"What happens when the trees stop growing? "

Why would a tree stop growing?

There are trees that are over a thousand years old and still converting CO2 to O2.

Destruction of the rain forest is contributing to the greenhaus effect but Obama says we should cut trees and make fuel out of them?

That makes me laugh.

The ultimate goal is to stop burning carbon based fuels altogether. Whatever money spent on developing this temporary source of energy should be spent on long term fix. Wind, solar, nuclear or hydroelectric.

In the mean time use the oil and gas we already have.

Actually the ultimate goal for energy generation is the Tokamak.