I realize you might have stopped your experiments but you can modify the local temperature for the trees by planting them at different elevations.
For every 10 blocks higher than sea level the average temperature gets colder by about 1.5C. This also applies to below sea level where the reverse is true and it gets warmer. The caveat to this is if you don’t have the “Underground Farming Allowed” option selected you aren’t supposed to be able to grow anything below 19ish blocks below sea level. But that still allows approximately +3C to the average temp. Edit: And if you were already building higher than sea level simply lowering it to sea level will increase the average temperature.
One of my goals is to someday make a tiered garden from y-level 0 to 256 or whatever the build height is set to at the time.
