There's several ways to fix overcrowding:
1) Use slavery to reduce the population of the city on production (never "whip" less than 2 population away as this doesn't do much to reduce population)
2) Build happiness buildings (coliseums, theaters)
3) Acquire more happiness creating resources (gold, gems and silk)
4) Increase your culture slider if you have coliseums and theaters
Forests and such have nothing to do with crowding, it is simply due to the number of people in the city (the numbers on the city). You can prevent unhappiness by not growing your city beyond the happiness that can be sustained, but usually I prefer to just whip away happiness with slavery so I can get some production usage out of the population.