It's not a legal fiction that ~"corporations are people." Corporations are literally individual owners, managers, employees, etc. with various personal rights and responsibilities. There is no forest but for the trees that compose it.
Now, the fact that we've also agreed that corporations can act as political actors is fucking stupid, but the intent behind corporate personhood is that without it, it's effectively impossible to hold corporations accountable in any kind of reasonably efficient fashion.
And, arguably, yes, the same argument applies to the legal personhood of a forest - that proving that the impact on a group of trees aggregates to something significant and legally actionable is unnecessarily time consuming to keep doing every time someone tries to argue their clear-cutting operation hasn't actually harmed anyone, so you assign legal personhood to the forest so you can say "you harmed the forest" in the same way you can say "you harmed the corporation."