[mlsu]

I know nothing about wood but does this mean that the heartwood has relatively wide ring spacing (since it grew when the tree was low and small?

[RoyalHenOil]

You see this in trees that have grown up in forest conditions. They sprout in a patch of sunlight, but then they eventually fill the space and can't grow as quickly anymore.

In trees that are grown in more open conditions with plentiful sunlight, water, and nutrients, the growth rings are more even because there is nothing holding back their growth and forcing them to slow down.