”Wood Defects” Knots, Spiral Grain, Juvenile Wood, and Reaction Wood FW1035 Lecture 6 Bowyer et al, Chapter 2, pp. 40-43, Chapter 6, pp. 109-129 ”Wood Defects” Knots, Spiral Grain, Juvenile Wood, and Reaction Wood

Grain Orientation in Wood Grain direction - direction of the long axis of longitudinal wood cells Spiral grain caused by anticlinal division with new primary cell wall formation in one direction only Interlocked grain genetically controlled common in elms may cause warping in lumber upon drying may exhibit nice figure in veneers. “ribbon figure”

Ribbon Figure Quarter-sawn lumber from tree stem with interlocked grain.

Knots are Branch Stubs Tight knots - incorporation of living branches into the stem knots are integral part of the surrounding wood = “intergrown knots” Loose knots – stem growth encases a dead branch may fall out upon lumber drying = “encased knots” “Grain” of wood deviates around knots – weak point

Juvenile Wood Short fibered xylem with high microfibril angles and low specific gravity. Wood produced during the first 5-15 years of growth As tree grows, the SG increases and the fibers lengthen. Gradual transition from juvenile wood to mature wood Caused by effects of hormones from apical meristems on cambium As cambium in stem becomes farther from and less influenced by the apical meristem, transition to mature wood

Physical Characteristics of Juvenile Wood that Affect its Use Cells are shorter than mature wood Thin cell walls and less latewood Leads to lower density and strength More spiral grain For Softwoods in Particular: Density 10-15% lower than mature wood Strength 15-50% lower than mature wood

Transition from Juvenile Wood to Mature Wood is Gradual! Not the same as transition from sapwood to heartwood, which is abrupt.

Juvenile Wood in Solid Wood Products Greater tendency for spiral grain Shrinkage often shows up to 10 times the longitudinal shrinkage of mature wood due to the greater S2 microfibril angle Trees with high juvenile wood content may yield only 20-50% as much high grade dimension lumber as older trees Sawmill loss may be reduced when planning specifically for cutting juvenile wood

Reaction Wood

Compression Wood and Tension Wood Reaction wood is called compression or tension wood in response to where it forms in a stem Softwoods form compression wood Hardwoods form tension wood to correct growth irregularity in a stem Reaction wood forms in branches of most trees Tension Compression

Reaction Wood in Softwoods and Hardwoods Compression Wood Softwoods Underside of branches or leaning stem Commonly in juvenile wood Appearance is similar in most species Tension Wood Hardwoods Top of branches or leaning stem Common in juvenile wood also Appearance and microanatomy is less consistent than for compression wood

General Appearance of Compression Wood Eccentric growth rings that appear to contain an abnormally large proportion of latewood in the widest portions Non-centrally located pith in stem Often darker color (red/brown)

Microanatomical Characteristics of Compression Wood Latewood longitudinal tracheids are most affected Rounded cross-section, rather than prismatic Intercellular spaces are present Greater cell wall thickness No S3 layer Larger S2 microfibril angle, ~45° Spiral cavities in S2 layer Longitudinal tracheids are 10-40% shorter Tips of tracheids are distorted

Effects of Compression Wood on Utilization Large longitudinal shrinkage (1-2%) causes warping and bending of boards upon drying Higher lignin content (average of 38% versus 29% in normal wood) gives lower pulp yields Lower strength properties than density would lead you to predict

General Appearance of Tension Wood Cut surfaces have “wooly” or fibrous appearance causes overheating and dulling of saws difficult to sand and finish

Appearance of Tension Wood in Stem

Microanatomy of Tension Wood Cell modifications are usually in the earlywood Most commonly affects fiber cells more numerous fibers, fewer rays, vessels, etc. secondary cell wall is significantly different

Microanatomical Differences Changes in the secondary cell wall almost entirely made of cellulose forms a floppy layer that is loosely attached to the primary wall gelatinous layer called the “G” layer

Appearance of the Gelatinous Layer

Utilization of Tension Wood Can produce good paper properties if pulping conditions are modified Good for “dissolving pulps” cellulose source for making cellophane, rayon, and nitrocellulose Solid wood products have lower quality tendency to ‘collapse’ upon drying higher longitudinal shrinkage 1-5x normal wood) may lead to warp and bending lower strength properties