Dryadism is a passive form of magic that anyone with the skill and training can learn. Unlike other forms of magic, it does not depend on the individual using their mana, but on the natural mana content of the world. This mana is unlimited, which makes it the strongest type of magic. A person absorbs this content through their physical form, using it as a conduit for the mana. They do not control it, but rather guide it in ways to perform the spell ( creating rain, parting a river, speaking to the dead, etc).
Dryadism depends on a number of factors depending on the spell:
certain locations where nature mana is thickest (stonehenge)
certain conditions or time (solar eclipse, full moon, a wednesday,)
certain steps must take place (chanting, animal sacrifice, drowning of a small child)
spells can include 1 individual or require dozens or even hundreds of dryads.
effects remain only as long as the spell is active. When the spell stops, the effects disappear and go back to normal.
As stated, dryadism doesn't involve the individual's mana at all. However, I need a person's mana content to play a role and somehow remain relevant enough to matter. How can I make this happen?