The Economist has a special section on forests--"the world's lungs." For the past 10,000 years we have been cutting down forests in order to clear the land for agriculture in order to support an ever larger (and generally better fed) population. More recently, we have intensified the "development" of land in order to raise cattle, and to grow plants for fiber and biofuels, not just food.
Across the world, forests and the soil beneath them absorb about a quarter of all carbon emissions.
This is an indispensable contribution to life as we know it, and forests offer many others, too. They house more than half the world’s species of animals, birds and insects...Indeed, the more that people learn about forests, the more perilous their mismanagement seems.
That forests regulate water run-off, mitigating risks of flooding and drought, has been recognised since ancient times. The ancients also understood that trees can increase rainfall and deforestation can reduce it. Cutting down trees leads to a reduction in evapotranspiration, which results in less downwind precipitation.