For general "large weapon" tips, check out "Be Nasty with a Boffer Polearm". Though that article does not directly address how to fight in closed spaces, its general tips are still relevant in a crowded setting.
Choose your battles
There are likely some places in the forest where you have full use of your weapon. Make sure your teammates know where to stand so that you can properly swing your weapon, and try to move your fights toward those areas.
Take advantage of the shield wall
If you have to be in a crowd, stand behind the front line. "Snipe" at any open targets as they approach your front line. "Don’t focus too hard on the person you’re engaged with. You can make an attack of opportunity against anybody within your reach. Swing when they swing." (Nerology)
Scorpion stance makes blocking easy while at the same time granting a great number of openings on your opponents legs. These openings are easy to take advantage of.
"Scorpion stance is a defensive posture. It’s a good defense against somebody moving directly towards you. Turn your shoulder towards your opponent and make your body as narrow as possible. Hold the polearm blade down, keeping your body behind the pole. One hand should be high, by the butt end. The other should be about halfway down the grip. Keep your legs wide and your posture low. From this stance, it’s easy to block your entire body and thrust at your opponent’s feet. You don’t need too much motion to block, just lean away from the attack and your polearm will already be in the right spot." (Nerology)
"The best defense is to not be there when they swing. Polearm is a very mobile style – if you’re getting hit, it’s probably because you’re standing still. Be a moving target." (Nerology) Keep your weight balanced between your feet so that you can easily step forward, backward, or to the side. If you're fighting a shield fighter, strafe around him towards his shield so that his reach will be decreased. Be aware of your surroundings, and move around to take advantage of the trees and other obstacles so that they don't impede your attacks.
Move your body
You have a lot of reach. That said, if you move your entire body as you move your weapon, it will seem like you have even more. Duck as you swing low, and you can add a foot to your swing. Jump back when you and your opponent swing at the same time - your swing will hit him, or at least his shield, but his will just hit air.
"Polearm style relies on speed and precision. You should only be hitting with the top six inches of the weapon. This is a 'lightest touch' style – use your wrists and forearms, not your biceps." (Nerology) I disagree, to a certain extent - you should use your arms and entire body, but your wrists and forearms will be moving the fastest.
Choke up on your weapon if you must. This will allow you to make smaller radius swings.
You may want to thrust and then swing - this will allow you to get past obstacles and still hit your opponent.