BC Ministry of Forests - Research Branch - Forest Productivity Section
Online Presentations: SYLVER Demo/Juvenile vs Mature
JUVENILE WOOD: A CLOSER LOOK
Now let's take a closer look at juvenile wood,
and compare it with mature wood.
WHERE IS IT FORMED?
Juvenile wood is added within and just below the live crown.
Production gradually shifts to mature wood as the distance from the
centre of the crown increases.
As the crown lifts, a core of juvenile wood develops within a shell of
denser mature wood which is relatively strong, stiff and less likely to
warp when cut into boards.
HOW IS IT CHARACTERIZED?
Here relative density, an important component of wood quality,
characterizes the juvenile to mature wood transition at breast height.
Pith to bark changes in fiber length and fibril angle are similar except
that the pattern of the latter is reversed.
The density drops briefly, and then rises rapidly before crossing an
arbitrary boundary which defined mature wood in the previous figure.
Note that the average density of old-growth Douglas-fir (0 - 250 yrs+)
is high (0.54) relative to its juvenile wood component.
Now watch TASS simulate the development of juvenile and mature wood.
TASS: UNTREATED STAND
Again, we've planted 1100 stems/ha.
Notice that the spacing of the rows and trees is realistically
imperfect at age 10.
SYLVER can also display longitudinal and transverse sections
through any tree.
Juvenile wood (red) predominates (99.6%) in this 25-year-old tree
because the crown covers most of the stem.
Now focus on the crowns ...
As the trees grow, competition for light kills the lower branches and forces
the crown to lift leading to the production of high quality mature wood
(yellow) by age 40.
Now watch the amount of mature wood increase as the stand grows,
reaching 55% by age 100.
HOW DOES IT PERFORM
Its lower density produces weak boards in terms of strength & stiffness,
but does gives us a more balanced pulp, although the yield is lower.
Its larger fibril angle increases lumber shrinkage, twist and warp.
WEAKER BOARDS - IMPACT ON LUMBER STRENGTH
Boards with 100% juvenile wood are 30% weaker than mature wood boards.
GREATER LONGITUDINAL SHRINKAGE
To qualify for SS or #1 grade, boards must have less than 0.56" of twist
per 8 feet of length - which is not limiting in terms of mean twist.
However, variation among boards will degrade up to 20% of the lumber to #2.
MORE BALANCED KRAFT PULP
On the positive side, kraft pulp from juvenile wood is more balanced
because it has lower tear strength, and higher tensile & burst strengths.
Juvenile wood pulp is more like spruce pulp, and therefore is suitable
for a broader range of paper products.
HOW IS IT MANAGED? - IMPROVING LUMBER QUALITY
How can we manage our second-growth stands to reduce the amount of
juvenile wood in the future resource.
HARVEST LATER
We can harvest later.
Extending the rotation from 70 to 250 years decreases the juvenile wood
content of Douglas-fir from from 50% to 25% in this TASS simulation.
Note that a delay of only 30 years will increase quality substantially.
PLANT MORE TREES
We can plant more trees per hectare.
Increasing the planting density from 500 to 2500 trees improves quality,
but will a comprehensive financial analysis support this practice?
PRUNE EARLY
Pruning elevates the live crown and hastens the production of mature
wood as well as clear wood.
MACHINE GRADE - OPERATION
Machines can grade lumber with a high juvenile wood content more
effectively then the human eye - but first, let's look at visual grading.
VISUAL GRADING (Updated title)
Here visual grading of 66 M bd ft returns $17,200.
But the strength & stiffness standards assume we're grading old growth.
What's new in managed stands is the potential loss in value of visually
graded lumber that fails to meet the strength requirements of the grade
because of relatively high juvenile wood content.
LUMBER DEGRADE
Strength & stiffness data for Douglas-fir reveal that lumber with:
0-39% juvenile wood meets the requirements of all visual grades.
40-89% JW qualifies if graded select structural, but boards graded #1
and #2 should be downgraded to #3 to reflect their lower strength.
90-100% JW fails in all grades, and should be downgraded as shown.
IMPACT OF GRADING OPTION ON LUMBER VALUE
Here we see a SYLVER simulation of the potential impact of juvenile wood
on the value of lumber from plantations.
The top curve assumes plantation-grown and old-growth lumber are equal.
The lower curve reflects potential losses in value if the grade shifts
depicted in the previous slide are applied to visual graded lumber.
The middle curve shows the potential recovery if lumber is machine
graded.
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