Tree species diversity increases carbon stocks in tropical montane cloud forests across successional stages

The tropical montane cloud forest (TMCF) is a structurally complex and hyper-diverse ecosystem that provides critical ecosystem services. However, it has undergone significant transformation and its role in carbon (C) storage remains poorly understood. This study aims to evaluate the current condition of the TMCF in Mexico, focusing on its structure, C storage, and tree diversity, and to assess the role of tree diversity in enhancing C stock across successional stages. We analyzed data from the Mexican National Forest and Soil Inventory 2015–2020. The data represented primary, secondary-arboreal, and secondary-shrubby TMCF from 174 sites distributed throughout the country. For tree diversity Hill numbers of order q = 0 (species richness), q = 1 (common species), and q = 2 (dominant species) were calculated per site. Basal area was 23.6 m²/ha in primary, 17.4 m²/ha in secondary-arboreal, and 12.8 m²/ha in secondary-shrubby TMCF. In total, 527 tree and shrub species were recorded, with 327 species in the primary, 266 in the secondary-arboreal, and 226 in the secondary-shrubby forest categories. Oaks were the most diverse (with 35 species), widely distributed, and dominant group, contributing 39 % of the basal area in primary, 31 % in secondary-arboreal, and 41 % in secondary-shrubby forest. Oaks accounted for approximately 39 % of the aboveground C in all forest categories, with 19.6 tons C/ha in primary, 15.0 tons C/ha in secondary-arboreal, and 13.6 tons C/ha in secondary-shrubby forest. C stock increased with tree richness, common and dominant species, in both primary and secondary-arboreal forests. In secondary-shrubby forest, C stock initially increased with tree richness, common and dominant species, but subsequently declined. For an average stand, each additional tree species could potentially increase C storage by 6 % in primary, 7 % in secondary-arboreal, and 13 % in secondary-shrubby forests. Old remnant trees from previous anthropogenic disturbances are common in secondary forests, playing a major role in C storage, regeneration, and resilience. Our findings underscore the critical role of tree diversity in enhancing C storage across successional stages in TMCF. Forest management strategies that prioritize the preservation of old remnant trees and promote multi-species plantations are essential for maximizing C sequestration and maintaining biodiversity.