I remember reading in an old book something along the lines of
"hemlock was considered a replacement for yew when yew was scarce". I don't remember the book, but I do remember reading it, as after I did I went out looking intentionally for some hemlock,
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. I suspect if this was really so, it was probably because hemlock looks like yew (from the outside I guess) and back then most people thought only yew would give a good longbow for some reason. I also remember laburnum being mentioned as well.