Well, the usage of
which in your combined sentence should be correct. It is a form of non-restrictive relative clause, which refers back to the whole main clause.
I found a case from http://en.wikipedia.org/wiki/Relative_clause#Finite_and_non-finite
A non-restrictive relative clause may have a whole sentence as its antecedent rather
than a specific noun phrase; for example:
* The cat was allowed on the bed, which annoyed the dog.
Here, which refers back (presumably) not to the bed or the cat, but to the entire
proposition expressed in the main clause, namely the circumstance that the cat was
allowed on the bed.
However, I'd like to suggest some modifications to your sentence for other reasons.
Conversely, automatic canopy detection often fails in dense scenes, which demands more specific solutions.
The first modification is to replace
is often failed to
often fails. I cannot say for sure the first form is wrong, but the second one is more concise to me.
The second modification is to use present tense in the relative clause, because I see no need to use past tense when you are stating a fact.
The third one is about connecting adjectives of the same degree of comparison together with
more efficient and more specific instead of
efficient and more specific.
And as Jim pointed out, only
more specific can be deduced from the main clause.