How did Avatar Earth become totally deforested?

[PunkMaister]

I do have to ask because it cannot be that people decided to mow down all national parks in fact if Obama was to declare that he is going to mow down Yellowstone, cover up the grand canyon to make a Stadium etc, people would have his head on a pike the next day, seriously!
It had to be a disease brought by climate change or something along those lines.