Which factors are essential for the growth of forests?

People who aren't cutting them down is a great way. It's dependant on what type of forest though really. Eg. In a rain forest it requires lots of water and humidity for the plants to thrive. In some ecosystems forests thrive after a forest fire. Soil fertility is also something I consider. Nitrogen rich soil. And of course for all forests: plenty of sunlight. So in summary: good soil, water, sunlight.