There's a thing:
Those are pine trees. You can easily identify them by the bark and needles. If you cut any pine tree in the world, you'll see exactly this. The core is more dense and the darker areas are there for transporting nutrients bottom up. Fully normal tree.
Nutrients are transported in the phloem, relatively thin layers just under the bark. Everything inside that is xylem, used for transport primarily of water upwards. So no, the cross sections of these trees is showing some impact to growth occurring to when the light to dark change can be dated. Some trees do have problems transporting water in their core - perhaps it's just this that we're seeing?
Those are pine trees. You can easily identify them by the bark and needles. If you cut any pine tree in the world, you'll see exactly this. The core is more dense and the darker areas are there for transporting nutrients bottom up. Fully normal tree.