The bark side of mangrove methane fluxes: Anatomical insights of the root of emissions in Rhizophora forests

Tropical wetlands are the largest natural source of methane (CH₄) globally. While soils are the primary CH₄ emitters, recent studies reveal that trees mediate soil-produced methane, highlighting potential underestimations in current methane and carbon budgets. In mangrove forests, the contribution of trees to local, regional, and global CH₄ emissions remains uncertain, despite their significant role as blue carbon reservoirs. Mangrove species exhibit unique physiological and morpho-anatomical adaptations—such as extensive aerenchyma tissues and lenticels—that facilitate gas exchange through their bark and roots. In this study, we assessed the spatiotemporal variation of CH₄ emissions from tree stems and stilt roots of three mangrove species (Rhizophora mangle, Avicennia germinans, and Laguncularia racemosa) across distinct ecological types (i.e., scrub, basin and hammock) during the rainy and dry seasons in the Ría Celestún Biosphere Reserve (Mexico). We also investigated the relationship between bark anatomical traits, aerenchyma development, and CH₄ fluxes. Our findings revealed that CH₄ emissions varied by species, tissue type, and season. Scrub R. mangle showed the highest CH₄ emission rates from both tree stems and stilt roots, particularly in near-ground tissues like third-order stilt roots with abundant bark aerenchyma, whereas basin mangrove forests had the lowest emissions, particularly in A. germinans and L. racemosa. Methane emissions increased during the rainy season and were positively correlated with bark (aerenchyma) proportion, lenticel density but negatively with wood density. To date, tree stem CH₄ emissions have been documented in eleven mangrove species globally. Our results underscore the need to refine local-to-global carbon models by integrating bark anatomy and tree-mediated CH₄ emissions. Moreover, mangrove trees can act as either CH₄ sources or sinks, depending on physicochemical and microenvironmental conditions. Understanding these dynamics requires a comprehensive approach rooted in plant physiology and anatomy.