Tree stem and soil CH4 and N2O fluxes from peat soils of the tropical cloud forest of Réunion Island

Tropical peatlands are important sources or sinks of greenhouse gases CH₄ and N₂O. However, comprehensive greenhouse gas flux assessments that incorporate different compartments of the ecosystem are scarce. The sensitivity of peatland greenhouse gas fluxes to hydrologic variability adds to the large uncertainty. Greenhouse gas dynamics of tropical Réunion Island peatland forests has not been previously studied. In addition, tree stems in tropical peatlands have been shown to emit CH₄ under waterlogged soil conditions, but further knowledge of these fluxes under different environmental and hydrological conditions is needed.

We aimed to quantify the fluxes of CH₄ and N₂O from tree stems and soil in high-altitude (1500-1600 m a.s.l.) cloud forest areas on peat soil on Réunion Island. Two study sites were examined during the dryer season – Plaine des Cafres and Forêt de Bébour. Stem fluxes were determined from tree heather Erica reunionensis (both study sites) and tree fern Alsophila glaucifolia (Plaine des Cafres) using static chamber systems mounted on tree stems, connected to trace gas analysers LI-COR LI-7810 (CH₄) and LI-7820 (N₂O), which measured gas concentration changes in chamber headspace during 10 minutes. Soil gases were sampled using static soil chamber systems at 20-minute intervals during one-hour sessions and analysed with gas chromatography (Shimadzu GC-2014). Soil environmental parameters were measured simultaneously with gas measurements at each site.

Preliminary results show that stems of Erica emitted negligible amounts of CH₄ (0.3 ± 2.71 (mean ± standard error) µg C m⁻² h⁻¹ at Plaine des Cafres and 0.58 ± 0.26 µg C m⁻² h⁻¹ at Forêt de Bébour) and small amounts of N₂O (6.25 ± 2.37 µg N m⁻² h⁻¹ and 1.43 ± 4.65 µg N m⁻² h⁻¹, respectively). Tree ferns took up CH₄ from the atmosphere (−21.45 ± 10.75 µg C m⁻² h⁻¹) but had negligible N₂O fluxes. Soils at both study sites were sinks of CH₄ (−12.41 ± 11.37 µg C m⁻² h⁻¹ and −21.5 ± 5.91 µg C m⁻² h⁻¹) and small sources of N₂O (1.06 ± 0.38 µg N m⁻² h⁻¹ and 0.37 ± 0.72 µg N m⁻² h⁻¹).

Our results indicate that stems of Erica reunionensis and Alsophila glaucifolia do not emit significant amounts of CH₄ and N₂O to the atmosphere in the tropical peatlands of Réunion Island during the dryer season. Nevertheless, it is crucial to further monitor greenhouse gas emissions for a longer period to clarify spatio-temporal dynamics in different environmental conditions. In addition, the consumption of CH₄ by tree ferns shows that variability of  greenhouse gas fluxes from stems of different tree species needs further attention to determine the contribution of trees to total ecosystem greenhouse gas budgets for improved global assessments.