Ayaka Sakabe

Determination of the gas exchange phenology in an evergreen coniferous forest from 7 years of eddy covariance flux data using an extended big-leaf analysis

We defined gas exchange phenology as the seasonality of the gas exchange characteristics of a forest canopy, and investigated how the gas exchange phenology could be directly detected from an eddy covariance (EC) dataset and its influence on the canopy fluxes within an evergreen Japanese cypress forest. For the detection of gas exchange phenology, we derived three bulk parameters of the extended big-leaf model (Kosugi et al. 2005) inversely from EC flux data over a 7-year period: surface conductance (gc), maximum rate of carboxylation of the “big leaf” (VCMAX), and intercellular CO2 concentration of the “big leaf” (CI). The relationship between gc and the vapor pressure deficit declined in winter and spring. The relationship between the daily ecosystem respiration and air temperature was greater in the spring than in the other seasons. The temperature dependence curve of VCMAX decreased substantially in the winter and was different from that of an evergreen broadleaved forest. A decrease in CI was occasionally coupled with the decrease in canopy gross primary production during April and August, indicating that stomatal closure was responsible for a decline in canopy photosynthesis. Gas exchange phenology should be quantified when understanding the determining factors of the seasonality of canopy fluxes at evergreen coniferous forests.

One year of continuous measurements of soil CH4 and CO2 fluxes in a Japanese cypress forest: Temporal and spatial variations associated with Asian monsoon rainfall

We examined the effects of Asian monsoon rainfall on CH4 absorption of water-unsaturated forest soil. We conducted a 1 year continuous measurement of soil CH4 and CO2 fluxes with automated chamber systems in three plots with different soil characteristics and water content to investigate how temporal variations in CH4 fluxes vary with the soil environment. CH4 absorption was reduced by the “Baiu” summer rainfall event and peaked during the subsequent hot, dry period. Although CH4 absorption and CO2 emission typically increased as soil temperature increased, the temperature dependence of CH4 varied more than that of CO2, possibly due to the changing balance of activities between methanotrophs and methanogens occurring over a wide temperature range, which was strongly affected by soil water content. In short time intervals (30 min), the responses of CH4 and CO2 fluxes to rainfall were different for each plot. In a dry soil plot with a thick humus layer, both fluxes decreased abruptly at the peak of rainfall intensity. After rainfall, CO2 emission increased quickly, while CH4 absorption increased gradually. Release of accumulated CO2 underground and restriction and recovery of CH4 and CO2 exchange between soil and air determined flux responses to rainfall. In a wet soil plot and a dry soil plot with a thinner humus layer, abrupt decreases in CH4 fluxes were not observed. Consequently, the Asian monsoon rainfall strongly influenced temporal variations in CH4 fluxes, and the differences in flux responses to environmental factors among plots caused large variability in annual budgets of CH4 fluxes.

Impacts of riparian wetlands on the seasonal variations of watershed-scale methane budget in a temperate monsoonal forest

Forest soils are considered a methane (CH4) sink because dry soils can oxidize CH4; however, previous studies on CH4 fluxes in humid temperate forests indicated a high spatial and temporal variability in CH4 fluxes, especially in CH4 emissions from wet soils close to riparian zones, which can turn the soil of a whole forest from a CH4 sink to a CH4 source. In this study, the spatial and temporal variability of soil CH4 fluxes was investigated in a Japanese coniferous forest, including a riparian wetland and a hillslope water-unsaturated forest floor, based on multipoint flux measurements using laser-based CH4 analyzers over a period of 2 years. We identified CH4 emission hot spots (60.2 ± 169.1 nmol m−2 s−1 from 117 sampling points) in the wetland in late summer, while the CH4 absorption rate in the forest floor was comparatively lower (−1.2 ± 1.4 nmol m−2 s−1 from 119 sampling points). The temporal variability of watershed-scale CH4 flux was amplified by a clear seasonal cycle of soil temperature and rainfall pattern under the Asian monsoon climate. The watershed-scale CH4 budget showed that the forest turned into a CH4 source during the summer owing to the high and variable CH4 emissions from the riparian wetland and the lower part of the hillslope. Overall, our results indicated that CH4 emissions from small riparian areas are important in controlling forest CH4 dynamics at a watershed scale.

Seasonal and diurnal patterns of soil respiration in an evergreen coniferous forest: Evidence from six years of observation with automatic chambers

Soil respiration (Rs) plays a key role in the carbon balance of forest ecosystems. There is growing evidence that Rs is strongly correlated with canopy photosynthesis; however, how Rs is linked to aboveground attributes at various phenological stages, on the seasonal and diurnal scale, remains unclear. Using an automated closed dynamic chamber system, we assessed the seasonal and diurnal patterns of Rs in a temperate evergreen coniferous forest from 2005 to 2010. High-frequency Rs rates followed seasonal soil temperature patterns but the relationship showed strong hysteresis. Predictions of Rs based on a temperature-response model underestimated the observed values from June to July and overestimated those from August to September and from January to April. The observed Rs was higher in early summer than in late summer and autumn despite similar soil temperatures. At a diurnal scale, the Rs pattern showed a hysteresis loop with the soil temperature trend during the seasons of high biological activity (June to October). In July and August, Rs declined after the morning peak from 0800 to 1400 h, although soil temperatures continued to increase. During that period, figure-eight-shaped diurnal Rs patterns were observed, suggesting that a midday decline in root physiological activity may have occurred in early summer. In September and October, Rs was higher in the morning than in the night despite consistently high soil temperatures. We have characterised the magnitude and pattern of seasonal and diurnal Rs in an evergreen forest. We conclude that the temporal variability of Rs at high resolution is more related to seasons across the temperature dependence.

Methane dynamics in a temperate forest revealed by plot-scale and ecosystem-scale flux measurements

Soils play important roles as CH 4 sources and sinks. CH 4 is produced in anoxic environments, including submerged soils, by methanogenic bacteria. On the contrary, CH 4 is oxidized by methanotrophic bacteria in upland soils. In general, forest soils are recognized as the efficient sinks for atmospheric CH 4, because of their CH 4 oxidation capacity in water-unsaturated soil (Le Mer and Roger, 2001). However, we hypothesized that forest ecosystems, especially in wet warm climates such as Asian monsoon climate, are not always CH 4 sink. In this study, we examined the CH 4 dynamics in a temperate Asian monsoon forest (Kiryu Experimental Watershed: 35 N, 136E), which included wet areas along riparian zones within the watershed. In order to reveal the spatio-temporal variations of CH 4 fluxes, we combined multi-point plot-scale CH 4 flux measurements using chamber methods and ecosystem-scale CH 4 flux measurements using a micrometeorological method, relaxed eddy accumulation (REA) method (Businger and Oncley, 1990; Hamotani et al., 1996, 2001). Intensive manual chamber measurements of CH 4 fluxes at 60 points in the wet areas and within the water-unsaturated forest floor, respectively, showed that the wet areas had a greater spatial and temporal variability of CH 4 fluxes than the forest floor. This indicates that accurate consideration of CH 4 fluxes from any wet areas is important in order to evaluate the CH 4 budget within the forests. From biweekly continuous manual chamber measurements of CH 4 fluxes at 9 points in the wet areas and the forest floor, respectively, hotspots of CH 4 emissions were observed during summer and fall immediately after intensive precipitation in the wet areas. On the other hand, in the forest floor, seasonal variations of CH 4 fluxes were not simply associated with temperature variations. In contrast, CH 4 absorption increased at some measurement plots in spring before intensive summer rainfall. In addition to the manual chamber measurements, we observed the environmental responses of CH 4 fluxes at a half-hourly time resolution, by using automated chamber measurements at three plots on the water-unsaturated forest floor. We found that the CH4 absorption flux was greatly weakened by summer intensive rainfall, but recovered and peaked after rainfall as the soil water content decreased. The responses of CH 4 fluxes to rainfall were different for each plot. In a dry soil plot with a thick humus layer, CH4 fluxes decreased abruptly at the peak of rainfall intensity, and it increased gradually after rainfall. In a wet soil plot and a dry soil plot with a thinner humus layer, such abrupt decreases in CH 4 fluxes were not observed, and CH 4 fluxes gradually switched from a sink to neutral following rainfall. Simultaneous measurements of CO 2 fluxes provided useful information when considering the controlling factors affecting complex CH 4 fluxes in terms of gas diffusivity and microbial activity. The ecosystem-scale CH 4 flux measurements revealed that the Japanese cypress forest switched seasonally between being a sink and source of CH 4, and the pattern differed year by year. CH 4 fluxes tended to be a source during summer and fall, and switched to a sink during dry period. At hourly to daily timescales, the CH 4 fluxes were sensitive to rainfall; rain events increased CH4 emission, decreased CH 4 absorption, or shifted CH 4 absorption to CH4 emission. The results show that the temperate forest containing riparian zone acted as a CH 4 source seasonally, through the increased CH 4 emission in the wet areas and/or the decreased CH 4 absorption on the water-unsaturated forest floor in response to changing soil temperatures and/or the soil water status. The Asian monsoon rainfall was found to strongly influence temporal variations in CH 4 fluxes at both plot-scale and ecosystem-scales.