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

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.