It depends. It doesn't penetrate very far into the trunk, but it doesn't have that much trouble with leaves and twigs. (Until they get rained on.) So you can have a line of sight that's pretty much blocked by trees visually, but quite clear in 2.4GHz.
It's not that simple. Attenuation increases when it rains and the tree gets wet, but whether the attenuation is enough to cut off your connection entirely depends on a lot of things. But sometimes, yes.