AsiaFlux (http://www.asiaflux.net/) was established in 1999 as the Asian arm of FLUXNET, the worldwide flux network, to develop collaborative research and data sets on the cycles of carbon and water in key ecosystems in Asia, to organize workshops and training on current and related global change themes, and to cultivate next generation scientists to be informed leaders with skills and perspectives. Over the past 20 years, AsiaFlux has developed into a regional research network composed of 28 member countries. More than 100 flux observation towers were built in Asia, covering diverse terrestrial ecosystems. Researchers of AsiaFlux have made great progress in flux observation, remote-sensing and ecosystem modeling, and made outstanding contribution to quantifying global carbon balance and understanding the functions of Asian terrestrial ecosystems. AsiaFlux will continue to lead flux-related research, contribute to the achievement of sustainable development in Asia, and provide an open forum for both field researchers (data providers) and remote-sensing / modeling researchers (data users) to encourage their communication and collaboration.
　AsiaFlux held a special workshop in 2019 in Takayama, Japan, to celebrate the 20^th anniversary with support from Gifu University and National Institute for Environmental Studies (NIES). In the workshop, a special session was organized to look back on the past and project the future of AsiaFlux. Seven speakers were invited and talked about principal research topics in AsiaFlux: 1) carbon dioxide (CO₂) flux observation, 2) water and energy fluxes, 3) soil respiration, 4) volatile organic compound exchange, 5) ecosystem processes, 6) remote sensing, and 7) terrestrial biosphere modeling. Based on these presentations, seven review papers were planned. This issue is composed of five review papers on water and energy fluxes (Kang and Cho, 2021), soil respiration (Sha et al., 2021), volatile organic compound (Tani and Mochizuki, 2021), ecosystem processes (Chang et al., 2021) and ecosystem modeling (Ito and Ichii, 2021). The other two review papers on eddy CO₂ flux and remote sensing will be presented soon in this journal, Journal of Agricultural Meteorology. We are grateful to all the authors of these impressive review papers and the editorial board of The Society of Agricultural Meteorology of Japan on behalf of AsiaFlux.

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　The eddy covariance (EC) technique-based observation system allows for researchers to determine latent and sensible heat fluxes, which are key components of the surface energy balance. The number of water and energy flux studies in Asia has increased as the number of flux measurement sites and the length of the observation periods have grown. To retrace the footprints of the AsiaFlux network and predict future research directions, we reviewed the progress in water and energy flux studies in Asia from the 1990s to the present day. This included studies on continuous evapotranspiration (ET) and surface energy balance measurements in various ecosystems, from the tropics to the polar regions. We also reviewed comparative experiments between the EC technique and other observation techniques including the use of a lysimeter or scintillometer, data processing techniques, connections between carbon and water fluxes, and multi-site syntheses. This paper discusses three remaining challenges that are hindering the derivation of scientific knowledge for ET and the surface energy balance, namely: the non-closure of the surface energy budget, imperfect compatibility between open- and closed-path gas analyzers, and difficulty in partitioning ET into evaporation and transpiration. If we leverage the advantages of the EC technique (i.e., high sampling rates of ≥ 10 Hz and continuous measurement capabilities), standardized methods for correcting and partitioning can be developed in the near future.

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　Soil respiration (R_s) is the largest flux of carbon dioxide (CO₂) next to photosynthesis in terrestrial ecosystems. With the absorption of atmospheric methane (CH₄), upland soils become a large CO₂ source and CH₄ sink. These soil carbon (C) fluxes are key factors in the mitigation and adaption of future climate change. The Asian region spans an extensive area from the northern boreal to tropical regions in Southeast Asia. As this region is characterised by highly diverse ecosystems, it is expected to experience the strong impact of ecosystem responses to global climate change. For the past two decades, researchers in the AsiaFlux community have meaningfully contributed to improve the current understanding of soil C dynamics, response of soil C fluxes to disturbances and climate change, and regional and global estimation based on model analysis. This review focuses on five important aspects: 1) the historical methodology for soil C flux measurement; 2) responses of soil C flux components to environmental factors; 3) soil C fluxes in typical ecosystems in Asia; 4) the influence of disturbance and climate change on soil C fluxes; and 5) model analysis and the estimation of soil C fluxes in research largely focused in Asia.

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　To elucidate the dynamic features of carbon sequestration in ecosystems under changing climates and various disturbance regimes, researchers must understand key ecosystem processes, such as carbon allocation and partitioning, organic matter decomposition, and nutrient cycles, as well as plant functional traits. Here, we reviewed the existing literature and conducted meta-analyses using available datasets from eddy covariance CO₂ flux sites in East Asia to clarify these ecosystem processes and attributes. Since the establishment of AsiaFlux in 1999, the number of flux tower sites has grown to 110 sites, spanning a large geographic extent in East Asia and covering diverse ecosystems embedded in large climatic gradients. Early publications relating to AsiaFlux described CO₂ fluxes from single sites, but over the last 20 years more ecosystem processes and attributes have been included in the study sites’ research programs. Among other advances, researchers have quantified the plant functional traits related to photosynthesis or ecosystem-scale gross primary production and thus demonstrated that CO₂ fluxes are controlled by plant traits; this quantification provides a basis for building ecosystem models. Additional means of understanding the carbon fluxes and pools of these ecosystems have been provided by biometric measurements beneath eddy covariance flux towers, partly on the basis of traditional forestry practices and the measurements of component carbon fluxes, such as respiratory fluxes and litter decomposition rates. Through meta-analyses, we demonstrate good correlations between these fluxes and mention the characteristics of carbon cycle processes in Asian forest ecosystems. By investigating nitrogen biogeochemical cycles at the flux sites, studies have shown that carbon fluxes are also controlled by nitrogen availability. The future success and progress of AsiaFlux could be promoted by further collaborations between this research community and other networks, such as long-term ecological research (LTER) networks, and the development of open databases.

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　Many VOCs are reactive in the atmosphere, may produce secondary organic aerosol (SOA), and keep photochemical ozone concentrations high by VOC-involved reactions. Accumulated studies have shown the importance of terrestrial ecosystems which can be sinks and sources of VOCs. The research progress in the exchange of volatile organic compounds (VOCs) between terrestrial ecosystems and the atmosphere was reviewed in this paper. Representative VOCs emitted from terrestrial ecosystems are low-molecular-weight oxygenated VOCs including methanol, acetone, formic and acetic acids, and terpenoids, including isoprene and monoterpenes. Terpenoid emissions have been intensively investigated from the leaf to the canopy level using advanced analytical systems, including proton-transfer-reaction mass spectrometry. Environmental factors, including temperature, light intensity, carbon dioxide and ozone concentrations, and water stress have been reported to affect terpenoid emissions from plants. The combined effects of these environments influence terpenoid emission additively or interactively, and are important in terms of VOC emission estimates against ongoing climate change. Isoprene is most abundantly released into the atmosphere among VOCs; the potential reasons why some plants release such large amounts of carbon as isoprene were summarized in this study. Among oxygenated VOCs, some compounds, including isoprene oxygenates methacrolein and methyl vinyl ketone, are bidirectionally exchanged, and both atmospheric chemical reactions and reactions under oxidative stress in leaves have been regarded as involved in bidirectional VOC exchanges. Bottom-up process-based models and top-down inverse models have been developed to estimate global and local terpenoid emissions. To validate the accuracy and precision of the models, the collection of additional in-situ ground truth data, such as long-term flux measurement data, at various sites is required. Otherwise, these models may still leave large uncertainties compared with CO₂ flux models that can be validated with a large number of ground truth flux data.

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　A wide variety of models have been developed and used in studies of land-atmosphere interactions and the carbon cycle, with aims of data integration, sensitivity analysis, interpolation, and extrapolation. This review summarizes the achievements of model studies conducted in Asia, a focal region in the changing Earth system, especially collaborative works with the regional flux measurement network, AsiaFlux. Process-based biogeochemical models have been developed to simulate the carbon cycle, and their accuracy has been verified by comparing with carbon dioxide flux data. The development and use of data-driven (statistical and machine learning) models has further enhanced the utilization of field survey and satellite remote sensing data. Model intercomparison studies were also conducted by using the AsiaFlux dataset for uncertainty analyses and benchmarking. Other types of models, such as cropland models and trace gas emission models, are also briefly reviewed here. Finally, we discuss the present status and remaining issues in data-model integration, regional synthesis, and future projection with the models.

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　Neural network (NN) models with environmental data and the extent of ventilator openings as inputs have the potential to estimate the number of air exchanges per hour (N) in real time of a naturally ventilated greenhouse. In this study, the intraseasonal and interseasonal applicability of an NN model was verified: whether the model trained in a specific period can be applied to different periods of the same and other seasons. First, the effect of data collection periods for model training and test within the same season on the estimation accuracy of N was examined. The estimation accuracy was lowered even though the model was applied to a period immediately following that used for model training. Adjusting the training dataset so that the relative distribution of the temperature difference inside and outside the greenhouse (∆T) approaches the relative distribution of the test dataset improves the estimation accuracy slightly. However, when the model was applied to interseasonal data, such training data adjustments did not improve the estimation accuracy. This indicates that the NN model needs to be further improved for practical use to estimate N of naturally ventilated greenhouses.

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