For every landscape except Anfauglith, I think the change is at least somewhat gradual.
For forests, I'm reminded of a line somewhere, describing Mirkwood or perhaps Taur-nu-Fuin. The trees strive darkly against each other, or something like that.
In pictures of European forests, the trees often appear to grow well apart from one another. I don't know if those are natural forests or plantations. But anyway, picture a clean forest with trees of all different sizes, including a lot of enormous old giants, but none are too close together, or tangled with each other. The forest floor is dim but not pitch dark, sprinkled with rosebushes and flowers and herbs and wholesome mushrooms. Beleriand was touched by the Valar and probably blessed by Melian, so imagine there's very little disease or parasites. It's all clean, bright greens and flowers.
Morgoth spreads his influence and green things "fell sick and rotted", as at the end of the Spring of Arda. So imagine in that forest, the old trees start getting dead limbs, rotting holes, parasitic fungi, black cankers, discolored and misshapen leaves. Insects start swarming and overeating. At the same time they're starting to rot, the old trees start growing new limbs at crazy angles, tangled with each other or hanging down like grasping hands, while new trees sprout up too close to each other and seem to fight. Poisonous plants and mushrooms, thorny stuff, and vines that strangle trees replace a lot of the flowers. The whole scene also becomes a bit greyer.
And we shouldn't forget how ambient music, and bird sounds, can help set a scene.
On grasslands, I imagine one of the distinctions is between abundant herds, and very few surviving animals. In a lake or wetland, lots of happy noisy birds vs. lots of rot and flies.
