How close can trees be to the track bed?

I've had more than one comment that the two skeleton trees seen in my previous post are too close to the track bed. I agree. In what might be deemed to be normal circumstances both trees are too close, but on this line there was more than one instance of trees being very close to the track bed. Maybe what makes the trees look closer than they will be when finished is they're not fixed, and one is leaning at jaunty angle towards the track.

I have a number of photos of this line and I am indebted to Chris Knowles-Thomas who took all the photographs below in July 1964.

The tree in the distance looks really close to the track bed.

I feel that the feature of trees being so close to the track bed helps to mark this line out to be a little different to most. From the start of building this "test plank" it has been my intenstion to somehow include some trees close to the track. The problem has been how many and where to place them on the layout without affecting the operation when using three link couplings....