The Forest Service wants to thin more than 1,800 acres to reduce the risk of fires.
Conservation groups say more research needs to be done on how it will impact the environment. They say there is no evidence thinning the forest will help with wildfires.
The lawsuit filed Thursday wants the courts to block the thinning forest.
There is also a plan to thin 107,000 acres in the Sangre de Cristo Mountains in the coming years.