After checking with local botanists I tried to grow some mesquite
(Prosopis glandulosa) from New Mexico here in South Carolina and they
won't grow. The leaves quickly develop black spots and fall off and
the plant dies. The same for Rhus trilobata.

I ask my botanist friends and they can offer very plausible theories -
"A fungus that requires high humidity attacks them" - but there seems
to be zero experimental data to back up the theories.

Look in Radford, "Manual of the Vascular Flora of the Carolinas" and
you find plants (e.g. Aesculus octandra) that grows in the mountains
but not below. Why? One can find theories galore but very little
proof.

Why is the tree line in the mountains so well defined? Is is the O2
level - or the temperature - or the winds or...?

I would think that in this cultural of debate over global warming, if
a botanist were to ask for (say, ~$50M) to create some green house
climates where these parameters could be explored so that various
theories could be tested, they might get a grant.

I'm surprised that botany isn't more of an experimental science.