I believe they cannot start with big trees and bushes because the there is not enough nutrients in the soil, so instead pioneer species pave the way for primary succession.
These fuels are nonrenewable because they are limited substances which cannot be produced at the same rate at which it is used. Also, they cannot be REused, hence the wording non-REnewable. Hope this answers your question.
The savanna is very dry and desert like. Grasslands get precipitation and have tons of grass, hence the name grasslands.
