The trunks I harvested had no heartwood, they were sapwood all the way through. This was Ostrya virginana (American Hop-Hornbeam). It made very nice bows, if designed appropriately.
I also made few bows from Carpinus caroliniana (Blue Beech or Musclewood, also called Hornbeam in some areas) which is a somewhat different, but also very tough wood. Hop-Hornbeam tends to grow straighter, and the wood is less prone to warpage.
IMHO some of the best info on wood and wood/sinew prods here on the forum is in posts by 'Mac'...he's no longer an active poster, but a bit of digging around should turn up some valuable stuff.