Fictional Ecology

So the book I’m reading has a moor and also some trees. I was confused for a while because moors shouldn’t have trees on them, but I eventually figured out that there is forest next to the moor. I’m still ecologically skeptical about this arrangement but at least there aren’t trees on the moor.

My ecological skepticism comes from my understanding of the concept of succession. Put it its most simple form succession means that plant communities are always in flux and that larger species will invade areas of smaller species. So grassland will become scrubland and scrubland will become woodland. Unless there is some reason why it can’t. Reasons include climate, grazing animals, fire, and soil conditions. For example before Europeans arrived the great plains of North America remained grassland because the plans were regularly burned by Native Americans and grazed by bison.

So when my book has moor next to the forest I wonder why the trees don’t take over the moor. I mean I could come up with several plausible reasons, but I’m finding myself distracted form the story by wondering about the ecology.