The pic in Jim's post is from the USFPL, it belongs to us, so no copyright problems, it appears in several of their publications and many others since.
The juvenile core of a tree contains some wonky wood that has several types of problems, we've discussed off axis microfibril angles and compression wood. This wood is weak, and shrinks longitudinally and has erratic extractive deposition. The juvenile core varies but is generally the first 12-25 years of growth. If the juvenile core is boxed within a timber then that low quality wood is in the neutral axis of the timber where stresses tend to be low...think about shear though.
In some of my fast growing white pine and in many SYP trees I've seen, that juvenile core could be on the faces of a large timber. This has been a problem with the "supertrees" but is also a function of cultural practices. There's a couple of ways of looking at it. If a tree puts on a bunch of growth fast and then as it matures it is putting on quality wood over a larger core then the board footage gain in the stand of high quality wood is maximized. Unfortunately we tend to harvest when the trees hit a merchantable size. That has been the case in the US lumber market so we end up with an abnormally high percentage of juvenile wood and we don't wait for the tree to put on that good wood. Hence the "In Grade" process that readjusted design values in the '90's, our resource was changing. If the trees grow slowly then they have a smaller juvenile core but are many more years to harvest. Juvenile wood typically causes bowing towards the heart upon drying.
In nature there are many exceptions, we've discussed ring porous wood being denser and stronger if fast grown. Dense woods as a rule move more too. Wood shrinks, air (relatively speaking) doesn't.
The growth stress that is usually released at or soon after sawing is in large part the tension that keeps a tree standing upright and varies by species and by tree. Locust, hickory and poplar can have amazing growth stress release on the mill. The release of this stress causes bowing away from the heart.
Drying stress;
Notice the heart centered cut in the diagram in Jim's post. It is thickest in the middle tapering towards the edges. Radial shrinkage is roughly half of tangential shrinkage. Notice the quartersawn piece at the 9 o'clock position has changed little in width but a good deal more in thickness. Quartersawn flooring is the most stable as far as opening and closing gaps seasonally for this reason but needs to be sawn thicker than normal or you lose pieces to skip dressing when planing it, it loses twice as much thickness during drying compared to flat sawn stock. The difference in radial and tangential shrinkage is what causes drying stress.
Think about what is going on there for a moment. I think most folks understand the vertical transport through cells up and down the axis of the tree. The rays that radiate, radially, from pith to bark are cells running horizontally through and across the vertical cells take care of horizontal transport. They are ported to the vertical cells by "pits", donuts of microfibrils swirl around the pits to form the aperature. These valves between the cells on the radial sides of the cells and the cross grain of the rays are what inhibit shrinkage in the radial direction where on the tangential faces, those facing the heart and bark have microfibrils oriented vertically in mature cells. So the tangential faces are pretty free to shrink where the radial sides are kind of locked up by the pits and the cross grain rays. Make concentric circles of people around a campfire, pass boards under their armpits. They can snug up sloser together shoulder to shoulder but cannot move towards the fire so easily. When the tangential shrinkage exceeds the tensile strength of the wood perp to grain a check forms. This is pretty much a guaranteed condition if the heart is in the piece.
If you can get a tree large enough to not only get you out of the heart but get you out of the juvenile core, If the tree has no reaction wood, If the tree has low growth stress and straight grain, and if you orient the cut well, pay attention to the branches and knot orientation too. THEN, a FOHC timber is a far superior timber to a boxed heart one, it is straighter, stronger and less likely to check. That is a tall order from my forest.
In the end every one is an individual, some are good, some are bad, some have a compact sound heart, some are juvenile all the way through, some are twisted and headed for the fire
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