Trees as net sinks for methane (CH4 ) and nitrous oxide (N2 O) in the lowland tropical rain forest on volcanic Réunion Island

Abstract

Trees are known to emit methane (CH₄ ) and nitrous oxide (N₂ O), with tropical wetland trees being considerable CH₄ sources. Little is known about CH₄ and especially N₂ O exchange of trees growing in tropical rain forests under nonflooded conditions. We determined CH₄ and N₂ O exchange of stems of six dominant tree species, cryptogamic stem covers, soils and volcanic surfaces at the start of the rainy season in a 400-yr-old tropical lowland rain forest situated on a basaltic lava flow (Réunion Island). We aimed to understand the unknown role in greenhouse gas fluxes of these atypical tropical rain forests on basaltic lava flows. The stems studied were net sinks for atmospheric CH₄ and N₂ O, as were cryptogams, which seemed to be co-responsible for the stem uptake. In contrast with more commonly studied rain forests, the soil and previously unexplored volcanic surfaces consumed CH₄ . Their N₂ O fluxes were negligible. Greenhouse gas uptake potential by trees and cryptogams constitutes a novel and unique finding, thus showing that plants can serve not only as emitters, but also as consumers of CH₄ and N₂ O. The volcanic tropical lowland rain forest appears to be an important CH₄ sink, as well as a possible N₂ O sink.

Keywords: basaltic lava flows; cryptogams; methane flux; nitrous oxide flux; soil; tree stem; tropical lowland rain forest; uptake.

Fig. 1

View of study set‐up and chamber systems used to determine greenhouse gas fluxes in the lowland tropical rain forest on volcanic Réunion Island. Overall view of the site: stem chambers installed in the vertical profile of tree stems (a–c), soil chambers in the vicinity of studied trees (a, b; marked with white circle; and d), volcanic rock chambers fixed on basaltic lava flow (c, marked with white asterisk), cryptogamic stem covers in incubation chambers (e, f), and portable greenhouse gas analyzer used for gas concentration measurements (a, b).

Fig. 2

Fluxes of methane (CH₄) (a), nitrous oxide (N₂O) (b), and carbon dioxide (CO₂) (c) from adjacent soil measured close to each studied tree and from volcanic rock surfaces of basaltic lava flows. The fluxes are expressed as medians (solid lines) and means (dashed lines) of measurements from soil positions in the vicinity of trees of each individual tree species (Syzbor, Syzygium borbonicum (n = 5); Dorape, Doratoxylon apetalum (n = 5); Antbor, Antirhea borbonica (n = 5); Hompan, Homalium paniculatum (n = 3); Mimbal, Mimusops balata (n = 3); Labcal, Labourdonnaisia calophylloides (n = 3)), from all studied soil positions (‘All soil positions’, n = 24), and from volcanic rocks (n = 8). The forest floor in the studied forest is approximately equally covered with soil and volcanic surfaces without soil cover (proportion roughly 50 : 50). Fluxes are expressed per m²of soil and volcanic rock area. Positive fluxes indicate trace gas emission; and negative fluxes trace gas uptake. The box boundaries mark the 25^th and 75^th percentiles and whiskers the 10^th and 90^th percentiles. Dots mark outliers. As indicated by the letter ‘a’ above each bar, there were no statistically significant differences among fluxes in adjacent soil of individual tree species at P < 0.05. One‐way ANOVA was applied for CH₄ fluxes and Kruskal–Wallis one‐way ANOVA on ranks was used for N₂O and CO₂ fluxes.

Fig. 3

Fluxes of methane (CH₄) (a), nitrous oxide (N₂O) (b), and carbon dioxide (CO₂) (c) from tree stems and cryptogamic stem covers. The fluxes are expressed as medians (solid lines) and means (dashed lines) of measurements from stems of six individual tree species (Syzbor, Syzygium borbonicum (n = 5); Dorape, Doratoxylon apetalum (n = 5); Antbor, Antirhea borbonica (n = 5); Hompan, Homalium paniculatum (n = 3); Mimbal, Mimusops balata (n = 3); Labcal, Labourdonnaisia calophylloides (n = 3)), from all studied trees and tree species (‘All trees’, n = 24), and from cryptogams (Pyrrhobryum spiniforme, Leucoloma capillifolium; n = 4). The trace gas exchange of cryptogams is presented as mean and median of all measurements under low light and dark conditions, because CH₄ and N₂O fluxes did not differ in relation to light conditions. The mean and median fluxes of CO₂ in cryptogams include both CO₂ emission and uptake measured under dark and light conditions, resulting in low gas exchange (more details in manuscript text). All fluxes, including fluxes from cryptogams, are expressed per m² of stem area. Positive fluxes indicate trace gas emission and negative fluxes trace gas uptake. The box boundaries mark the 25^th and 75^th percentiles and whiskers the 10^th and 90^th percentiles. Dots mark outliers. Statistically significant differences among fluxes in individual tree species at P < 0.05 are indicated by different letters above bars. One‐way ANOVA was applied for CH₄ and N₂O fluxes and Kruskal–Wallis one‐way ANOVA on ranks was used for CO₂ fluxes.

Fig. 4

Fluxes of methane (CH₄) (a), nitrous oxide (N₂O) (b), and carbon dioxide (CO₂) (c) from tree stem vertical profiles. The fluxes are expressed as medians (solid lines) and means (dashed lines) of measurements from trees of three tree species: Syzygium borbonicum (n = 3), Doratoxylon apetalum (n = 3) and Antirhea borbonica (n = 3). The measurements were performed at three stem heights of c. 0.4, 1.1 and 1.8 m aboveground. All fluxes are expressed per m² of stem area. Positive fluxes indicate trace gas emission and negative fluxes trace gas uptake. The box boundaries mark the 25^th and 75^th percentiles and whiskers the 10^th and 90^th percentiles. As indicated by the letter ‘a’ above each bar, there were no statistically significant differences in fluxes among stem heights at P < 0.05. One‐way ANOVA was applied for CH₄ and N₂O fluxes and Kruskal–Wallis one‐way ANOVA on ranks was used for CO₂ fluxes.

Fig. 5

Relationships between methane (CH₄) vs carbon dioxide (CO₂) stem fluxes (a) and nitrous oxide (N₂O) vs CO₂ stem fluxes (b). All six tree species (24 trees in total) are included. All fluxes are expressed per m²of stem area. Positive flux values indicate gas emission and negative values indicate gas uptake.

Fig. 6

Fluxes of methane (CH₄) (a, b), nitrous oxide (N₂O) (c, d) and carbon dioxide (CO₂) (e, f) from tree stems and adjacent soil expressed per stem or soil surface area unit (a, c, e) and scaled up to unit ground area of the tropical rain forest (b, d, f). The fluxes are expressed as medians (solid lines) and means (dashed lines) of measurements from all studied trees of six tree species (n = 24) and all studied soil positions (n = 24). Positive fluxes indicate trace gas emission; negative fluxes trace gas uptake. The box boundaries mark the 25^th and 75^th percentiles and whiskers the 10^th and 90^th percentiles. Statistically significant differences in fluxes at tree stem and soil level at P < 0.05 are indicated by different letters above the bars. Mann–Whitney rank‐sum test was applied for the gas flux pairs. The contributions of stem fluxes of six tree species to the soil fluxes (equal to 100%) are expressed as percentages of the soil flux.