Foliar methane and nitrous oxide fluxes in tropical tree species

Methane (CH₄) and nitrous oxide (N₂O) are critical biogenic greenhouse gases (GHGs) with global warming potentials substantially greater than that of carbon dioxide (CO₂). The exchange of these gases in tropical forests, particularly via foliar processes, remains poorly understood. We quantified foliar CH₄ and N₂O fluxes among tropical tree species and examined their potential association with the leaf economics spectrum (LES) traits. Sampling within Lawachara National Park, Bangladesh, we used in-situ measurements of foliar CH₄ and N₂O fluxes employing off-axis integrated cavity output spectroscopy (CH₄, CO₂ and H₂O) and optical feedback-cavity enhanced absorption spectroscopy (N₂O) analyzers. Leaves were measured under dark, low, and high (0, 100, and 1000 μmol·m^-2·s^-1) light conditions. Surveyed tree species exhibited both net foliar uptake and efflux of CH₄, with a mean flux not different from zero, suggesting negligible net foliar emissions at the stand level. Plant families showed differences in CH₄, but not N₂O fluxes. Consistent efflux was observed for N₂O, with a mean of 0.562 ± 0.060 pmol·m^-2·s^-1. Pioneer species exhibited a higher mean N₂O flux (0.81 ± 0.17 pmol·m^-2·s^-1) compared to late-successional species (0.37 ± 0.05 pmol·m^-2·s^-1). Pioneer species also showed a trend toward a higher mean CH₄ flux (0.24 ± 0.21 nmol·m^-2·s^-1) compared to mid-successional (-0.01 ± 0.26 nmol·m^-2·s^-1) and late-successional species (-0.05 ± 0.28 nmol·m^-2·s^-1). Moreover, among all leaf traits within the leaf economic spectrum, a significant positive relationship was observed between leaf N₂O flux and total leaf nitrogen. Our results suggest that pioneer tree species significantly contribute to net CH₄ and N₂O emissions, potentially counteracting the carbon sequestration benefits in regenerating tropical forests. These findings indicate that accurate GHG budgeting should include direct measurements of foliar CH₄ and N₂O fluxes. Moreover, the results suggest that forest conservation and management strategies that prioritize late successional species will better mitigate GHG emissions.