Mioko Ataka

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.

CO2 efflux from leaf litter focused on spatial and temporal heterogeneity of moisture

Leaf litter respiration (RLL) was directly measured in situ to evaluate relationships with the water content in leaf litter (WC), which is distributed heterogeneously under natural conditions. To do so, we developed a small, closed static chamber system using an infrared gas analyzer, which can measure instantaneous RLL. This study focuses on the measurement of CO2 effluxes from leaf litter using the chamber system in the field and examines the relationship between RLL and WC among seven broadleaf species in a temperate forest. The measurements focused on the position of leaves within the litter layer, finding that both RLL and WC were significantly higher in the lower layer. The value of RLL increased with increasing WC, and the response of RLL to WC was similar among all seven species. Moreover, the temporal variation in WC differed among three species and was associated with leaf litter thickness. The observed heterogeneity in WC induced by the physical environment (e.g., position and thickness of leaf litter) affects the variation in WC and, therefore, both RLL and the decomposition rates of organic matter in the litter layer.

Differences between coarse woody debris and leaf litter in the response of heterotrophic respiration to rainfall events

Heterotrophic respiration strongly influences carbon cycles at the ecosystem and global scales. We used an automated chamber system to measure the heterotrophic respiration of coarse woody debris (CWD) and leaf litter in a secondary broadleaved forest in southern Kyoto Prefecture. This system, which targeted only organic matter, could detect heterotrophic respiration responses to changes in environmental factors, especially rainfall events. The temporal trends and responses of respiration to environmental factors differed dramatically between CWD and leaf litter. CWD respiration showed a clear diurnal change corresponding to changes in CWD temperature and a clear decrease during rainfall events. Leaf litter respiration did not change clearly but increased at the beginning of rain events and returned to pre-rain rates when soil water content declined. The temporal patterns of the residuals between the observed respiration and baseline respiration, developed from the temperature–response curves under pre-rain conditions, differed between CWD and leaf litter respiration. The typical trend in CWD respiration response to rainfall events was a clear decrease and then gradual increase in the residuals after the event. The response of leaf litter respiration to wetting was an increase in the residuals during rainfall events and then a gradual decrease during drying. The difference in the responses of these respirations to wetting and drying processes are likely caused by differences in the physical characteristics of the CWD and the leaf litter layer. Measurements targeting only organic matter using an automated chamber system could detect the responses of heterotrophic respiration to environmental factors.

Using Capacitance Sensors for the Continuous Measurement of the Water Content in the Litter Layer of Forest Soil

Little is known about the wetting and drying processes of the litter layer ( layer), likely because of technical difficulties inherent in nondestructive water content (WC) monitoring. We developed a method for continuously measuring the WC of leaf litter (the “LWC method”) in situ using capacitance sensors. To test variants of this approach, five (for the LWC_5) or ten (for the LWC_10 method) Quercus serrata leaves were attached around capacitance sensors. The output voltage used for each LWC method was linearly correlated with the gravimetric WC (LWC_5: ; LWC_10: ), producing different slopes for each calibration line. For in situ continuous measurements of WC in the layer, two sensors were used, one placed on top of the layer and the other at the boundary between the and mineral layers. The average continuous WC of the layer was then calculated from the output voltage of the two sensors and the calibration function, and this value was linearly correlated with the gravimetric WC . However, because the layer characteristics (e.g., thickness, water-holding capacity, and species composition) may differ among study sites, appropriate approaches for measuring this layer’s moisture properties may be needed.

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.

In Situ CO2 Efflux from Leaf Litter Layer Showed Large Temporal Variation Induced by Rapid Wetting and Drying Cycle

We performed continuous and manual in situ measurements of CO2 efflux from the leaf litter layer (R LL) and water content of the leaf litter layer (LWC) in conjunction with measurements of soil respiration (R S) and soil water content (SWC) in a temperate forest; our objectives were to evaluate the response of R LL to rainfall events and to assess temporal variation in its contribution to R S. We measured R LL in a treatment area from which all potential sources of CO2 except for the leaf litter layer were removed. Capacitance sensors were used to measure LWC. R LL increased immediately after wetting of the leaf litter layer; peak R LL values were observed during or one day after rainfall events and were up to 8.6-fold larger than R LL prior to rainfall. R LL declined to pre-wetting levels within 2–4 day after rainfall events and corresponded to decreasing LWC, indicating that annual R LL is strongly influenced by precipitation. Temporal variation in the observed contribution of R LL to R S varied from nearly zero to 51%. Continuous in situ measurements of LWC and CO2 efflux from leaf litter only, combined with measurements of R S, can provide robust data to clarify the response of R LL to rainfall events and its contribution to total R S.

Relationship between fine-root exudation and respiration of two Quercus species in a Japanese temperate forest

Plants allocate a considerable amount of carbon (C) to fine roots as respiration and exudation. Fine-root exudation could stimulate microbial activity, which further contributes to soil heterotrophic respiration. Although both root respiration and exudation are important components of belowground C cycling, how they relate to each other is less well known. In this study, we aimed to explore this relationship on mature trees growing in the field. The measurements were performed on two canopy species, Quercus serrata Thunb. and Quercus glauca, in a warm temperate forest. The respiration and exudation rates of the same fine-root segment were measured in parallel with a syringe-basis incubation and a closed static chamber, respectively. We also measured root traits and ectomycorrhizal colonization ratio because these indexes commonly relate to root respiration and reflect root physiology. The microbial activity enhanced by root exudation was investigated by comparing the dissolved organic carbon (DOC) and microbial biomass carbon (MBC) between rhizosphere soils and bulk soils. Mean DOC concentration and MBC were ca two times higher in the rhizosphere soils and positively related to exudation rates, indicating that exudation further relates to the C dynamics in the soils. Flux rates of exudation and respiration were positively correlated with each other. Both root exudation and respiration rates positively related to ectomycorrhizal colonization and root tissue nitrogen, and therefore the relationship between the two fluxes may be attributed to fine-root activity. The flux rates of root respiration were 8.7 and 10.5 times as much as those of exudation on a root-length basis and a root-weight basis, respectively. In spite of the fact that flux rates of respiration and exudation varied enormously among the fine-root segments of the two Quercus species, exudation was in proportion to respiration. This result gives new insight into the fine-root C-allocation strategy and the belowground C dynamics.

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.