How do evergreen tree leaves and needle litter increase the soil acidity?

Question

I know that these plants do good with very little nutrients in the soil but how does this help there conditions.

Answer 1

some tree, cedar are a acidic plant, and evergreens are of this family in nature.  The only plant that grows under a evergreen tree and lives is a rose bushes that I know of.

Answer 2

Evergreen needles and litter contain soluble tannic acid which leaches into the soil.

This acid might 'free up' some minerals bound into insoluble carbonates.