I can see this question has already been answered but I'm going to have to strongly disagree with the answer congusbongus gave. I'm not trying to kick up a fuss so let me say why I think its very possible for a forest kingdom of some sort to exist.
When you say forest kingdom I assume you mean a fairly sophisticated state based on a high population density and a separate class of aristocracy that did not need to farm due to the amount of produce grown by others that could support such a class, who were then able to provide a ruling structure to weld the state together as well as control external defense, all of this existing in a forest.
There is increasing evidence that something pretty much exactly like I just described actually did exist in reality, specifically in the Amazonian basin prior to contact with the Europeans. The main evidence for this is the fact that huge areas of the Amazon have so called 'Terra Preta' (black soil), earth with high charcoal content. Terra Preta usually also has signs of broken pottery, as well as other human remains such as middens, really it implies heavy human habitation over a long period, supporting a much higher population level than today. This has been recognized by scholars all over the world:
In fact, from all directions the idea that the Amazonian basin was a relatively low density wilderness before Europeans arrived is under attack, there have been clear indications that big towns and vast amounts of villages once existed in the amazon:
There have even been huge geoglyphs etched into the land discovered from the air, as well as with radar and laser analysis:
Even roads have been discovered:
a bit more info:
Now this is all extremely controversial but the evidence is building up endlessly that a complex civilization with a much higher population than we thought possible was spread throughout the Amazon, one that supported major public works and complex trade between them. There's more as well, Francisco de Orellano was the first European to travel through the rain-forest and write about it. For centuries his account has mostly been dismissed as fantasy but in light of these recent discoveries there has been a re-evaluation. Even if we don't take everything he says as absolute truth there are still important clues as to the makeup of the Amazonian civilization, for one there were way more people than would be attested for later, they were experienced in war and had a complex aristocracy. Even today the nomadic descendants of the Amazonian civilizations are known to often have a bafflingly complex social structure and landless aristocracy compared to almost all other Nomadic peoples. This is probably a clue of the past society's social complexity.
If you want to know more about the Amazon's past let me recommend a couple of books and documentaries:
BBC4 "Unnatural History":https://www.youtube.com/watch?v=HUXLim2HIvU
1491: New Revelations of the Americas Before Columbus: http://www.amazon.com/1491-Revelations-Americas-Before-Columbus/dp/1400032059
The Amazon was still a forest, but the basis of the agriculture was on trees (in any event it was tough to clear large areas of the forest without iron tools, none of which existed in the Amazon before the Europeans), Charles C. Mann's article in the Atlantic has more info here:
The BBC4 documentary also goes into the nature of the agriculture as well.
(BTW, in case your wondering what happened to all of the one time inhabitants they seem to have been extremely hard hit by the epidemic of old world diseases after the Europeans arrived, which may have killed up 90% of the people living there and caused their society to crumble)
Anyway, sorry for being so long winded but this is a hugely controversial topic so I wanted to be thorough. To sum it up, YES, it really is possible to create a complex society within a dense forest, and with the use of trees to cover for field crops. In our reality, kingdoms and complex societies DID exist in the Amazonian basin.