Generally for me its not just root space that matters ,but also air space. As trees/plants grow they will start touching each other and ultimately shading one another .
That's a good point. My intention is to manage sunlight and circulation over time. This is for the portion of my yard that is dedicated to a hedge and I'm packing it in a bit for more rapid privacy. So for root space, I guess it would be center to center, and for air space, I would factor in other considerations?
I suppose if leave more space for the canopy ,u ultimately have more space for the roots between each tree. Another consideration that can be taken into acount is the spread of the root perimeter. I know from personal experience that the roots ot trees spread in a much wider diameter than the canopy.
Pc: now I saw your question is for a hedje ,in which I would assume u can pack the plants for a faster result ,but can also spread the apart more ,in which case they will also close to each other but will take more time. What plants are we talking about, fruiting or decorative ? If they are fruiting and your goal is also the fruit I would plant them with more distance, if they are ornamentals ,plant them as close as u want .