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Dr. Andrea Lloyd says:

We want to know how old all the trees in this plot are, and since seedlings are so small, we can't core them. You could cut them down to count the rings, but then they'd be dead and so we wouldn't be able to monitor their future survivals. So we want a non-destructive way of figuring out how old this seedling is, so what we're doing is counting the number of annual internodes. So a spruce produces one-- one internode per year, so in each year's growth it produces this whorl of branches and then a stem. And so then we can count each whorl of branches, or if the branches are gone, it leaves a little scar that looks like a constriction or a line around the stem. And we can just count those and get an estimate of how old the seedling is, um, that's comprable to the estimate we get of how old big trees are.