In OP's context, tree is being used as an attributive noun, which obviously requires a "real" noun to follow it. Technically speaking, possible nouns with the required meaning (the act or fact of being obscured) include...
obscuration, occultation, hiding, concealment, etc.
...but personally I think the entire construction is "awkward", and it would be better to rephrase. Apart from anything else, "the tree obscuration of the mist" could just as easily be interpreted as...
1: [the tree obscuration of] the mist (the mist obscures the trees)
or
2: the tree [obscuration of the mist] (the trees obscure the mist)