Influence of the density of scattered trees in pastures on the structure and species composition of tree and grass cover in southern Tabasco, Mexico

The rapid deforestation of Tabasco due to extensive livestock farming has resulted in a decrease in the original forest cover. Silvopastoral systems represent an option for reversing this problem. A comparative study was conducted to determine the influence of tree density on the structure and species composition of scattered trees in pastures (STP) as well as the herbaceous coverage. A total of 16 livestock systems with 3 STP densities were selected: low (from 24 to 49 individuals ha⁻¹), medium (from 53 to 85 individuals ha⁻¹) and high (from 129 to 212 individuals ha⁻¹. The diameter at breast height, total height, clear bole height and crown area of all trees in each plot were measured. Shannon's and Simpson's index values were determined along with the relative importance values (RIVs), and the herbaceous cover under the tree canopy was measured in dry and rainy seasons. A total of 64 species representing 31 botanical families were found. The structure and composition of the STPs are correlated with arboreal density, and thus, species diversity was greater under low and medium densities. The tree species with the highest RIVs were Ceiba pentandra (L.) Gaertn., Guazuma ulmifolia (Lam.), and Tabebuia rosea (Bertol.) DC. in low-density STPs; Cordia alliodora (Ruiz & Pav.) Oken., Schizolobium parahyba (Vell.) S.F. Blak., and T. rosea in medium-density STPs and Swietenia macrophylla (King.), Gmelina arborea (Roxb.), and C. alliodora in high-density STPs. The herbaceous cover was lower under high arboreal densities in both seasons. The STPs at high densities are inferred to be more specialized because they present less arboreal species diversity and structure than low-density STPs, and these properties also affect the coverage of the associated herbaceous component.