OK, so here is a question to assembled guitar-violin-cello-fiddle enthusiasts.
The world is getting very, very slightly warmer since the 1970’s (in September the global average temperature was 0.14 degrees C higher than it was in 1975, but 2 degree warmer than the very cold Little Ice Age of the 1625-1700 when the Thames river routinely iced over each winter.)
More important than temperature though, we have added more CO2 to the air, which IS increasing the rate of growth of ALL plants worldwide by 12% to 27% faster, thicker, greener with more leaves and thicker trunks. More fruit, more grain, more spinach and more algae and trees.
Pine, for example, is now growing so fast that the structural wood engineers have to “de-rate” its strength from the 1970-1980 wood that was growing more slowly.
So, if 2x4’s are weaker because the grain structure is “looser” (tree rings are further apart with larger cells), what happens to the tone and resonance and frequencies of the notes coming from guitars and violins made from “looser” wood?
Well temperature and humidity can cause a string instrument to go out of tune, for sure, but most everyone is tuning those instruments to electronic tuners nowadays so the actual notes they are playing are going to be the same as always. Can’t say what it might do to the resonances or tone of the instrument though.
Tone is really only affected in “acoustic” type instruments. This includes anything that gets its done from hollow areas in the instrument body.
A solid body bass or guitar depend more on strings, pickups and playing style. There are actually a lot of youtube videos demonstrating this with A/B comparisons.
Pine isn’t used much in instruments but this may be an issue in other woods. Luthiers use what works best. In the cheap instruments it’s pretty irrelevant, though.
Simple answer:
Grow trees in enclosed spaces, strictly meant for instruments, and restrict their CO2.
In fact, no idea why nobody has done this.
Let’s us two guys get rich.
Denser woods in general tend to be used. And the CO2 levels have risen (perhaps, based on proxy data) by 100 ppm in the past 150 or so years. I'm not sure that CO2 levels haven't declined since the 1970s.