I am confused by the concept of coexistent planes. For example, about the Plane of Shadow in the Manual of Planes we read: If a wizard travels onto the Plane of Shadow from a forest, she first sees an equivalent shadow forest. If she starts underwater, she appears within a shadowy sea that behaves like a Material Plane ocean.
So the wizard arrived in the Plane of Shadows see something similar what he saw in the Material Plane, but still it is obviously another places; this brings the egg-chicken problem, what influences what? What would happen if somebody burns down the forest in one of the two planes?

I'd like to imagine some kind of symbiotic relation between the planes, but I am not sure. How do you imagine this stuff in your games?

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I acknowledge the existence of multiple planes in a realm, and interconnected realms in my main canons. Without going into too much detail, there's a few fundamental concepts I have for it.

For multiple planes in a realm, it's along the lines of differing wavelengths; every entity is the sum of its soul, and the soul is capable of existing in multiple planes. It is a physical form that binds the soul to one plane or another. Two primary ones exist; the others are more like different dimensions. The main one is material, where all physical actions occur, and the second is minus the material part where souls directly interact (most are incapable of moving to this second level). The forest, being material, doesn't actually exist in the second plane, but the souls of the trees do. If you "burn down the forest" in this plane, the trees would exist without souls; there would be no special attributes to them, and they would be influenced by what happened in the other plane. Those attributes are another discussion altogether, but that's how I approach it.

Physical materials in the second plane are just representations and aren't actually affected in the material plane. I'll use computing as an example... the material plane is the hard drive where everything actually exists, on the other side is RAM where everything is temporary, 100% changeable and ultimately can be unloaded and reloaded altogether.

@helenedwards is a bot who reposts threads from Giant in the Playground forum to here. I know this because I've seen the original posts and they're all from different people. So there's no point in replying.

Well, I suppose the lack of sig and the username are somewhat indicative. Oh well. Lets make a discussion out of it anyways... call me the OP if you like.

We could do that. Now, the fake-OP's question is specifically about the Plane of Shadows as seen in D&D cosmology, and that question seems to have a fairly clear answer: the Plane of Shadows is stated to be constantly shifting, gradually changing shape, which makes creating an exact map impossible. That implies to me that the Plane of Shadows is mirroring the Material Plane, not the other way around, since the Material Plane is the "solid" one. Ergo, if you were to burn down a forest in the Plane of Shadow, it would probably reform to remain consistent with its material counterpart.

As for general, I prefer to take a nihilistic approach to world-building and not objectively confirm the existence of souls (as a separate thing from the body), the afterlife, or any such thing. If alternate planes are involved, they probably overlap with the known plane in the sense that opening a portal in plane A will consistently bring you to the equivalent location in plane B, but they don't generally mirror one another. As such, I've never faced the fake-OP's issue.