There are various survival topics we seldom see solutions for. Mouth care in the wild is one of them. Even when surviving, where most likely a sugar-based diet won't be an issue, there are still some things that need to be addressed and having the knowledge will be helpful.
#1 Dogwood Stems
Dogwood stems can be used as a chew stick or toothbrush.
#2 Birch Twigs
Birch twigs can also be used the same way.
#3 Wood Ashes
Paste of wood ashes works as toothpaste. Rinse thoroughly to avoid irritating gums.
#4 Pine Needles
Gargle of pine needle extract (from soaking pine needles) works to freshen breath somewhat and has some antiseptic properties.
Oak tanin (from soaking bark) is also antiseptic.
Flossing, or more likely in a survival situation, picking your teeth is almost as important as some form of brushing. Keeping food out of the spaces between teeth as well in the hollows of molars goes a long way. Carve some tooth picks or use thorns from non toxic plants (i.e. wild roses, hawthorns)
Simply rub clean fingers through the teeth until they feel clean (sound like rubbing a clean wet plate). Also eliminate any food between the teeth with a small, non-toxic stick.
Juniper twigs for brushing teeth.
Source: Seldom Addressed Survival Tips