Heartwood formation begins with a bit of a lag, then tends to put on growth at a steady rate to the rate of growth of the tree. The other way you might think about it is this, cells live a certain number of years on average. How long? Count the rings of sapwood
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My white pine can have monster sapwood, our first 35 years or so on an open site can yield 2-4 rings per inch of rank growth, then the roots hit the rock of the mountain and the growth slows to 8-10 rings/inch, going through those zones the sapwood width will vary tremendously in inches but very little in number of rings.
I've been wandering in my stand limbing the pines up, they're mostly in the 25-30 year old range. In general they are running about the same height and diameter but the supressed trees in the stand are half the size of the normal ones so they will have more rings per inch and a narrower sapwood band. They're also struggling and not good timber, well neither are the big boys, they'll be sheathing or panelling. It's the northern red oak and white oak at their feet that I'm trying to show some light. Besides looking up, look down.
