the 6 trees on fire

To me, it symbolized the attitude of the rich and drunk. The rich factor allowed them to do what they wanted and not realize how destructive they were and the drunk factor showed them partaking in another activity that blinded them from what they were really doing.

Here's my take,
The six trees were lit on fire as Jonathan and Barthalomew were exchanging arguing points. As they fought verbally and their conflict escalated, trees were being ignited, thus symbolizing that the conflict is getting more serious.
6 of them corresponds to Jonathan's number which is also the same number we see in the Prisoner to represent individuality which is a ky theme of the movie.

The obvious point that they are decadant party-goers just out for a cheap thrill and insensitive to trees (life, suffering, things of great value) that took decades to grow is also there on the surface. That, along with the fact they they are also unaware and insensitve to the greater issues being discussed as they romp around.

The tree scene is simply a symbolic reinforcement of the struggle between humanity (Jonathan) and corporate utopia (trees), specifically they represent the actual corporations.

There are 6 trees. During the concession/flaming trees scene, we see 6 different trees get shot. (starting at 01:02:40) At 01:03:50, we see the first long shot which reveals all six trees. Four are burning (2 to go.) We then see the next two trees get shot, right before the final lines in the argument. As we hear the crowd yelling "Jonathon" asynchronously over the remainder of the scene, we see one more shot, but it goes in the dirt (no more trees get shot.) At the end of the tree scene, all 6 are shown burning in a long shot.

These are the corporations (majors) mentioned by Bartholomew in the first conversation with Jonathan E (22:31)

1. Transport
2. Food
3. Communications
4. Housing
5. Luxury
6. Energy

There may be some subtext here about decadence, environmental abuse, drunk rich people with guns or the aesthetic futility of neat rows of pine trees, but the main message here is a foreshadowing of Jonathan prevailing in the end.

Before the New York game, we see the 6 executives (trees) voting to kill Jonathon. (01:29:18) He survives, instead. Trees lose.

This continuity of six cannot be a coincidence, although it seems Jewison himself does not realize this, according to his explanation of the tree scene. Nothing in film happens by accident, so I doubt that's the case. I think he just didn't want to tell us and let the movie speak the message for itself.

It's all right there in front of us.

Mindless destruction, as others have talked about. I first saw the movie when it premiered at the Cinerama Dome in Hollywood, and I can tell you that scene was absolutely shocking, a lot more than the actual rollerball scenes were. Nothing drove home more the idea of a decadent society that was so powerful that a tiny gun like that could make a tree go up in flames, and yet be so completely without a sense of purpose.

What others haven't talked about is how the scenes of the trees being destroyed is intercut with Bartholomew telling Jonathan that he didn't need to understand why he had to leave the game. If Jonathan hadn't felt the need to understand, he would have been no better than the people destroying the trees. The irony of the entire picture is that of all people, someone who perpetrates mindless violence in the rollerball rink is the only one who cares.

It is symbolic of the spoilt and wasteful children the all the controlling corporate nanny state created with no value systems except acting immediately on their own desires. The woman cries in the closing of that scene as a small child would when they realise they have killed their goldfish by taking it out of its bowl and are then overtaken bya mixture of confusion, loss and shame.

6 was Jonathan's jersey number.