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Central South University of Forestry & Technology Key National Discipline on Wood Science and Technology First-class Course in Hunan Province Wood Science Professor WU Yiqiang Chapter 8 Variability of Wood Within a Species (木材株内变异) Chapter 8 Variability of Wood Within a Species(同种树 木的变异性) 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 teee are never identical and are similar only within broad limits. All dimensional and physical characteristics of wood within trees are varible;i.e.,they exhibit a range od values .The variations which are present can be related to radical or axial position of the sample either within an individual growth increment or over a series of growth increments within the tree. ⅠSummation of Principal Factors Affecting Variavility of Wood Properties in Individaul Trees(影响单株树木 木材性质变异的主要因子) A.Variability in Cellular Characteristics (细胞特性的变异) The curves representing variation in cell dimensions from pith to bark show greater consistency than those for any other wood characteristic within trees. The general shape of these curves shows minimum values at or close to the pith(髓心）.The magnitude of the cell dimensions increases in the early years near the pith during the period in which most of the cambial maturation(形成层) occurs .As the cambium becomes physiologically mature,the slopes of the curves relating cell dimensions to the age of the cambium become less steep or may show no change.In old age ths slopes of the curves may decline when the cambium again produces cells with reduced dimensions. This general type of pith-to – bark(从髓心到树皮) variation is particularly evident for those axial elements which undergo postcambial elongation .Where postcambial elongation is considerable,as in softwood tracheids and hardwood fibers ,the curves exhibit steep slopes near the pith(Fig7- 5).Limited postcambial elongation ,as in vessel elements ,is reflected in the relatively low slopes of the curves near the pith. Tracheid diameters（管 胞直径） and cell wall thickness in the softwoods increase from pith to bark but with much less change than shown for tracheid lengths（管胞长度） (Fig.7-8)In the hardwoods the patterns of change in fiber diameters and wall thickness are generally similar to those in softwoods but are less consistent. Tracheid and fiber lengths within growth increments increases from the group upward reaching a maximun below the crown.Length of tracheids and fibers then decreases toward the top of the tree to a minimunm typical of the newly formed cambium in the crown region.The maxium tracheids or fiber length in trunk occurs in the outermost layers of wood at a point ranging from one-third to one-half the total tree height above the ground,as is apparent in Fig.7-6 for softwoods and in Fig7-10 for hardwoods. Within individual increments（年轮） the general pattern is that of a regular change in the lengths of softwood tracheids(管胞) ,hardwood fibers纤维) ,and vessel elements(导管) in the radial direction from early to late wood.The minimum lengths are found in the early wood,and the maximum is reached in the late wood. B Variability in Specific Gravity and Related Properties(密度及有关性质的变异 (1 )The radial direction （横向） Average specific gravity in the majority of softwoods shows a general increase from pith to bark.This is related to the increasing percentage of dense latewood in successive increments from the pith outward. In the diffuse-porous(散孔材) hardwoods about two-thirds of the species studied exhibit increasing specific gravity from the pith outward,which appears to be related to fiber characteristics.In the ring- (环孔材)and semi-ring –porous(半环孔材) hardwoods decreasesing specific gravity from pith to bark can be related to increasing early-wood percentage in the same direction in about half of the species studied. (2) The longitudinal direction (树轴方向) Average specific gravity in the majority of the softwoods decrease from the ground upward. Variation of specific gravity with height within the tree in hardwoods follows by one of the three patterns:it may decreases upward;it may decrease in the lower trunk and increase in th upper trunk;or it may increase from the ground upward. C.Juvenile and Mature Wood(幼龄材与成熟材) The stem of the tree may be devided into two regions on the basis fundamenttal differences in the structure and properties of wood.Juvenile wood （幼龄材）forms in a cylindrical column acout the pith as the result of prolonged influence of the apical meristems in the region of the active crown on wood formation by cambium .As the tree crown moves father upward in a growing tree,the influence of the apical meristems on a given cambial region decreases and mature wood（成熟材） is formed. Juvenile and Mature Wood must be considered as two distinct populations in the same tree.The mature wood possesses characteristics which are considered normal for the speciec,whicle juvenile wood has structural characteristics and physical properties inferior to those in the mature wood of the same tree.This is evident for fiber and tracheid lengths in Figs7-5 and 7-10 It should be noted that the primary basis for defining juvenile is the cell structure and properties of the wood .Juvenile wood in plantation-grown (人工林) treescan be related to fast growth near the pith,but wide rings are not necessarily associated with juvenile wood in all trees. ⅡVariabtlity in Wood Properties among Trees of the Same Species(同树种木株间材性的变异) 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 moditied as the result of long-term growing conditions existing on different sites ,even in rather small areas.The different conditions on different sites ,even in rather small areas .The differences may also arise from inherent characteristics transmitted from parent trees. Environmental and Geographical Influenced on Variability among Trees(环境及地理因素引起株间变异) Growing conditions 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 are domimant 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.Latitude and altitude ,in an unspecified interaction of mearn temperature and available moisture ,have also been shown to have major deeects on variability in wood propertied within species. Ⅲ Variability of Wood Structuere Within Tree(单 株内木材构造的变异) A.Variation of Cell Dimensions Stem Cross Sections(树干横切 面细胞尺寸的变异). 1.Cellular Variability Within Growth Increments(生长轮内细胞 的变异) a Length of Softwood Tracheids and Hardwood Fibers (针叶树 材管胞内的变异) These are at a minimum within the early-wood zone .They increase to a maximum length within the late wood,or near the transition from early to late wood,and finally decrease at the end of the year’s growth. b.Vessel Element Lengths(导管分子的长度) These vary across the width of growth increments from early early to late wood in two characteristic patterns. In the ring-porous woods a plot of vessel element lengths shows an approximately parabolic curve of change in length across the growth ring.The diagram in Fig7-2 for Fraxinuz pennsylvanica Marsh（北美白蜡树） .indicates short vessel elements in the early-wood zone but minimum lengths near the middle of the growth increment. In the diffuse-porous woods available data indicate that there is only a slight increase in length of vessel elements across the width of the growth increment (Fig7-3). 3.Microfiberillar Angles in the Cell Walls (细胞壁的微纤丝)＼ Microfiberillar Angles in the Cell Walls of softwood tracheids and hardwood fibers are inversely related to the lengths of the fibous cells.In consequence,microfibrillar angles vary across the growth increment from a maxmiunm in the early wood to a minimum in the late wood .Fi.g7-4 2.Variability in Wood Structure among Growth Increment (生长 轮间木材构造的变异性) a.Cell Length Variation in Stem Cross Sections(树干横切面上细 胞长度的变化)The basic curve forms across a radius from pith tio bark for softwood tracheid lengths and hardwood fibers ,both of which exhibit appreciable postcambial elongation ,are shown in Fig7-5 b.Cell Diameter Variation from Pith toBark(从髓到树皮细胞直 径的变化) In softwoods the tracheid cross-section dimensions increase systematically from the pith outward .The rate of increase is considerably less than that for legtn increases in the tracheids(Fig7-8) B.Dimensional Changes in Cells along the Stem Length(沿树干 方向细胞的尺寸变化) 1.Variation in Length of Cells(细胞长度的变异)The maximum length of fibers ,and the stem height at which they occur in a growth increment,increases regularly from the first up to the thirtieth growth increment in the hardwood trees shown in Fig7- 10. ２Diameter Variations of Along the Stem (沿树干高度细胞直径 的变化) Tracheid diameters within growth increments of softwoods increase in the lower trunk and then decrease toward the top in a pattern resembling that for tracheids-length variation along the stem axis. ⅣVariations in Chemical Composition of Cell Walls 细胞壁化 学组成的变异) A .Polysaccharides (多糖类) 1.Cellulose Content (纤维素含量) Cellulose content of cell walls within growth increments has received veryattention,and most of the studies have been made on softwoods .The most complere report of intra-increment variation is for Fseudotsuga menbziesii(Mirb)(北美黄杉)Franco(Fig7-12) ２．Hemicelluloses （半纤维素） Within a growth increment of Pinus resinosa Ait.(北美赤 松)analysis of the simple sugars has shown that xylose in the early wood appears to be 1 or 2 percent higher than in the late- wood portion;the percentages are reserved for mannose. B.Lignin (木质素) 1.Variation of Ligin content within Crowth Increments (年轮中 木质素含量的变异) 2.Lingin-content Variation within the Tree (单株树木木质素含量的变异) In softwoods the lignin content shows a general decrease of 1.5 to 3 percent from the pith to the bark. C.Extractives (抽提物) Variations in extractives within the stem are principally associated with their presence in heartwood,except for resins.Resin content in softwoods is reported tio the highest near the pith at the butt of the tree ,decreasing outward and upward in the stem.
"Variability of Wood Within Species"