@PhantomNVD - I was also confused about how tree planting ensures the electricity grid runs entirely from renewables.
However, looks like that sentence is a list of things to do for the govt i.e. "...hoping to make the nation greener by: 1) planting 100 million trees each year, 2) ensuring the electricity grid runs entirely from renewable energy, and 3) spending more money on cycle ways and rail transport."
The trees soak up carbon dioxide for decades. So they help balance emissions. As long as you don't burn then, the CO2 remains locked up. Make chairs and tables and build houses.