In OP's context, ***tree*** is being used as an [*attributive noun*][1], 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)


  [1]: http://grammar.about.com/od/ab/g/Attributive-Noun.htm