There are some common errors in placing a tree on a canvas in an outdoor painting. Usually these are wrong. I try never to say always, but................. Above is a scene with a tree that is placed well on the canvas. Below the tree ends right at the rabbet, that's the edge of the frame and not a freakish Englishwoman's misbegotten issue. It is seldom a good idea to jam a tree or anything else up against the top of a painting.
If you do have to chop the top of a tree of to make a painting do it boldly, like this. It is best to end the top of a tree well short of the frame or run boldly past it, like the example below.
Below is another problem child.
This tree "stands" on the frame. That looks artificial too. This happens because what might look like enough of a gap between the base of the tree and the bottom of the canvas, might not be enough when placed into a frame. The frame will cover up to a quarter of an inch at the bottom of the picture .
It is easy for these errors to happen because the trees are so large and the canvas is small. As you paint them you keep unconsciously making them bigger and they assume a size larger than that you intended. It sometimes works to draw a line at their top and resolve you will not enlarge them past that line.