Wet canopy photosynthesis in a temperate Japanese cypress forest

This study aimed to reveal the mechanism and significance of wet canopy photosynthesis during and after rainfall in temperate coniferous ecosystems by evaluating the influence of abaxial leaf interception on wet canopy photosynthesis. We used the eddy covariance method in conjunction with an enclosed-path gas analyser to conduct continuous ecosystem CO2 flux observations in a Japanese cypress forest within the temperate Asian monsoon area over 3 years. The observation shows that wet-canopy CO2 uptake predominantly occurred during the post-rainfall canopy-wet period rather than the during-rainfall period. Then, the measured canopy-wet net ecosystem exchange was compared with the soil-vegetation-atmosphere transfer multilayer model simulations under different parameter settings of the abaxial (lower) leaf surface wet area ratio. The multilayer model predicted net ecosystem exchange most accurately when it assumed the wet area ratio of the abaxial surface was 50% both during and after rainfall. For the wet canopy both during and after rainfall, the model overestimated CO2 uptake when it assumed no abaxial interception in the simulation, but underestimated CO2 uptake when it assumed that the entire abaxial leaf surface was wet. These results suggest that the abaxial surface of the Japanese cypress leaf is only partly wet to maintain stomatal openness and a low level of photosynthesis. These results allow for an evaluation of the effect of rainfall on forest carbon circulation under a changing climate, facilitating an improvement of ecosystem carbon exchange models.

Insights into the mechanism of diurnal variations in methane emission from the stem surfaces of Alnus japonica

Recent studies have suggested that in certain environments, tree stems emit methane (CH₄ ). This study explored the mechanism of CH₄ emission from the stem surfaces of Alnus japonica in a riparian wetland. Stem CH₄ emission rates and sap flux were monitored year-round, and fine-root anatomy was investigated. CH₄ emission rates were estimated using a closed-chamber method. Sap flux was measured using Granier-type thermal dissipation probes. Root anatomy was studied using both optical and cryo-scanning electron microscopy. CH₄ emissions during the leafy season exhibited a diurnally changing component superimposed upon an underlying continuum in which the diurnal variation was in phase with sap flux. We propose a model in which stem CH₄ emission involves at least two processes: a sap flux-dependent component responsible for the diurnal changes, and a sap flux-independent component responsible for the background continuum. The contribution ratios of the two processes are season-dependent. The background continuum possibly resulted from the diffusive transport of gaseous CH₄ from the roots to the upper trunk. Root anatomy analysis indicated that the intercellular space of the cortex and empty xylem cells in fine roots could serve as a passageway for transport of gaseous CH₄ .

Identifying dominant environmental predictors of freshwater wetland methane fluxes across diurnal to seasonal time scales

While wetlands are the largest natural source of methane (CH₄ ) to the atmosphere, they represent a large source of uncertainty in the global CH₄ budget due to the complex biogeochemical controls on CH₄ dynamics. Here we present, to our knowledge, the first multi-site synthesis of how predictors of CH₄ fluxes (FCH4) in freshwater wetlands vary across wetland types at diel, multiday (synoptic), and seasonal time scales. We used several statistical approaches (correlation analysis, generalized additive modeling, mutual information, and random forests) in a wavelet-based multi-resolution framework to assess the importance of environmental predictors, nonlinearities and lags on FCH4 across 23 eddy covariance sites. Seasonally, soil and air temperature were dominant predictors of FCH4 at sites with smaller seasonal variation in water table depth (WTD). In contrast, WTD was the dominant predictor for wetlands with smaller variations in temperature (e.g., seasonal tropical/subtropical wetlands). Changes in seasonal FCH4 lagged fluctuations in WTD by ~17 ± 11 days, and lagged air and soil temperature by median values of 8 ± 16 and 5 ± 15 days, respectively. Temperature and WTD were also dominant predictors at the multiday scale. Atmospheric pressure (PA) was another important multiday scale predictor for peat-dominated sites, with drops in PA coinciding with synchronous releases of CH₄ . At the diel scale, synchronous relationships with latent heat flux and vapor pressure deficit suggest that physical processes controlling evaporation and boundary layer mixing exert similar controls on CH₄ volatilization, and suggest the influence of pressurized ventilation in aerenchymatous vegetation. In addition, 1- to 4-h lagged relationships with ecosystem photosynthesis indicate recent carbon substrates, such as root exudates, may also control FCH4. By addressing issues of scale, asynchrony, and nonlinearity, this work improves understanding of the predictors and timing of wetland FCH4 that can inform future studies and models, and help constrain wetland CH₄ emissions.

Ecosystem-scale methane flux in tropical peat swamp forest in Indonesia

Data on ecosystem-scale methane (CH₄ ) fluxes in tropical peatlands are currently lacking in the global CH₄ budget. Although the waterlogged Indonesian peatlands contain the largest share of peat carbon in South-East Asia, ecosystem-scale CH₄ budgets have not yet been reported, although these peatlands have the potential to emit CH₄ . We observed 1-year variations in the ecosystem-scale CH₄ flux in an undrained secondary peat swamp forest in central Kalimantan, Indonesia, using the eddy covariance method. We found that the peat swamp forest switched from being a CH₄ sink during the dry season (as low as -8.9 mg C m^-2  day^-1 ) to a source of CH₄ during the wet season (up to 10.7 mg C m^-2  day^-1 ), and this was dependent on changes in the groundwater level (GWL). The high GWL during the wet season enhanced the anaerobic CH₄ production in the surface layer that had more labile organic matter. However, the CH₄ emission also increased when the GWL dropped during dry spells in the wet season. The annual CH₄ budget in the studied tropical peat swamp forest (0.09-0.17 g C m^-2  year^-1 ) was much lower than that in northern, temperate, and subtropical wetlands. We found that CH₄ fluxes had almost no effect on the global warming gas budget of the peat swamp forest, and values were only a few percent less than the CO₂ fluxes at the same site. In addition, we conducted anaerobic soil incubation experiments to examine the effect of land-use change on CH₄ production. The results indicated much higher CH₄ production potential in undrained forest soil than in drained or drained and burned ex-forest soils. However, although CH₄ production decreased in drained soils relative to undrained soils, conserving pristine peat swamp forests with high GWLs is important to suppress global warming because CO₂ emissions increase in drained peatlands.