They do indeed, that being said in a roundabout way and assuming normal gameplay. Technically speaking if you weren't producing any pollution then you could destroy every tree on the map and wouldn't prompt the aliens to attack until you got to the trees close to their bases. Destroying trees in the base game simply allows for less pollution to be absorbed before it slowly creeps towards alien bases.

The amount they absorb is already small enough that it can easily be overcome hence why pollution can escape forests and why you can receive attacks from aliens in the forests, but it's still larger than the terrain pollution generation and those small amounts add up as spreads farther.

My suggestion was more about creating a more direct response from the aliens. Instead of allowing a little more pollution to spread potentially towards them, it would create more pollution thus bring able to directly prompt an attack if too many trees are destroyed.

The only thing I'm not certain about is I believe pollution has data about where it came from and I believe aliens target the biggest source of the pollution that prompted their attack. If this is how they work and you then added destroying trees to this list of potential sources of pollution then the aliens would be prompted to attack a clearing a trees with nothing in it. Presumably assuming normal gameplay the player would be destroying these trees so they could make way for something they'd be building there, so those buildings could be attacked, but still it would then be theoretically possible to have them scramble to attack nothing. And you could also prompt these attacks remotely with artillery.