The end paper product (paper, paperboard, tissue etc.) dictates the species, or species blend, that is best suited to provide the desireable sheet characteristics, and also dictates the required fiber processing (chemical treatment, heat treatment, mechanical 'brushing' or refining etc.).
In North America, virgin (non-recylced) wood fiber is primarily extracted from hardwood (deciduous) trees and softwood (coniferous) trees, of which there is an abundant and (if properly managed) virtually inexhaustible supply.
These treated wood fibres (which are often combined with other additives) are then processed into a network of wood fibers, which constitutes the sheet of paper.