I get conflicting results, several sources quote the first statement, I only found the second statement once:
Internet sources report that wood from the following trees is suitable for smoking: AVOCADO, BAY, CARROTWOOD, KIAWE, MADRONE, MANZANITA, GUAVA, OLIVE, BEECH, BUTTERNUT, FIG, GUM, CHESTNUT, HACKBERRY
, PIMIENTO, PERSIMMON, and WILLOW. The ornamental varieties of fruit trees (i.e. pear, cherry, apple, etc.) are also suitable for smoking.
No one can deny the smell of bad wood. Beware of soft wood, cedar, pine, hackberry
, elm, and the like. Always test-burn unknown wood on a campfire, and then determine if that would be a taste you would want on your food. Some wood is quite toxic. Never burn treated lumber of any kind. Good smoke comes from hardwoods that bear a nut or a fruit. Oak, hickory, mesquite, pecan and various fruit trees have established themselves in the BBQ fuel inventory. Beware of unknown woods!