> all it should take is a system of valves,

That would work, but it's not how to works apparently. According to this veritasium video, it's because of "negative pressure" aka tension.

https://www.youtube.com/watch?v=BickMFHAZR0

I recommend watching, I think it's one of the best veritasium Dereck has ever produced.

Also, wasn’t the 250 ppm atmospheric CO2 concentration prior to the start of the Industrial Revolution a historic low as far a geological-time goes?

I suppose that’s not particularly relevant for more recent old growth tall trees that seem to have got by fine in a colder Earth.

But it’s easy to imagine a warmer, wetter, Earth with higher atmospheric CO2 concentrations being more conducive to taller tree growth.

On the other hand, I probably don’t really know what I’m talking about, not my area of expertise.