Trade and traderoutes in M-E

[Tuor in Gondolin]

About the third leg of the trade routes (Fornost to
Gondor/Calenardhon), until c. 2000 of the Third
Age it would seem to have had the potential of being
not only a sort of Royal Road (like the Persian road from
their capitals through Asia Minor) but also a path of
commerce. Indeed into the Second Age also the Noldor in
Hollin and Moria dwarves would probably participate in
some form in trade and travel.

And continuing trade and travel potential is seen in Butterbur
observations after the War of the Ring.

It all comes from those newcomers and gangrels that
began coming up the Greenway last year, as you may remember;
but more came later. Some were just poor bodies running away
from trouble; but most were bad men, full o'thievery and mischief.

and Gandalf's prediction:

...the Greenway will be opened again, and his messengers
will come north, and there will be comings and goings, and the evil
things will be drivenout of the waste-lands. Indeed the waste in time
will be waste no longer, and there will be people and fields where once
there was wilderness.

There are several things here. One, who were these refugees and where
did they come from? Dunland? Perhaps a few. Rohan, maybe a handful of
Wormtongish- but doubtful. Perhaps people from some areas of South
Gondor going through Anfalas?
Two, Gandalf's comments suggest not a naturally barren land in Minhiriath
and Enedwaith but one almost unnaturally depopulated (shades of the
Dust Bowl?) Perhaps some New Deal CCC (Civilian Conservation Corp)
replantings and soil management was called for

The point is the basis for a trade route is there for a good bit of the Second
and Third Ages, and even in a form of Dark Ages for the region (including not
seeing to upkeep of fords) you'd think it would be marginally kept open by
entrepreneurs. Btw, had tobacco use spread to Gondor (perhaps Thorongil
brought the vice there ).

And then there's that potential sea route, Forlindon to Pelargir...

[The Mouth of Sauron]

I think I remember reading somewhere that Tharbad had a garrison of soldiers and engineers from Gondor until about a century before the War of the Ring. That would suggest that the maintenance of the bridge there was considered vital for North/South trade.

[Legate of Amon Lanc]

Originally Posted by Tuor in Gondolin

There are several things here. One, who were these refugees and where
did they come from? Dunland? Perhaps a few. Rohan, maybe a handful of
Wormtongish- but doubtful. Perhaps people from some areas of South
Gondor going through Anfalas?
Two, Gandalf's comments suggest not a naturally barren land in Minhiriath
and Enedwaith but one almost unnaturally depopulated (shades of the
Dust Bowl?) Perhaps some New Deal CCC (Civilian Conservation Corp)
replantings and soil management was called for

You basically answer yourself with the second remark: I always believed that apart from a few Dunlendings, these men were mainly from Enedwaith or Minhiriath, exactly those bits of "depopulated" people. Mainly, remember that Barliman says that at first mostly they were poor guys running away from trouble. What trouble, one must ask? The Dunlendings had no reason to run away from trouble, unless they were persecuted in their homeland because they disliked the alliances with Saruman. But I could imagine that the Dunlendings on the rise, supported by Saruman, could become a threat to the neighbouring peoples - and that includes the primitive and simple folk of fishermen and hunters (as said in the UT) in Enedwaith/Minhiriath (I never remember which one is which).

And then there's that potential sea route, Forlindon to Pelargir...

Yes, but in late TA nobody used it, probably since the fall of Arnor. There was this big army of Gondor coming to defeat the Witch-King, and they sailed to Lindon, but there did not seem to be any more contact reported later.

Originally Posted by The Mouth of Sauron

I think I remember reading somewhere that Tharbad had a garrison of soldiers and engineers from Gondor until about a century before the War of the Ring. That would suggest that the maintenance of the bridge there was considered vital for North/South trade.

I don't think was that way by then. It's in the UT, but I don't think there was any powerful garrison there by that time. It was just some population remaining there, and after the great floods (2912 TA) the city was completely deserted.

Okay, I actually found the quotes. As for the population of the regions:

Originally Posted by Unfinished Tales, The History of Galadriel and Celeborn, Appendix D

Since the Great Plague of the year 1636 of the Third Age Minhiriath had been almost entirely deserted, though a few secretive hunter-folk lived in the woods. In Enedwaith the remnants of the Dunlendings lived in the east in the foothills of the Misty Mountains; and a fairly numerous but barbarous fisher-folk dwelt between the mouths of the Gwathló and the Angren (Isen).

And here is the one about Tharbad and its garrison:

Originally Posted by Unfinished Tales, The History of Galadriel and Celeborn, Appendix D

A considerable garrison of soldiers, mariners and engineers had been kept there until the seven*teenth century of the Third Age. But from then onwards the region fell quickly into decay; and long before the time of The Lord of the Rings had gone back into wild fenlands.

Seventeenth century of the Third Age - that's actually quite a long time ago and Gondor was at its heights still back then. After the demise of the Northern Kingdom, it gradually lost its importance to keep a big garrison in Tharbad.

[Pitchwife]

Originally Posted by Tuor

Btw, had tobacco use spread to Gondor (perhaps Thorongil
brought the vice there).

It doesn't look like pipeweed was ever exported any farther south-east than Isengard. Théoden's bafflement at the sight of Merry and Pippin smoking clearly indicates that he was unfamiliar with the custom (LotR Book III, The Road to Isengard):

'For one thing', said Théoden, 'I had not heard that they [=halflings] spouted smoke from their mouths.'

As for Gondor, the Prologue quotes Merry's Herblore of the Shire as saying,

The Men of Gondor call it sweet galenas, and esteem it only for the fragrance of its flowers.

Confirmed by Aragorn in LotR Book V, The Houses of Healing:

If your pack has not been found, then you must send for the herb-master of this House. And he will tell you that he did not know that the herb you desire had any virtues

Anyway, even if the Gondorians had picked up the habit of smoking, they would have had no need to import tobacco from the Shire. Again from Merry's Herblore in the Prologue:

It grows abundantly in Gondor, and there is richer and larger than in the North