Attila

...on the subject of spacing,
it really depends on the species. I like to keep members of a particular species together. So, for instance, I grow young trident maples very crowded. I have about a hundred of them growing in a relatively small area, so densely packed that I can barely stick my hand between them. It doesn't matter because they grow up to the sky and next year I will chop them down to about ten inches.
With pines, it's a different story. I will use the low branches for future styling, so I want to make sure that they are not overshadowed by each-other. Otherwise, those branches may weaken and die off. So, neighbouring pines don't cross each other's branches.
Some trees like more shade, so I grow them closer to taller ones. So, there is no general rule. Each one has a different story.

Attila