I think all plants should count as coverage. Personally don't think you should adjust such features just to allow/build an elevated viewing area.
I lower a lot of trees in the ground just for looks or to mix it up.
Would be strange that the same tree would sometimes be counted as coverage and sometimes not.
Also I think this would be difficult to implement. Trees don't have a height meter (like the barriers). They are just objects.
Okay I did not communicate this well enough..
only plants and bushes, I meant, Trees should be left as they are (I know they are plants as well, that's the confusion), its the flowers, cacti, bushes etc. that should have this, but not the trees. Its just that instead of calculating their coverage as "cylinders" they should be calculated as spheres.