There's nothing unusual there. Typically natural forests grow out in stages, with different generations of trees replacing each other.
Because they actually killed off everything, the "older" trees are not propagating there because they are not adapted for that. That's also why natural forests have clearings that can last for a while.
What's unusual (or "unnatural") is that it's not just the macro level that was annihilated, but even the subterranean fungal network. Fire won't do that (unless the soil is very thin). Drought won't.
What is happening is apparently the entire cycle of repopulation of a food source for the most fundamental of ecosystem anchors.
What surprising is that this should be the equivalent of planting a garden with fuly sterilized soil. As someone else noted, why aren't wind-borne spores and nematode corpses revitalizing the subterranean ecosystem?
That is probably why you can see some low scrubby undergrowth in the Google Earth view, but nothing that needs to put down deep roots has yet returned.
If there's enough radioactive material and it is mobile enough (due to ground water or wind driven mixing) to stay near the surface it could sterilize any organic material that comes in faster than it can accumulate.
"An area of oak-pine wood was selected East of Upton, and a tower was constructed that could raise and lower a canister from underground that contained radioactive source material, allowing for controlled dosage levels that emanated in a radius from the tower. The canister contained Cesium-137, which would emit ionizing gamma radiation without making the surrounding area radioactive itself."
