Because on a larger scale, trees of the same species will have very specific microbiomes. In the past, most of the studies focused on ecology scales.

I'm sorry, but I don't quite understand. When you say "very specific microbiomes" do you mean similar microbiomes? I.e. on a larger scale there is much more across-specifies microbiome variability than within-species microbiome variability? Or have I misunderstood?