Composition and diversity of the vegetation in four sites of Mexico’s Northeast Tekijät: Ramírez-Lozano, Roque G., Domínguez-Gómez, Tilo G., González-Rodríguez, Humberto, Cantú-Silva, Israel, Gómez-Meza, Marco V., Sarquís-Ramírez, Jorge I., Jurado, Enrique