I think it is Bruce Hoadley's Understanding Wood (2000) that suggests that, after an average number of years, a tree converts the innermost pair of annual sapwood growth rings to heartwood, and each year thereafter converts the innermost pair of annual sapwood growth rings to heartwood. On page 12 of his book is a chart of a few hardwoods showing for example Northern catalpa averages 1-2 sapwood rings, black cherry 10-12, black walnut 10-20, sugar maple 30-40, and black tupelo 80-100.
I've always guessed that as a tree grows the moisture carrying capacity of the sapwood increases at a greater rate than the moisture demand (leaf load) as an explanation for converting sapwood to heartwood, but that is just a guess.
Anyone know if wood scientists have a generally accepted explanation?
That is a great question Don, and I've been wondering and thinking about this for a lot of years now myself. Of course I'm not a scientist, so can only share some of my observations.
Some species like Eastern Red Cedar, Juniperus virginiana, can have a huge variation when they grow fast, they can have a wider band than one hanging off a cliff growing slowly. I happened to have found the oldest tree of this species back around 1985. After turning a few pieces from it, I counted ~500 rings, and contacted Dave Stahle, a PHD Dendrochronologist at UofA Fayetteville AR. He couldn't believe I'd found any that old, since he'd been studying, looking for the oldest for 20 years at that point. When I brought him a nicely polished cookie, we quickly became friends. He had a student needing a master's thesis, so turned my sample and location over to him, so I have a copy of that, but my sample was 150 years older than anything either had found within the state of AR.
One thing I quickly noticed in this wood were the woodpecker pecks. When I cut through the actual peck, I can make out the fractured wood V shape of the end of the Yellow-bellied Sapsucker beak. Around that zone the fiber will be totally saturated with the gum filled fiber which has solidified as the tree heals over. Behind that will always be a little patch of sapwood which never turned to heartwood, I suppose because the air and or any residue from the production of new wood in the cambium is blocked out from passing through down to where it would normally deposited in the edge of the sapwood - heartwood line.
Some years later I was ask to make a ceremonial Mace for UCA in Conway from the remains of a really old cedar from near the entry of one of the old halls which had been destroyed by a storm. Whomever saved me the wood, just picked up a big limb which had been dead on the top half for many years, while still alive on the lower half. I wasn't able to use much of it, so I just carved the torch flame from it, and used rosewood for the rest. Most of it ended up in my wood stove. But the last piece I picked up to throw in, something caught my eye. I noticed that at both edges where the living wood kinda hooks around as it grows, where AIR can permeate the dead wood, and get in behind the growing new wood, the new growth was going directly into heartwood, without any sapwood layer, which leads me to postulate that the thickness of the sapwood layer is related to the thickness of the bark.
It is not at all uncommon to find steps in the sapwood to heartwood transition, rather than one year of new growth adding one year of sapwood is just an approximation.
I like getting really large trees for my sawmill. Some have been from the National Cemetery not far from here. One big Post Oak which started growing ~ 1880, had been hit by lightning many times. The worse strike was in the early Summer of 1945, which was one of the hottest years on record for here. Every day for two months was over 100º F without a drop of rain in the whole state. I figured whenever the storm which ended this drought hit, there was so much energy stored in the hot earth that it must have been an incredible strike. One side had about a 4" x 4" section blown out of the side of the tree, with 12 other smaller scars all the way around that ring. It took about 7 years for the scar tissue to bridge that gap. When I got this tree around 2003, I could tell it had been hit by lightning long before, because the bark was thinner over that scar. After the tree had bridged and healed, the wood from there out for the next 42 years was normal, there was an offset in the sapwood > heartwood transition, which would indicate something to do with air passing through the thinner bark.
Another question I've wanted answered it does the process gradually happen all year long, or when does it occur?
One of my favorite turning woods, and most likely to have burls here in LR is Red Mulberry. I have gotten it on a couple of occasions where there was a light ring which seemed to be in transition, and I think it was early Spring, but it's been too long to be sure. However while turning, I was thinking how neat it would be to have a sample showing it only half as dark. However, while we turn green wood, there is a lot still going on as far as moisture movement, and by the time the vessel was completed and dried, I guess the direct exposure to air and the moisture within the sapwood finishing the process of transporting whatever it had to that layer, ended up just as dark as the other heartwood.
I keep thinking I should do a simple experiment to prove this. I have some timberland down where I grew up near our cabin. All it would take is to shave down the bark in maybe a 10" circle, then take some sort of metal or plastic and bond it on with a thick bed of latex. Wait a few years then cut through it to see the results. If any of you have read this far, and have a tree you intend to cut, feel free to try this and report back.
I hope my feedback helps, and feel free to ask if you have any questions. This also ties in with what I've written about Ring-shake, which occurs at the sapwood to heartwood transition line at the END of a drought. When the tree has been dried down enough to shrink the heartwood, then the rain comes back, the sapwood being outside to get the moisture first plus being much more absorbent expands then pops, or rips away from the heartwood. Tension perpendicular to the grain is the weakest property of all in almost every species.