Hi there,
I would second that - within the game, planting trees in an area can be a little element of strategy and not just serve to making this really beautiful game even more beautiful: Trees could be used as a kind of barrier to siege engines if planted right (may need some micro management though). Sieges can not break through and the barrier can only be removed by citizens chopping wood.
Also establishing new buildings can be blocked (the area would need to be cleared first).
For this to work trees need to be "plantable" in a free area and not on a plantation that can be destroyed. (The principle is something many were using in the past when playing settlers 3 to block the adversary from establishing buildings nearby own borders...)
I know this would be modifying the game quite a bit. Just saying...
