Smooth slabs and grassy meadows are good sliding layers for an avalanche. Boulder fields, shrubs, and small trees can be good anchors for the snow until they become buried, but must be assessed cautiously. Shrubs can bend over time due to snow creep and become much less effective later in the season. Boulders can change the temperature gradient in a snowpack and weaken the base. These anchors must be close together and numerous to anchor the slope.
Trees offer good anchors and protection in avalanche terrain only if they are close together.
In general if you can comfortably downhill ski through the forest, then it is open enough to potentially avalanche. On rare occasions a large slab avalanche can destroy well forested areas.