Using so called negative forms or space is a popular method to relax from concentrated observation on drawing outlines of a subject. Instead of following the outline of the tree branches one can shift the view to the empty space in between. Those closed forms of sky patches are often easier to outline than following the complicated grid of the branches. The results of such a way to draw are remarkable different form the standard way. A combination of both may deliver a most vivid and unique result. These roller pen sketches are quick studies using a lot of blind drawing too.
There is a great explanation of drawing negative space by Robert Gardiner, the University of British Columbia, Vancouver, Canada, on the net.
If this method is applied consequently a very realistic result in drawing a tree top can be expected. The black and white study below was generated solely by drawing the patches of sky seen between the branches grid of an old oak tree against the evening sky. This drawing was done on an Iphone using a photograph as background layer. The white strokes were set on a half transparent layer on top of the photograph.