Which tree species produces the nuts called pecans?

The tree species that produces pecans is Carya illinoinensis.

Carya illinoinensis is a hickory in the walnut family, Juglandaceae. It is native to the south-central United States and northern Mexico, especially fertile bottomlands near the Mississippi River. The tree can reach 20–40 metres and produces a husked fruit whose edible seed is the pecan.

Although people call it a nut, a pecan is botanically a drupe: its outer husk develops from floral tissues, while the familiar hard shell surrounds the seed. This makes it different from a true botanical nut. Carya ovata, one of the alternatives, is shagbark hickory and does not produce the commercial pecan.

Indigenous peoples gathered wild pecans long before commercial orchards developed. Spanish explorers encountered them in the 16th century, while large-scale cultivation in the United States began in the 1880s. Pecans are now strongly associated with pecan pie, pralines, snack foods and smoked meats.