Thanks for posting!
It is about dendrochronology. Before RC dating, dendrochronology involved looking at tree rings, and measuring them as a way of matching old wood structures with old surviving trees of the same type, etc, as a method of dating structures. I believe its first use was actually in the SW US, in the Four Corners area.
When RC dating came along, using it to calibrate dendrochronology led to the discovery that the variable output of the Sun led to variation in the initial level of C14 in each tree ring. The outcome was, dendrochronology survived as a way to calibrate RC dating, rather than the other way around.
What Manning's done here is a sort of return to one of the past critiques of pre-RC dendrochronology, that is, that in very dry conditions trees don't add a visible ring; same kind of critique applies to trees that grow closer to a steady water supply such as a stream.
Manning's reference in the article to high and low chronologies is actually tied into the supposed mid-2nd m BC eruption of Thera, but the one thing I like about him is, he's been jamming RC dating down Egyptology's throat. As you may know, Zahi Hawass has claimed that RC dating "doesn't work" in Egypt -- the reason is, the conventional pseudochronology (which Manning is actually a part of) is largely wrong -- in the New Kingdom, only the Nubian (25th) dynasty is basically dated correctly.