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You won't believe it, but your haiku is very similar to a haiku I wrote a few years ago. Trees look like sentinels to me too!

Oh, @bananafish! I think the coincidence is in the soul of the tree. In fact, in the trees that are tall and rise towards the sky.
The trees of wide and leafy crown have vocation of nest.
The skinny ones, of branches in uprights, serve to contemplate themselves in the afternoons... and so on.
You should place that haiku around here, and so it accompanies the sentinels of mine.
A hug!