Isn't it just more common to start sentences like this one with "There are..."?
Australians would typically say, "Lots of tall trees are in these green forests", although "there are a lot of tall trees in these green forests" and "There are lots of tall trees in these green forests" are also fairly common.
'In these green forests are many tall trees'. Not accepted. Why?
Can we substitute "lots of" for "many"? It may be different in other parts of the English-speaking world, but in Australia we rarely, if ever, start a sentence with "many".