Foliar Methane and Nitrous Oxide Fluxes: A Comprehensive Study in Tropical Forest Ecosystems

Abstract

Comprising 45% of global forest cover, tropical forests are pivotal in the GHG budgets. Emerging research highlights the significance of tropical trees as CH4 sources, yet tree-foliage emissions have been minimally investigated. Moreover, the N2O fluxes from tropical tree foliage remain almost completely unexamined. Objectives: This study presents a comprehensive survey of foliar CH4 and N2O fluxes across tropical forest tree species using integrated output spectroscopy and a purpose-built cuvette system for accurate in-situ flux rate measurements. It tests two key hypotheses: (1) broadleaf trees in well-drained soils of tropical forests exhibit foliar CH4 oxidation; (2) foliar CH4 and N2O flux patterns vary systematically among ecological and phylogenetic groups. Methods: We measured foliar fluxes from 120 trees across 40 species within Lawachara National Park, Bangladesh, an upland mixed-tropical-evergreen forest, prioritizing diverse shade-tolerant canopy trees. We utilized a dynamic leaf chamber (CS-LC7000) with continuous gas flow and portable CH4 (LGR 915-001) and N2O (LI-7820) analyzers, alongside concurrent measurements of CO2 and H2O flux. In addition to gas flux data, our study incorporated leaf trait measurements (of leaf mass per area and leaf N content). Results: Across all samples, the mean CH4 flux of 0.016 nmol m-2 s-1 did not display a significant deviation from zero (t = 19.44, df = 827, p > 0.05). In contrast, the mean N2O flux 0.54 nmol m-2 s-1, exhibited a significant elevation above zero (t = 19.42, df = 827, p < 0.001), indicating notable N2O emissions on average. Methane flux varied among species and various ecological successional groups, namely pioneer, mid-successional, and late successional species (F = 5.99, df = 2, p < 0.01). Pioneer species, which were sources of CH­4, demonstrated significantly higher CH4 flux compared to both mid (p < 0.01) and late successional (p < 0.05) species, which both acted as weak CH4 sinks. All ecological groups were sources of N2O, with significant variations among the ecological successional groups (F = 12.97, df = 2, p < 0.01). Pioneer species were identified as the highest emitters of N2O, followed by mid and late-successional species. A comparative CH4 flux analysis among the 28 families revealed significant variability (F = 47.7, df = 27, p < 0.01), with certain species acting as sources and others as sinks of CH4. Notably, 11 families were classified as CH4 sources, while the remainder functioned as sinks. Meliaceae emerged as having the highest average CH4 emissions, and Thymeliaceae the greatest CH4 consumption. Similarly, a distinct variation in N2O flux was observed among families (F = 6.57, df = 27, p < 0.01), with Sapindaceae showing the highest, and Rubiaceae and Euphorbiaceae the lowest N2O emissions. Conclusions: This study on foliar CH4 and N2O fluxes in tropical forests reveals trees' crucial role in greenhouse gas emissions. Pioneer species emerge as major emitters of both CH4 and N2O, suggesting that foliar emissions of these GHGs may be pronounced in secondary forests, and hence the importance of conserving intact forests dominated by later-successional species.