It has been more than 40 years since I graduated from forestry college, but I was as interested in gun stocks back then as we are now. I paid a lot of attention in tree physiology class. Think about how heartwood differs from sapwood. IIRC, growth (a series of biochemical processes)takes place in the cambium and in the sapwood and metabolic wastes are stored in the cells of heartwood. The figures and coloring that we admire are in reality internally-excreted and internally-stored metabolic wastes.
The tree's growth is controlled by available sunlight, by available water and the nutrients and minerals available in the soil within the range of the roots. That range is generally the same as the area covered by the tree's branches and leaves.
Consider the isolated walnut tree. Animals have been penned for many scores of years in the area surrounding a certain walnut. That walnut's heartwood is more highly figured than walnut from nearby trees of similar age but grown where animals are not confined. The presence of animals is the observable variable. It seems reasonable to assume that the isolated walnut tree had more of some critical mineral compounds available to it during specific times in its life than did the other nearby walnut trees.
If walnut from certain locations in Iowa shows a lot of red with some black streaking, that color pattern can probably be tied to one or a few types of soil in that part of Iowa. The ISU library probably has a room full of detailed analyses of Iowa soils and their chemical properties. Some obscure academician may have studied that particular phenomenon.
The Oregon wood probably comes from alluvial (valley or lowland) soils from west of the Cascades. Many of those soils have a very high component of volcanic material, a very different mix than the Iowa soils. Add to that a very different rainfall pattern and different depositional patterns occur in the heartwood. The Oregon trees are under moisture stress for at least half of each annual growing season. That almost never happens in Iowa. All living things are influenced by their environment.
