Spacing is indeed important, and if you are going to grow walnut on a plantation, you want to manage spacing over the first 20 years of the plantation pretty cl

Spacing is indeed important, and if you are going to grow walnut on a plantation, you want to manage spacing over the first 20 years of the plantation pretty closely, with very dense competition in the early years to force straight upward growth, followed by thinning to allow space for crowns to gather enough light to maintain growth rates. You don't have to do that with most conifers, which tend to grow straight single boles regardless, but you do with most other North American hardwoods, including timber maples, and most species of oak.