There's not much you can do about protruding roots. If you cut or bury the roots, you can injure or kill the trees. The best thing you can do for the trees and your mower is to eliminate grass around the roots. Cover the area with loose mulch, such as pine needles or shredded bark. Or plant a low-maintenance, shade-tolerant ground cover in the area to ramble among the roots, eliminating the need to mow. Suitable ground covers for shady areas include ajuga (Ajuga), barrenwort (Epimedium), lungwort (Pulmonaria), sweet woodruff (Galium odoratum), periwinkle (Vinca minor), and wild ginger (Asarum).