Questions

I want to plant some native Irish trees in about half an acre of land to create a small dense wood. I have no plans to ever cut down these trees once sown.I was thinking about common ash (Fraxinus excelsior) and common alder (Alnus glutinosa) as an option. I dug a few holes in the field and on average there's about 1.5-2 feet of depth in clay (good clay soil) but past that the ground is quiet rocky. So I was wondering if the trees that I am thinking about sowing are suitable for this area and how far apart these trees (or perhaps other trees) should be planted in order to let them grow to their full size. Have you a particular native irish tree that you'd like to see growing?

I was planning on planting some common ash trees (Fraxinus excelsior) in a field, but I read on a webpage that they were poisonous to cattle and sheep. Is this true? Are there any of the native irish trees which are poisonous to cattle and sheep?

I was thinking of planting about 15 or so wych elm trees along a wall in a field. I heard that they are less prone to attack by Dutch elm disease than other elms but I was still wondering if it is a good idea to plant them. Is there another type of elm that's not prone to Dutch elm disease? If it is safe to plant them how far apart should I space them? What type of soil suits the elm?