I am not sure that I fully agree with FumbleFingers' assertion that
The collection of vases is priceless, some being over 2000 years old
is extremely unnatural. I agree I'd prefer as a matter of style
The collection of vases, some [being] over 2000 years old, is priceless.
But the key point is that "some being over 2000 years old" is a modifier of "vases" that explains "priceless. I am not sure that it is helpful to think of "some" as an independent subject in this case, but I will not argue about what people call things.
Leaving aside the fact that "demolishing" makes no sense in this context, your variant does not work because you are not modifying "trees on the west" with "grown," but introducing a whole new, contrasting subject. Moreover, if you meant "decreasing," "some" does not make sense if you are talking about the entire population.
The population of evergreens growing in the west of the district is stable, but that growing in the east is declining
is grammatical. And I'd prefer to say that "that" is the subject of "is" and "growing" modifies that subject. But let's not argue about names.