Ryuichi Hirata

CO2 and water vapor exchange of a larch forest in northern Japan

In the northern part of East Asia, forests dominated by larch are extensively distributed and probably play an important role in the global carbon cycle. However, a knowledge of the CO2 balance of larch forests based on long-term flux measurements is very restricted in East Asia. Thus, a long-term flux measurement has been started in 2000 at a larch plantation on a flat terrain in Hokkaido, Japan to obtain more information on the CO2 and energy balances of larch forests. From September 2000 to August 2001 the net ecosystem CO2 exchange (NEE) changed seasonally in accordance with the annual cycles of phenology and climate. NEE was negative for six months of the growing season, May—September; the larch ecosystem was a carbon sink with a peak intensity of –0.38 mol m−2 d−1 for this period. In the leafless season from November to April the forest ecosystem was a carbon source with an intensity ranging between 0 and 0.05 mol m−2 d−1. Annual NEE from September 2000 to August 2001 was −24.4 to −32.4 mol m−2 yr−1 (=−293 to −389 gC m−2 yr−1); this value is compatible with those reported from other temperate forests. Annual evapotranspiration for the same period was 367 mm, which was only 29% of annual precipitation.

Influences of various calculation options on heat, water and carbon fluxes determined by open- and closed-path eddy covariance methods

ABSTRACT Synthesis studies using multiple-site datasets for eddy covariance potentially contain uncertainties originating from the use of different flux calculation options, because the choice of the process for calculating half-hourly fluxes from raw time series data is left to individual researchers. In this study, we quantified the uncertainties associated with different flux calculation methods at seven sites. The differences in the half-hourly fluxes were small, generally of the order less than a few percentiles, but they were substantial for the annual fluxes. After the standardisation under current recommendations in the FLUXNET communities, we estimated the uncertainties in the annual fluxes associated with the flux calculations to be 2.6±2.7 W m−2 (the mean 90% ± confidence interval) for the sensible heat flux, 72±37 g C m−2 yr−1 for net ecosystem exchange (NEE), 12±6% for evapotranspiration, 12±6% for gross primary productivity and 16±10% for ecosystem respiration. The self-heating correction strongly influenced the annual carbon balance (143±93 g C m−2 yr−1), not only for cold sites but also for warm sites, but did not fully account for differences between the open- and closed-path systems (413±189 g C m−2 yr−1).

A review of tower flux observation sites in Asia

Aggregating and sharing the metadata of flux observation sites results in a strong collaboration among various fields of study. Such data sharing will also be a part of the future design of a tower flux observation network in Asia. The aim of this review is to comprehend the state of tower flux observation sites in Asia. There are 109 tower flux observation sites in Asia including 51 forest sites. There are more new sites under construction in Asia than in America and Europe. These sites range from the taiga in Siberia to the rainforest in Southeast Asia, and from the equatorial to polar Koeppen climate zones. There are many highly humid areas in Asia, not only at low latitudes but also at middle latitudes. This climate condition has developed unique vegetation such as lucidophyllous (evergreen broadleaf) forest, which is distributed in warm areas with high precipitation in the growing season. However, there are only a few observations taking place in lucidophyllous forest. Rice paddy fields are also unique land cover in Asia. It is important to accumulate long-term data for rice fields with their management records, because plant activity depends highly on both climate conditions and land-use management. Flux data, especially net ecosystem exchange and related elements, are used for widespread studies not only within the flux-research community but also in other fields of study, for example remote sensing. At present, however, both the quantity and quality of the data are not sufficient for these studies. Regarding the quantity, there are many recently established sites that have not published data yet; regarding quality, flux data include uncertainties caused by methodological problems. Flux researchers are required not only to obtain flux data but also to improve their quality. Meanwhile, data users must understand there are still uncertainties in flux data.

Exploring Simple Algorithms for Estimating Gross Primary Production in Forested Areas from Satellite Data

Algorithms that use remotely-sensed vegetation indices to estimate gross primary production (GPP), a key component of the global carbon cycle, have gained a lot of popularity in the past decade. Yet despite the amount of research on the topic, the most appropriate approach is still under debate. As an attempt to address this question, we compared the performance of different vegetation indices from the Moderate Resolution Imaging Spectroradiometer (MODIS) in capturing the seasonal and the annual variability of GPP estimates from an optimal network of 21 FLUXNET forest towers sites. The tested indices include the Normalized Difference Vegetation Index (NDVI), Enhanced Vegetation Index (EVI), Leaf Area Index (LAI), and Fraction of Photosynthetically Active Radiation absorbed by plant canopies (FPAR). Our results indicated that single vegetation indices captured 50–80% of the variability of tower-estimated GPP, but no one index performed universally well in all situations. In particular, EVI outperformed the other MODIS products in tracking seasonal variations in tower-estimated GPP, but annual mean MODIS LAI was the best estimator of the spatial distribution of annual flux-tower GPP (GPP = 615 × LAI − 376, where GPP is in g C/m2/year). This simple algorithm rehabilitated earlier approaches linking ground measurements of LAI to flux-tower estimates of GPP and produced annual GPP estimates comparable to the MODIS 17 GPP product. As such, remote sensing-based estimates of GPP continue to offer a useful alternative to estimates from biophysical models, and the choice of the most appropriate approach depends on whether the estimates are required at annual or sub-annual temporal resolution.

Dataset of CarboEastAsia and uncertainties in the CO2 budget evaluation caused by different data processing

The datasets of net ecosystem CO2 exchange (NEE) were acquired from 21 forests, 3 grasslands, and 3 croplands in the eastern part of Asia based on the eddy covariance measurements of the international joint program, CarboEastAsia. The program was conducted by three networks in Asia, ChinaFLUX, JapanFlux, and KoFlux, to quantify, synthesize, and understand the carbon budget of the eastern part of Asia. An intercomparison was conducted for NEE estimated by three gap-filling procedures adopted by ChinaFLUX, JapanFlux, and KoFlux to test the range of uncertainty in the estimation of NEE. The overall comparison indicated good agreement among the procedures in the seasonal patterns of NEE, although a bias was observed in dormant seasons depending on the different criteria of data screening. Based on the gap-filled datasets, the magnitude and seasonality of the carbon budget were compared among various biome types, phenology, and stress conditions throughout Asia. The annual values of gross primary production and ecosystem respiration were almost proportional to the annual air temperature. Forest management, including clear-cutting, plantation, and artificial drainage, was significant and obviously affected the annual carbon uptake within the forests. Agricultural management resulted in notable seasonal patterns in the crop sites. The dataset obtained from a variety of biome types would be an essential source of knowledge for ecosystem science as well as a valuable validation dataset for modeling and remote sensing to upscale the carbon budget estimations in Asia.

Diurnal variations and vertical gradients of biogenic volatile and semi-volatile organic compounds at the Tomakomai larch forest station in Japan

Volatile and semi-volatile organic compounds (VOCs and SVOCs) in the atmosphere control oxidative capacity of the air and formation of organic aerosols. To investigate sources, variations and fluxes of VOCs and SVOCs in the forest atmosphere, samples were collected in a larch forest at different heights in Tomakomai, Japan, from 3 to 5 September 2003 and were analyzed for VOCs (isoprene and α-pinene) and SVOCs (n-nonanal, n-decanal, glycolaldehyde, hydroxyacetone, glyoxal and methylglyoxal) using GC-FID and GC-MS. Isoprene (8–851 pptv) showed a diurnal variation with a maximum in daytime, whereas diurnal pattern was indistinct for α-pinene (6–145 pptv). SVOCs showed diurnal variations similar to isoprene, suggesting their origin to be biogenic. Denuder samplings of SVOCs demonstrated that up to 60% of SVOCs were present in aerosol particles. n-Nonanal and n-decanal were more abundant at lower altitude, suggesting their primary emissions from vegetations. In contrast, other four SVOCs showed an opposite or no trend and are considered as oxidation products of biogenic precursors such as isoprene. The averaged upward fluxes of isoprene and α-pinene were obtained to be 34.8 and 9.8 ng m-2 s-1, respectively. Magnitudes of SVOC fluxes were comparable to those of α-pinene, indicating that SVOCs affect budgets of organic gases/aerosols over the forest.

Evaluation and improvement of MODIS gross primary productivity in typical forest ecosystems of East Asia based on eddy covariance measurements

Gross primary productivity (GPP) is a major component of carbon exchange between the atmosphere and terrestrial ecosystems and a key component of the terrestrial carbon cycle. Because of the large spatial heterogeneity and temporal dynamics of ecosystems, it is a challenge to estimate GPP accurately at global or regional scales. The 8-day MODerate resolution Imaging Spectroradiometer (MODIS) GPP product provides a near real time estimate of global GPP. However, previous studies indicated that MODIS GPP has large uncertainties, partly caused by biases in parameterization and forcing data. In this study, MODIS GPP was validated using GPP derived from the eddy covariance flux measurements at five typical forest sites in East Asia. The validation indicated that MODIS GPP was seriously underestimated in these forest ecosystems of East Asia, especially at northern sites. With observed meteorological data, fraction of photosynthetically active radiation absorbed by the plant canopy (fPAR) calculated using smoothed MODIS leaf area index, and optimized maximum light use efficiency (εmax) to force the MOD17 algorithm, the agreement between predicted GPP and tower-based GPP was significantly improved. The errors of MODIS GPP in these forest ecosystems of East Asia were mainly caused by uncertainties in εmax, followed by those in fPAR and meteorological data. The separation of canopy into sunlit and shaded leaves, for which GPP is individually calculated, can improve GPP simulation significantly.

The role of carbon flux and biometric observations in constraining a terrestrial ecosystem model: a case study in disturbed forests in East Asia

The process of confining unnecessary freedom is a step toward advanced ecosystem modeling. This study demonstrates the importance of carbon flux and biometric observation in constraining a terrestrial ecosystem model with a simple optimization scheme. At the selected sites from AsiaFlux network, a simultaneous optimization scheme for both carbon flux and biomass was compared with carbon flux-oriented and biomass-oriented optimization schemes using the Biome-BGC model. The optimization scheme oriented to either carbon flux or biomass provided simulation results that were consistent with observations, but with reduced performance in unconstrained variables. The simultaneous optimization scheme yielded results that were consistent with observations for both carbon flux and biomass. By comparing long-term projections simulated by three schemes, it was found that the optimization oriented only to carbon flux has limited performance because misrepresented biomass significantly affected a projection of carbon exchange through heterotrophic respiration. From these experiments, we found that (1) a process model like Biome-BGC is capable of reproducing both carbon flux and biomass within acceptable proximity, (2) constraining biomass is importance not just because it is one of carbon cycle components, but also it significantly affects simulations of carbon flux. Thus, it is important to invest more effort to improve simulation of biomass as well as carbon flux.

Spatial and seasonal variations of CO2 flux and photosynthetic and respiratory parameters of larch forests in East Asia

Abstract Larch (Larix spp.) forests are predominantly distributed across high latitudes of Eurasia. They potentially have a strong influence on the terrestrial carbon and energy cycles, because of their vast area and the large carbon stocks in their peat soils in the permafrost. In this study, we elucidated intersite variation of ecosystem photosynthetic and respiratory parameters of eight larch forests in East Asia using the CarboEastAsia carbon flux and micrometeorology dataset. These parameters were determined using the empirical relationship between the carbon fluxes (photosynthesis and respiration) and micrometeorological variables (light and temperature). In addition, we examined leaf area index (LAI) determined by Moderate Resolution Imaging Spectroradiometer (MODIS) remote sensing data to explain the intersite variation. Linear or exponential relationships with annual mean temperature or seasonal maximum LAI at the study sites were found for the annual carbon fluxes (gross primary production [GPP] and total ecosystem respiration [RE]) as well as for four of the five seasonal maximum values of determined photosynthetic and respiratory parameters (maximum GPP at light saturation, initial slope of the light-response curve, daytime respiration, and RE at the reference temperature of 10°C). Phenological indices, such as start day of the growing season, growing season length and growing season degree days explained much of the intersite variation of GPP and RE of the studied larch forests; however, the relationship between MODIS LAI and photosynthetic or respiratory parameters implies that the intersite variation in GPP and RE was caused not only by the temperature variation (abiotic factor), but also by the variation in the photosynthetic and respiration activity by vegetation (biotic factor) through the change in leaf (or whole vegetation) biomass. Our analysis shows that MODIS LAI serves as a good index to explain the variation of the ecosystem photosynthetic and respiratory characteristics of East Asian larch forests.

Mitigation Effect of Farmyard Manure Application on Greenhouse Gas Emissions from Managed Grasslands in Japan

Applying manure can lead to decrease of chemical nitrogen (N) fertilizer use and increase of soil carbon (C) sequestration. The effect of manure application on net ecosystem C balance (NECB), methane (CH4) and nitrous oxide (N2O) emissions and global warming potential (GWP) was examined at four managed grasslands on Andosols in different climatic regions in Japan for 3 years. At adjoining manure and fertilizer plots in each site, net ecosystem exchange (NEE) and CH4 and N2O fluxes were measured by the eddy covariance method and dark chamber methods, respectively. Manure application decreased fertilizer N application rate in manure plot to 65–88 % in fertilizer plot. NECB (= NEE−C applied in manure + harvested C) was higher in fertilizer plot (1.9 ± 0.9 MgC ha−1 year−1) than in manure plot (−1.8 ± 1.8 MgC ha−1 year−1), indicating that the soils in fertilizer plots lost C. There was no significant difference in harvested C between fertilizer and manure plots (4.3 ± 0.8 and 4.1 ± 0.6 MgC ha−1 year−1, respectively). NEE showed more CO2 uptake in fertilizer plots (−2.4 ± 1.1 MgC ha−1 year−1) than in manure plots (−1.6 ± 0.7 MgC ha−1 year−1), but manure application could compensate for the shortage in NEE. CH4 emission was close to zero, while the N2O emission was greater in manure plots (6.2 ± 3.7 kgN ha−1 year−1) than in fertilizer plots (3.6 ± 3.2 kgN ha−1 year−1). The difference of GWP between manure and fertilizer plots showed a negative relationship with manure C application rate (y = −4.45 ln(x) + 2.84; R 2 = 0.85; p < 0.01), indicating that manure application rate more than 2 MgC ha−1 year−1 can mitigate global warming in the Japanese grasslands.