And this is why people who don't know science should ask questions first.

The tree ring data, after a certain point in history, is no longer a reliable indicator of local temperature changes. There is no explanation for why trees no longer reflect local temperature changes. We know that they no longer reflect local temperatures, presumably, because we have other more reliable measurements available now (measurements that are not available further back in history).

There is NO SINGLE EXPLANATION for the discrepancy. This is NOT the same as there being no explanation.

What happens instead is that if you look in specific localities you can find specific explanations. Increased acidity here. Drought conditions there. Insect infestations in another. And in yet another the correlation with temperatures remains.

What happened is that after 1960 due to large scale changes in environment in many localities the correlation broke down, so the overall correlation broke down. But there is no single explanation because in different areas the specific reasons differ.

This leaves us with a conundrum: is there any period in which alternative measurements and tree-ring measurements correspond with one another? Is the divergence between our measurements and the tree-rings something we can trace? Is it sudden or gradual? If there is no period in which there is a correspondence between our measurements and the tree-rings, what does this say about our assumption that tree-rings have ever accurately captured local temperatures?

Actually there is about a century of comparative data before that which shows a good correlation. That is why we are willing to project temperatures back farther using that record.

Now there is the possibility that there were previous periods which saw a similar breaking of the correlation. This question is a question of scientific debate.

It seems that the implication is that changes in the environment have rendered tree-rings unreliable. But if we're arguing that tree-rings are unreliable because of environmental change (i.e. the chemical changes to the composition of the atmosphere and water cycle that contribute to global warming), and we're seeking to explain environmental change, we're begging the question (assuming the explanandum in the premise of our argument).

There is no begging the question when we are also studying the mechanisms involved.

Until a causal mechanism for the divergence between tree-rings and other measured temperatures can be specified, how can they legitimately be discarded as evidence, without invoking tautological reasoning?

You are making two large assumptions here. The first is that there is a single causal mechanism. The second is the assumption that we don't actually understand the mechanisms involved. Both assumptions are wrong.

Look. Scientists aren't exactly stupid people. When they spend a lifetime studying something, you should assume that they've asked the obvious questions and try to find out the answers they have found rather than assuming that they are stupid and you're the first to notice the glaring issues.

Unfortunately climate scientists are also humans who are under constant assault from people who can't and won't be bothered to actually learn the record. People who feel they are under constant assault engage in defensive behavior. And people who are being defensive tend to circle the wagons and have difficulty in remaining open with outside groups.

When you encounter such defensive behavior I strongly recommend trying to understand what the defensive person's position is rather than immediately piling on and validating their defensiveness. If they are wrong, this approach leaves you in a better position to figure that out. But if they have a point, you'll be more likely to figure that out as well.