Just tossing out a couple random ideas here. Probably can't even be implemented in Civ IV, but maybe in Civ V?
1. Reforestation: should be allowed, but take quite a while... how long does it take a newly planted forest to reach maturity? A worker would be given the task for a turn or two on an unimproved tile, and then several turns later (4? 5?) the forest would appear; this additional time could not be sped up by putting additional workers on the job. Would hopefully not be as exploitable as immediately becoming forested, chopped, reforested, chopped, etc.
2. Introduce an algorithm similar to Global Warming whereby the planetary environment can take a hit if overall forestation falls below a certain percentage. Percentage would differ based on map type and size, and jungles would count as more than forests for this calculation. Of course, this would be similar to the "pollution" of old, so many people would not like it... so make it so it can be turned off. Would pretty much require some form of reforestation ability to exist, so the environment could be stabilized if this happens.
