It is the only bending art to have not been learned from an original source and the only one that the Avatar cannot learn. The first humans to use this art were given this ability from the spirits of the forests soon after taking refuge to keep the forests safe. Woodbending resembles earthbending, in that its users are diverse, persistant and enduring benders, and are known to deploy neutral jing; waiting for the right moment to attack. Using this ability to their advantage, this tribe has kept their forests safe for unknown centuries.
Woodbending is the only bending art that was given to humans by it's source, not learned. When the first woodbenders travelled to the north-east of the earth kingdom for a new settlement at a time of war, they came to the Anjing Forest and not before long, were intended to always protect their new home. That was when the spirits gave them the ability to manipulate the wood around them. The wood tribe has kept the forest safe for a number of centuries.
There is no known force that can negate a woodbender's power, except their power is limited to the amount of wood around them. Woodbenders can also bend specialised forms of wood such as paper, but it works less efficiently than wood, but only slightly.