It's woodland *edge* - ie an open space on one side. Open spaces, left to themselves, don't stay open. The next stage in success is scrub (eg brambles and blackthorn) followed by trees which will eventually shade out the brambles and blackthorn.

Trees put a lot of energy into developing a strong trunk to be self supporting, so that they can over-top everything else and grab all the light. Everything else is to a certain extent opportunistic, making use of either temporary gaps in the tree cover, or living in areas where there isn't enough soil cover for trees (eg mountain tops, cliffs). They avoid the need to put all that effort into building trunks, but have to snatch what they can, where they can.

This is a gross simplification!