Shijie Han

Response of surface air temperature to small-scale land clearing across latitudes

Climate models simulating continental scale deforestation suggest a warming effect of land clearing on the surface air temperature in the tropical zone and a cooling effect in the boreal zone due to different control of biogeochemical and biophysical processes. Ongoing land-use/cover changes mostly occur at local scales (hectares), and it is not clear whether the local-scale deforestation will generate temperature patterns consistent with the climate model results. Here we paired 40 and 12 flux sites with nearby weather stations in North and South America and in Eastern Asia, respectively, and quantified the temperature difference between these paired sites. Our goal was to investigate the response of the surface air temperature to local-scale (hectares) land clearing across latitudes using the surface weather stations as proxies for localized land clearing. The results show that north of 10 N, the annual mean temperature difference (open land minus forest) decreases with increasing latitude, but the temperature difference shrinks with latitude at a faster rate in the Americas [ 0.079 ( 0.010) C per degree] than in Asia [ 0.046 ( 0.011) C per degree]. Regression of the combined data suggests a transitional latitude of about 35.5 N that demarks deforestation warming to the south and cooling to the north. The warming in latitudes south of 35 N is associated with increase in the daily maximum temperature, with little change in the daily minimum temperature while the reverse is true in the boreal latitudes.

Comparison of eddy covariance and chamber-based methods for measuring CO2 flux in a temperate mixed forest

Two methods, eddy covariance and chamber-based measurements, were employed to measure the net ecosystem CO(2) exchange in a mature temperate mixed forest in 2003. The eddy covariance system was used as a reference, which was compared with the chamber-based method. Based on chamber fluxes, the ecosystem had a gross primary production of 1490 g C m(-2) year(-1), 90% of which was released as efflux back into the air via respiration of the entire ecosystem. This was comprised of about 48% from soil surface CO(2) efflux, 31% from leaf respiration and 21% from stem and branch respiration. Net ecosystem exchange (NEE), estimated from the sum of daily component fluxes, was 146 g C m(-2) year(-1). Ecosystem respiration (ER), estimated from the sum of daily ecosystem respiration, was 1240 g C m(-2) year(-1). NEE was 9.8% of actual gross primary production (GPP). The eddy covariance estimates of NEE, ER and GPP were 188, 1030 and 1220 g C m(-2) year(-1), respectively. The eddy covariance estimation of NEE was higher than that of the chamber-based estimation by 22.5%. On a daily basis, NEE of the scaled chamber measurements was in acceptable agreement with eddy covariance measurement data with R(2) values of 0.71. The discrepancy between the measurement of the two methods was greater in the non-growing season primarily due to the lack of spatial variability in the scaled chamber estimates and weak atmosphere turbulence by eddy covariance measurements. There are many uncertainties for determination of absolute values of ecosystem component flux. More detailed experiments and related theoretical studies are needed in the future.

Responses of Fine Roots and Soil N Availability to Short-Term Nitrogen Fertilization in a Broad-Leaved Korean Pine Mixed Forest in Northeastern China

Knowledge of the responses of soil nitrogen (N) availability, fine root mass, production and turnover rates to atmospheric N deposition is crucial for understanding fine root dynamics and functioning in forest ecosystems. Fine root biomass and necromass, production and turnover rates, and soil nitrate-N and ammonium-N in relation to N fertilization (50 kg N ha−1 year−1) were investigated in a temperate forest over the growing season of 2010, using sequential soil cores and ingrowth cores methods. N fertilization increased soil nitrate-N by 16% (P<0.001) and ammonium-N by 6% (P<0.01) compared to control plots. Fine root biomass and necromass in 0–20 cm soil were 13% (4.61 vs. 5.23 Mg ha−1, P<0.001) and 34% (1.39 vs. 1.86 Mg ha−1, P<0.001) less in N fertilization plots than those in control plots. The fine root mass was significantly negatively correlated with soil N availability and nitrate-N contents, especially in 0–10 cm soil layer. Both fine root production and turnover rates increased with N fertilization, indicating a rapid underground carbon cycling in environment with high nitrogen levels. Although high N supply has been widely recognized to promote aboveground growth rates, the present study suggests that high levels of nitrogen supply may reduce the pool size of the underground carbon. Hence, we conclude that high levels of atmospheric N deposition will stimulate the belowground carbon cycling, leading to changes in the carbon balance between aboveground and underground storage. The implications of the present study suggest that carbon model and prediction need to take the effects of nitrogen deposition on underground system into account.

Distribution of Soil Organic Carbon Fractions Along the Altitudinal Gradient in Changbai Mountain, China

Understanding the responses of soil organic carbon (SOC) fractions to altitudinal gradient variation is important for understanding changes in the carbon balance of forest ecosystems. In our study the SOC and its fractions of readily oxidizable carbon (ROC), water-soluble carbon (WSC) and microbial biomass carbon (MBC) in the soil organic and mineral horizons were investigated for four typical forest types, including mixed coniferous broad-leaved forest (MCB), dark coniferous spruce-fir forest (DCSF), dark coniferous spruce forest (DCS), and Ermans birch forest (EB), along an altitudinal gradient in the Changbai Mountain Nature Reserve in Northeast China. The results showed that there was no obvious altitudinal pattern in the SOC. Similar variation trends of SOC with altitude were observed between the organic and mineral horizons. Significant differences in the contents of SOC, WSC, MBC and ROC were found among the four forest types and between horizons. The contents of ROC in the mineral horizon, WSC in the organic horizon and MBC in both horizons in the MCB and EB forests were significantly greater than those in either DCSF or DCS forest. The proportion of soil WSC to SOC was the lowest among the three main fractions. The contents of WSC, MBC and ROC were significantly correlated (P < 0.05) with SOC content. It can be concluded that vegetation types and climate were crucial factors in regulating the distribution of soil organic carbon fractions in Changbai Mountain.

Multiyear precipitation reduction strongly decreases carbon uptake over northern China

Drought has been a concern in global and regional water, carbon, and energy cycles. From 1999 to 2011, northern China experienced a multiyear precipitation reduction that significantly decreased water availability as indicated by the Palmer Drought Severity Index and soil moisture measurements. In this study, a light use efficiency model (EC-LUE) and an ecosystem physiological model (IBIS) were used to characterize the impacts of long-term drought on terrestrial carbon fluxes in northern China. EC-LUE and IBIS models showed the reduction of averaged GPP of 0.09 and 0.05 Pg C yr-1 during 1999-2011 compared with 1982-1998. Based on the IBIS model, simulated ecosystem respiration experienced an insignificant decrease from 1999 to 2011. The multiyear precipitation reduction changed the regional carbon uptake of 0.011 Pg C yr-1 from 1982 to 1998 to a net source of 0.018 Pg C yr-1 from 1999 to 2011. Moreover, a pronounced decrease in maize yield in almost all provinces in the study region was found from 1999 to 2011 versus the average of yield from1978 to 2011. The largest maize yield reduction occurred in Beijing (2499kgha-1yr-1), Jilin (2180kgha-1yr-1), Tianjing (1923kgha-1yr-1), and Heilongjiang (1791kgha-1yr-1), and the maize yield anomaly was significantly correlated with the annual precipitation over the entire study area. Our results revealed that recent climate change, especially drought-induced water stress, is the dominant cause of the reduction in the terrestrial carbon sink over northern China.

Estimation of the gross primary production of an old-growth temperate mixed forest using eddy covariance and remote sensing

Continuous flux data from CO2 flux sites can be used to improve our understanding of leaf phenology and validate the algorithms of satellite‐ based carbon cycling models. In this study, we conducted a simulation of the Vegetation Photosynthesis Model (VPM) using the Enhanced Vegetation Index (EVI) and the Land Surface Water Index (LSWI) derived from the 8‐day Moderate Resolution Imaging Spectroradiometer (MODIS) surface reflectance product, as well as site‐specific air temperature, biological temperature, and photosynthetically active radiation (PAR) data. Gross primary production (GPP) estimates derived from the VPM were compared with field observations of a flux tower in an old temperate mixed forest in northeastern China during 2003–2005. Time series data for the EVI have a stronger exponential relationship with the GPP (R 2 = 0.74, n = 67, p<0.01) than those for the Normalized Difference Vegetation Index (NDVI) (R 2 = 0.62, n = 67, p<0.01), indicating a different light use efficiency during the different stages of foliage development. In comparison to the flux tower GPP, the VPM‐predicted GPP captured the onset of the growing season well, and their seasonal dynamics were generally consistent in terms of phase in the peak growing season, while the end date of the growing season was 8–16 days earlier than that of field measurements. The annual forest GPP estimated from the flux tower observations varied from 1312 g C m−2 (grams of carbon per metre squared) to 1490 g C m−2 in the three observation years from 2003 to 2005, which is less 10% different from the VPM‐based annual GPP. These results demonstrate the potential of the satellite‐driven VPM for scaling up the GPP of forests at the CO2 flux tower site, a key issue for the study of the carbon budget at regional scales.

Soil Nematode Responses to Increases in Nitrogen Deposition and Precipitation in a Temperate Forest

The environmental changes arising from nitrogen (N) deposition and precipitation influence soil ecological processes in forest ecosystems. However, the corresponding effects of environmental changes on soil biota are poorly known. Soil nematodes are the important bioindicator of soil environmental change, and their responses play a key role in the feedbacks of terrestrial ecosystems to climate change. Therefore, to explore the responsive mechanisms of soil biota to N deposition and precipitation, soil nematode communities were studied after 3 years of environmental changes by water and/or N addition in a temperate forest of Changbai Mountain, Northeast China. The results showed that water combined with N addition treatment decreased the total nematode abundance in the organic horizon (O), while the opposite trend was found in the mineral horizon (A). Significant reductions in the abundances of fungivores, plant-parasites and omnivores-predators were also found in the water combined with N addition treatment. The significant effect of water interacted with N on the total nematode abundance and trophic groups indicated that the impacts of N on soil nematode communities were mediated by water availability. The synergistic effect of precipitation and N deposition on soil nematode communities was stronger than each effect alone. Structural equation modeling suggested water and N additions had direct effects on soil nematode communities. The feedback of soil nematodes to water and nitrogen addition was highly sensitive and our results indicate that minimal variations in soil properties such as those caused by climate changes can lead to severe changes in soil nematode communities.

How recent climate change influences water use efficiency in East Asia

Water use efficiency (WUE), defined as the ratio of gross primary productivity to evapotranspiration, is an important indicator of the trade-off between water loss and carbon gain. We used a biophysical process-based model to examine the relative importance of climate-induced changes in meteorological factors and leaf area index (LAI) on the changes in WUE in East Asia. Validation showed that our simulation could capture the magnitudes and variations of WUE at 18 flux sites in Asia. Regional results indicated that the highest WUE occurred in boreal forests at high latitudes and the lowest WUE in desert areas of China. Changes in meteorological factors negatively affected WUE in the northwestern, northern, and eastern study regions. Changes in LAI had determinant impacts on changes in WUE in most areas except for those with sparse or low-density vegetation (e.g., western interior China, southeast island countries) where meteorological factors dominated. We conclude that, aside from the impact of meteorological factors on WUE, climate-induced changes in LAI may play a prominent role in regulating WUE changes.

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.

Global parameterization and validation of a two-leaf light use efficiency model for predicting gross primary production across FLUXNET sites

Light use efficiency (LUE) models are widely used to simulate gross primary production (GPP). However, the treatment of the plant canopy as a big leaf by these models can introduce large uncertainties in simulated GPP. Recently, a two-leaf light use efficiency (TL-LUE) model was developed to simulate GPP separately for sunlit and shaded leaves and has been shown to outperform the big-leaf MOD17 model at six FLUX sites in China. In this study we investigated the performance of the TL-LUE model for a wider range of biomes. For this we optimized the parameters and tested the TL-LUE model using data from 98 FLUXNET sites which are distributed across the globe. The results showed that the TL-LUE model performed in general better than the MOD17 model in simulating 8 day GPP. Optimized maximum light use efficiency of shaded leaves (epsilon(msh)) was 2.63 to 4.59 times that of sunlit leaves (epsilon(msu)). Generally, the relationships of epsilon(msh) and epsilon(msu) with epsilon(max) were well described by linear equations, indicating the existence of general patterns across biomes. GPP simulated by the TL-LUE model was much less sensitive to biases in the photosynthetically active radiation (PAR) input than the MOD17 model. The results of this study suggest that the proposed TL-LUE model has the potential for simulating regional and global GPP of terrestrial ecosystems, and it is more robust with regard to usual biases in input data than existing approaches which neglect the bimodal within-canopy distribution of PAR.