Structure and diversity of secondary tropical dry forests in Mexico, differing in their prior land-use history.
We compared the structure and diversity of secondary tropical dry forests, growing in sites cleared by bulldozer 30 years ago and subjected to different land uses prior to abandonment: (a) housing development (HD), (b) pasture (P), and (c) no use (NU). We sampled two forest sites representing each of the three former land use types (i.e., a total of six sites) and measured the diameter at breast height (dbh) of all stems with dbh ≥1 cm and at least 2 m in height. All individuals were identified to species and classified into each of four dbh size classes: 1-3, 3-5, 5-10, and >10 cm. No major differences in structure or diversity were evident among land use types. Total tree densities varied from 2575±55 to 4745±985 individuals/ha and total basal areas ranged from 12.5±2.6 to 14.6±0.3 m2/ha (mean and S.E.). No significant difference (p<0.05) were found among land use types. Overall, the most abundant tree size class was the 1-3 cm dbh (47% of the total number of trees), whereas trees greater than 10 cm dbh were the least abundant (4%). We found significantly higher density and basal area (p=0.01) in NU than in HD and P only in the smallest trees (1-3 cm dbh). We identified a total of 87 species in 28 families. The Leguminosae plant family had the greatest species richness and number of individuals in all land use types. The HD land use type showed lower total species richness than P and NU only with two of the four nonparametric estimators. Observed species richness decreased as dbh increased in all land use types. At least 5 legumes were among the 10 most dominant species in all land use types, of which the non-native Mimosa arenosa was the most dominant one. It represented between 42 and 52% of the total basal area in each land use type. The secondary forests have reached, on average, 74 and 22% of the total density and basal area, respectively, of an average primary tropical dry forest of Mexico. The high number of individuals with 1-3 cm dbh (48% of the total and 83% of the species) suggest that regeneration is occurring in all land use types, although to different degrees. The absence of small individuals of M. arenosa and a high abundance of primary forest species at NU, suggest that M. arenosa is being replaced at these sites.