Is this a thing? Or has the tropical madness finally set in?

#1
I've just done a quick spot of research on Mt Teide in the Corona Forestal, in which we measured the stemflow of 32 trees in the Forest, and the stemflow of 32 trees on the volcanic slope that's set up shop next door.

We found that you get more stemflow on the Volcanic Slope then you do in the Forest. But you also get more stemflow on trees that are bigger.
Am i right in thinking i can fire up SPSS and run a test that will say "The relationship between Stemflow and Tree size is Significant, AND the relationship between Stemflow and Location is Significant, but the findings are more likely to be due to Location, then they are to tree size (or one location having more big trees for example)"?

Does this test exist? or am i just hoping that this kind of test is possible because it would be useful?

Thanks for your time!