Question about materials not being affected by lighting

I’ve been looking for some answers to this problem and can’t seem to figure it out. I’ve played with some assets as I’m new to UE4 and there are some meshes like these trees shown that are not affected by the amount of light in the scene. The leaves on the tree to the left for example does not get darker as the scene gets darker like the tree on the right. If anyone could think help I’d greatly appreciate it.