I think an easier to understand way to look at it is that every square meter of forest is enough to offset 1 or 2 watts of carbon based energy consumption if we assume that the trees don't die and decay, no forest fires, etc. We know that's not enough to sustain a modern civilization because past civilizations were dependent on energy from wood and coulndn't sustain a much lower standard of living even with a much lower population density. By contrast solar can sustain on the order of 100 watts per square meter and can do it even in areas without enough rainfall for forests.