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 mu mol center dot m(-2)center dot 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 center dot m(-2)center dot s(-1). Pioneer species exhibited a higher mean N2O flux (0.81 +/- 0.17 pmol center dot m(-2)center dot s(-1)) compared to late-successional species (0.37 +/- 0.05 pmol center dot m(-2)center dot s(-1)). Pioneer species also showed a trend toward a higher mean CH4 flux (0.24 +/- 0.21 nmol center dot m(-2)center dot s(-1)) compared to mid-successional (-0.01 +/- 0.26 nmol center dot m(-2)center dot s(-1)) and late-successional species (-0.05 +/- 0.28 nmol center dot m(-2)center dot 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.