Potential and efficiency of seed production of Pinus engelmannii Carr., in three seed stands of the state of Durango, Mexico Autores: Bustamante-García, Verónica, Prieto-Ruíz, José Ángel, Merlín-Bermudes, Enrique, Álvarez-Zagoya, Rebeca, Carrillo-Parra, Artemio, Hernández-Díaz, José Ciro