces of wood even from the same tree are never identical and are similar only within broad limits. These measurable variable in wood are influenced by several factors: Changes in the cambium as its ages; Genetic controls that govern the form and growth of the tree; Environmental influences, such as seasonal or geographical conditions, or nutrient supply.

Topical highlights Ⅰ. Summation of principal factors affecting variability of wood properties in individual trees （株内变异） Ⅱ. Variability in wood properties among trees of the same species （种内株间变异）

Ⅰ. Summation of principal factors affecting variability of wood properties in individual trees 1. Variability in cellular characteristics All dimensional and physical characteristics of wood within trees are variable, i.e., they exhibit a range of values. The variations can be related to radial or axial position of the sample within the tree, as shown in the following topographic map of fiber length.

Variation pattern in radial direction Variation pattern in height within a increment Distribution of fiber length in the tree trunk

2. Variability in specific gravity and related properties Softwood Average specific gravity in the majority of the softwoods increases from the pith to the bark and decreases from the ground upward. Hardwood In the hardwoods a little over half of all the species investigated shows increasing specific gravity from the pith outward. Variation of specific gravity with height within the tree follows one of three general patterns: — decrease upward; — decrease in the lower trunk and increase in the upper trunk; — increase from the ground upward.

3. Juvenile and mature wood 幼龄材与成熟材 The stem of the tree may be divided into two regions on the basis of fundamental differences in the structure and properties of wood ， i.e., juvenile wood and mature wood. 3-1 Definition and conception of juvenile wood — Juvenile wood has been defined as secondary xylem produced by cambial regions near the growing point, which are influenced by activity in the apical meristem. — Juvenile wood exists not only in young trees, but also in mature trees, even in over-grown trees. 3-2 Location of juvenile wood Juvenile wood forms in a cylindrical column about the pith.

3-3 Formation of juvenile wood Juvenile wood forms as the result of prolonged influence of the apical meristem in the region of the active crown on wood formation by the cambium. As the tree crown moves farther upward in a growing tree, the influence of the apical meristem on a given cambial region decreases and finally mature wood is formed. 3-4 Demarkation between juvenile and mature wood There is no sharp demarcation between juvenile and mature wood. Instead a gradual transition in properties occurs from the tree center outward. Because of the gradual change in wood properties, it is unclear as to where juvenile wood ends and adult wood begins. The location of this boundary depends upon the property or properties used to define the zone. Generally, researcher use fiber length as the indicating index, as shown in the following figure.

3-5 Main differences from mature wood Cells are shorter. Cell walls are thinner. Wood density is lower. Wood strength is lower. Microfibril angle is greater. There appears to be a greater tendency for spiral grain. Shrinkage is greater along the grain and smaller across the grain.

Ⅱ. Variability in wood properties among trees of the same species Variability in properties of normal wood among trees of the same species may be greater or smaller than that within individual trees being compared. The normal-wood characteristics may be modified as the result of long-term growing conditions existing on different sites, even in rather small areas. The differences may also arise from inherent characteristics transmitted from parent trees.

1. Environmental and geographical influences on variability among trees 环境和地理因素对株间变异的影响 Growing condition such as competition within the stand or soil fertility can cause major differences in the magnitude and patterns of variability in wood properties in trees of the same species growing in the same stand. For example, trees which are dominant may show one pattern of variability in wood, and neighboring trees of the same species that are suppressed may have quite different characteristics. The same is true for trees, which are open-grown or forest-grown. Geographical location, which is in reality a reflection of mean temperature and rainfall differences, may be a cause for variation between trees in a species.

2. Modifications of variability in wood characteristics 材性变异的改良 The possibilities for modifications and control of wood characteristics have been recognized for some time in the practice of forestry. However, most of the forest management practices in the past have been directed toward improving the rate of diameter and height growth in trees, without much regard to the quality of the wood. Such silviculture practices are an important means of increasing wood production, and they can be applied to existing stands of timber. In softwoods some of these manipulations have also resulted in incidental modifications of specific gravity and cell dimensions.

2-1 Silviculture treatment / 营林措施 In general, the treatments which have been shown to have major effects on some wood properties are either the of followings. To manipulate the spacing of trees To control the knot-free length of the trunk To stimulate the growth by supplying the needed nutrients or water.

2-2 Tree-improvement programs 林木改良项目 Silvicultural treatment of stands of uncontrolled seed origin may result in some improvement of wood qualities associated with the rate of growth. However, silvicultural treatments cannot yield any modification of the trees’ cellular characteristics that can be transmitted to their progeny. This can only be achieved by selection of superior seed sources or by breeding for improvement in the cellular characteristics, growth rate or form of trees. Mean specific gravity of wood is a property, which can be increased. Such increase, even if they are apparently small, can result in significant changes in wood production. For instance, an average increase of only 0.02 in specific gravity of the wood of southern hard pines increases the modulus of rupture of the wood about 1000 psi and the dry weight of pulpwood by 100 pounds or more per cord.

Reflection and practice: 1.Definition of juvenile wood? 2.Formation of juvenile wood? 3.Location of juvenile wood? 4.The main differences between juvenile wood and mature wood? 5.How to demark the juvenile wood region?