Productivity and Carbon Sequestration in Pure and Mixed Tropical Forest Plantations in Western Mexico

Commercial forest plantations (CFPs) provide timber and ecosystem services, particularly carbon (C) sequestration, but the performance of native tropical hardwoods in pure versus mixed systems is still poorly understood. We evaluated growth, productivity, biomass, and C storage in 17-year-old plantations of Tabebuia rosea, T. donnell-smithii, and Swietenia humilis in western Mexico. Four plantation systems were assessed: pure T. rosea (PPT1), pure T. donnell-smithii (PPT2), mixed T. rosea + T. donnell-smithii (MPT1T2), and mixed T. donnell-smithii + S. humilis (MPT2S). Tree structure (DBH, height, basal area, volume), litter layer, and soils (0–15 cm) were measured. Thirty trees per species were destructively sampled to develop species-specific biometric models. Model performance was evaluated with adjusted R², RMSE, and residual analysis. PPT1 was the most productive system (39.8 m³ ha⁻¹; 55 Mg C ha⁻¹), while PPT2 had the lowest values (20.5 m³ ha⁻¹; 45.1 Mg C ha⁻¹). MPT1T2 increased basal area (+29.8% vs. PPT1) and litter layer C (3.3 Mg C ha⁻¹; +190% vs. PPT2) but did not surpass PPT1 in standing volume. Soil C was highest in PPT1 (36.5 Mg C ha⁻¹). Biometric models achieved high accuracy (R² = 0.91–0.99), confirming DBH as a reliable predictor of biomass and C. We conclude that pure T. rosea maximizes short-term productivity and soil C, whereas mixed systems diversify C allocation by enhancing litter layer pools. These findings highlight the complementary roles of pure and mixed CFPs and provide reliable models for C accounting in tropical hardwood plantations.