It fuckin wimdy

It's against the deer, mostly. We killed all the wolves centuries ago and now there's heaps of antlery bastards that run about and eat every sapling

...that said, Hirta is its own case. There's been nobody living there to plant any trees for almost a century, and trees have seemingly never grown there. It's a pretty harsh and rugged place

It's a combination of too many herbivores with no carnivores to balance their population, and thousands of years of deforestation