I've just tested it and any spirit from wisp projector vanishes when the ring is removed, even when the character has global +skills. In fact, the +skills don't benefit the level of spirits from charges. One interesting fact though, is that if the character is a duid with at least one point in the spirit, the spirit is retained when the ring is removed and the level of the spirit is unchanged. So a druid with one point in a spirit and no +skills could get a higher spirit by using a ring and removing it.
Edit: Correction, a druid doesn't even have to have a point in the skill to retain the wisp.
Edit #2: If a spirit is cast from heart of the oak with a non-druid, it vanishes when weapons are switched (so heart of the oak is no longer actively equipped). However, if the character also has wisp projecor equipped, the spirit is retained at the original level.
Edit: Correction, a druid doesn't even have to have a point in the skill to retain the wisp.
Edit #2: If a spirit is cast from heart of the oak with a non-druid, it vanishes when weapons are switched (so heart of the oak is no longer actively equipped). However, if the character also has wisp projecor equipped, the spirit is retained at the original level.