If I recall correctly from Hoadley, "Understanding Wood":
- for softwoods, slower growth results in harder wood (and I think it's
because the earlywood is harder than latewood in evergreens, so you
want thin bands of latewood)
- for hardwoods, faster growth results in more latewood which is harder
(and I recall a series of pictures of a ring-porous wood which
demonstrated this phenomenon)

(I'm posting from google groups; I dunno how to force google to quote
message text)

- Daniel H