Softwoods: Characteristics and Uses

Softwoods

• Trees exhibit needle or scale-like foliage
• Trees have an 'excurrent' form with a dominant trunk and lateral branches
• Softwoods are distributed worldwide but are less common than hardwoods
• North America maintains a large standing inventory of softwoods
• Softwoods are foundational to the forest products industry in North America
• Softwoods are used in structural construction lumber because of their moderate density, straight grain, minimal warp when dried, and ease of nailing
• Softwoods are used in pulp and paper due to their long fiber cells
• They are also used in plywood and oriented strand board (OSB) industries

Cell Types in Conifer Wood

• The dominant cell is the longitudinal tracheid, making up 90-95% of the wood volume
• Longitudinal tracheids measure 3-4mm long, elongating 75-100 times their diameter, thus having a large aspect ratio
• These tracheids appear in radial rows or "ranks"
• Denser latewood results from smaller radial diameters (flattening of the cell) and thicker cell walls
• Rays are generally uniseriate, except when a ray contains a resin canal, which is termed a fusiform ray
• Resin canals are found in these 4 genera: Pinus (Pines), Picea (Spruces), Larix (Larches), and Pseudotsuga Douglas-fir

Earlywood/Latewood Transition

• Earlywood/Latewood can have either an abrupt or gradual transition
• Radial diameter varies with position in growth ring

Longitudinal Tracheids

• Longitudinal tracheids have similar shapes and proportions among softwood species
• They have conductive and support functions
• Bordered pit pairs connect longitudinal tracheids at radial walls
• Earlywood longitudinal tracheids contain 50-300 bordered pits per cell
• Latewood longitudinal tracheids contain 30-50 bordered pits per cell

Spiral Thickening in Longitudinal Tracheids

• Also known as "helical thickening"
• Smooth inner cell walls are typical
• Some species have spiral thickening on inner walls
• Spiral thickening looks like fine, helical ridges of microfibril bundles
• Spiral thickening is found in Douglas-fir and western yew
• Spiral thickening is a very useful diagnostic

Pitting in Softwoods

• Fluids are transported between cells through pits; softwoods have no vessels like hardwoods
• There are 3 types of pit pairs
• Bordered pits are the most common pit type in softwoods; the xylem is composed of longitudinal tracheids
• Appearance of half-bordered pit pairs between ray parenchyma and longitudinal tracheids is important for microscopic ID, called "crossfield pitting"

Bordered Pit Pair Structure - Conifers

• Earlywood longitudinal tracheids contain 50-300 bordered pits per cell
• Latewood longitudinal tracheids contain 30-50 bordered pits per cell

Bordered Pit Pair Aspiration

• Response to injury, drying, or conversion of sapwood to heartwood
• Fibers of the margo are flexible enough to distort in response to pressure
• A bubble moving into the pit pair moves the torus, causing it to contact the inside surfaces of the pit aperture and seal the pit pair
• Aspiration affects wood processing

Bordered Pit Pairs

• Most gymnosperms have a torus, whereas most hardwoods do not
• The primary purpose of bordered pit structure is to reduce the spread of gas bubbles (cavitation)
• Pit aspiration is associated with softwoods
• Small pit aperture size is associated with hardwoods

Longitudinal Parenchyma

• Also called strand or axial parenchyma
• Formed by fusiform cambial initials
• Appear as chains of short, vertically oriented cells, microscopically in a longitudinal section
• Appear as fine dark broken lines on longitudinal surfaces that can be seen with a hand lens
• These are often present in late wood, spars
• May have dark-colored contents
• Can compose as much as 1-2% of the wood volume in redwood (Sequoia spp.), cedars (Thuja spp.), and pines (Pinus spp.)
• Typically only visible with a hand lens in redwood and bald cypress

Resin Canals/Ducts

• They conduct resin secreted by specialized parenchyma cells, known as epithelial cells, lining the canal opening
• Seals wounds from insects or mechanical damage
• They occur oriented in the longitudinal and radial directions (within fusiform rays)

Resin Canals

• Present in Pinus spp., Larix spp., Picea spp., and Pseudotsuga menziesii
• Useful for wood ID
• May need magnification to see
• Appearance varies with the presence/absence of resin
• You can tell if a wood sample is from a species with resin canals if normal longitudinal resin canals and fusiform rays occur together!
• Their occurrence in pines is large and numerous (all growth rings and even distribution)
• They occur in spruces, larches, and Douglas-fir as smaller, more sporadic distributions which may not be in all rings and are commonly in tangential groups of two to several
• Pines have thin-walled epithelial cells that are easily damaged in cutting
• Spruces, larches, and Douglas-fir have thick-walled epithelial cells
• The resin canals act as a tree's defense system that female mountain pine beetles nibble a tree and assess resin amount and pressure before attacking trees
• Resin canals allow foresters a simple, quick method of thinning and enhancing stand resistance by removing more susceptible trees
• Number and size of resin canals are the best two predictors of tree survival
• Resistant trees have up to 55% more resin canals than those attacked and dead trees
• Trees with large resin canals are more likely to survive

Traumatic Resin Canals

• Traumatic resin canals may occur in species that do not normally have resin canals
• They are a response to injury
• Appear as a single, continuous line along a growth ring tangentially
• Have a different appearance from normal resin canals
• Almost always longitudinal

Conifer Rays

• Almost always uniseriate, with the exception of fusiform rays, which has a resin canal in it
• Non-fusiform rays are composed of ray parenchyma and/or ray tracheid cells and can be homogeneous (one cell type) or heterogeneous (two or more cell types)
• Ray tracheids are usually found in one or more rows at the top and bottom of a heterogenous ray

Heterogeneous Rays

• Heterogeneous rays consist of:
  • Ray Tracheids
  • Ray Parenchyma
  • Ray Tracheids
• Dentate ray tracheids occur in hard pines

Cross-Field Pitting

• Intersection of a longitudinal tracheid and a ray parenchyma cell produces a half-bordered pit
• On the radial plane, in conjunction with ray tracheids, we see distinctive pitting patterns types
• Types of cross field pitting include Fenestriform or windowlike, Pinoid, Piceoid and Cupressoid-Taxodioid

Fusiform Rays (Heterogenous)

• Horizontal resin canals
• Occur in all species that have longitudinal resin canals
• Contain a resin canal within a ray
• May contain three cell types: ray parenchyma, ray tracheids, and epithelial cells

Important Concepts

• Microanatomy of softwoods is relatively simple, consisting mostly of longitudinal tracheids with uniseriate rays
• Non-traumatic resin canals are unique features of species in four softwood genera
• The bordered pit pairs structure allows them to close (aspirate) in response to injury, and also happens when the wood is dried
• The microscopic appearance of the rays on the radial plane (cross-field) shows many species- or genus-specific features