No proportional increase of terrestrial gross carbon sequestration from the greening Earth

Earth observations from space have shown a widespread greening over the land surface since the beginning of this century. However, how this substantial greening translates to gross carbon sequestration or gross primary production (GPP), especially in the context of climate change, is not well established. The greening could lead to a proportional increase in GPP by enhancing the absorbed photosynthetic active radiation. Climate warming may further alleviate temperature stress in cold regions, increasing GPP. However, high temperature and low precipitation following the ongoing warming may increase climate stress, reducing GPP. Global GPP dynamics are further complicated by human‐induced land use change such as deforestation. To date, a consensus on global GPP dynamics and the driving forces are still elusive. Here we investigated terrestrial GPP dynamics and the respective contributions of climate change and vegetation cover change (VCC) from 2000 to 2015. A series of experiments were designed to disaggregate the effects of climate and VCC on global GPP, aiming to understand how recent greening translated into gross carbon sequestration. Our study highlights the potential vulnerability of terrestrial gross carbon sequestration under climate and land use changes and has important implications in the global carbon cycle and climate warming mitigation. This work, funded by NASA Carbon Cycle Science, has been published in Journal of Geophysical Research: Biogeosciences (2019).

Research Highlights

• Global terrestrial GPP did not increase in proportion to the greening on Earth
• Enhanced global terrestrial GPP largely contributed by nonforests, especially cropland
• Contrasting GPP changes in forest and cropland may have shifted the distribution of land carbon sink

Read More: Y. Zhang, C. Song, L. E. Band, G. Sun, J. Li, 2019. No proportional increase of terrestrial gross carbon sequestration from the greening Earth. Journal of Geophysical Research: Biogeosciences https://doi.org/10.1029/2018JG004917

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Niño-Southern Oscillation-induced variability of terrestrial gross primary production during the satellite era

Terrestrial gross primary production (GPP) is the largest carbon flux entering the biosphere from the atmosphere, which serves as a key driver of global carbon cycle and provides essential matter and energy for life on land. However, terrestrial GPP variability is still poorly understood and difficult to predict, especially at the annual scale. As a major internal climate oscillation, El Niño‐Southern Oscillation (ENSO) influences global climate patterns and thus may strongly alter inter-annual terrestrial GPP variation. Using a remote sensing‐driven ecosystem model with long‐term satellite and climate data, we comprehensively examined the impacts of ENSO on global GPP dynamics from 1982 to 2016, focusing on lag effects of ENSO and their spatial heterogeneity. Our study linking the ENSO cycle to spatiotemporal variations of GPP could improve our understanding of inter- annual variability and climatic drivers of global GPP, potentially leading to improvements of both short‐term forecasts of vegetation productivity and longer‐term projections of climatic influences on the carbon cycle. This work, funded by NASA Carbon Cycle Science, has been published in Journal of Geophysical Research: Biogeosciences (2019).

Research Highlights

• The composite ONI in the previous‐year's August‐October shows the strongest correlation with global annual GPP
• ENSO effects on annual GPP exhibit diverse seasonal evolutions, and the timing of peak ENSO influences are heterogeneous across the globe
• TRENDY ensemble appears to miss the major lag effects of ENSO identified in the satellite‐derived GPP and top‐down‐based land carbon sink

Read More: Y. Zhang, M.P. Dannenberg, T. Hwang, C. Song, 2019. El Niño-Southern Oscillation-induced variability of terrestrial gross primary production during the satellite era. Journal of Geophysical Research: Biogeosciences https://doi.org/10.1029/2019JG005117

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Reanalysis of global terrestrial vegetation trends from MODIS products: Browning or greening?

Terrestrial vegetation is a key component of our biosphere and significantly regulates global carbon, water and energy exchanges between the land and the atmosphere. Systematically monitoring global vegetation dynamics can help us to better understand basic biogeochemical processes, and their possible feedbacks to the climate system, thus improve our ability to predict, mitigate and adapt to future global climate change. One focus of my research is on characterizing such dynamics on different spatial and temporal scales using high-quality digital optical remote sensing data. This work, funded by US National Science Foundation and Natural Science Foundation of China, has been published in Remote Sensing of Environment (2017).

Research Highlights

• A significant greening Earth was detected by MODIS Terra-C6 VIs.
• The widespread browning areas in Terra-C5 VIs were likely artifacts of sensor degradation.
• A greening Earth suggests a positive feedback of terrestrial vegetation under global environmental change

Read More: Y. Zhang, C. Song, L. E. Band, G. Sun, J. Li, 2017. Reanalysis of global terrestrial vegetation trends from MODIS products: browning or greening? Remote Sensing of Environment. https://doi.org/10.1016/j.rse.2016.12.018

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Development of a coupled carbon and water model for estimating global gross primary productivity and evapotranspiration based on eddy flux and remote sensing data

The terrestrial biosphere on Earth is breathing continuously by inhaling carbon dioxide from the atmosphere and exhaling water vapor into the atmosphere. However, these 'invisible' gas exchanges, playing an important role in regulating climate system, are highly dynamic and have long been difficult to quantify. One direction of my research is to develop eco-hydrological model based on ground observations from global flux tower data and remote sensed data from different sources of satellite products (e.g. NASA, ESA). Related work has been funded by National Science Foundation, USDA Forest service, and NASA Carbon Cycle Science. The recent developed Coupled Carbon and Water (CCW) model has been published in Agricultural and Forest Meteorology (2016).

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Understanding moisture stress on light use efficiency across terrestrial ecosystems based on global flux and remote-sensing data

Ecosystem light-use efficiency (LUE) is a key biophysical parameter characterizing the ability of plants to convert absorbed light to carbohydrate where the gaseous CO₂ in the atmosphere removed by plant growth is stored. However, the responses of LUE to environmental regulations, especially moisture stress, are poorly understood and weakly represented in current ecosystem models, leading to large uncertainties in the estimation of ecosystem carbon sequestration. Another direction of my research is to investigate the influences of water stress on LUE and the representations of different water stress indicators based on site-level flux tower data and remote sensing data. The relevant work, funded by U.S. National Science Foundation, USDA Forest Service and Chinese Natural Science Foundation, has been published in Journal of Geophysical Research (JGR)-Biogeosciences (2015).

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Effects of land-use/land-cover and climate changes on terrestrial net primary productivity in the Yangtze River Basin of China

Human-induced land use/cover change (LUCC) and climate change represent the most dynamic aspects of global environmental changes and have profound impacts on ecosystem carbon sequestration. However, the effects of these two factors on terrestrial carbon sequestration are always lumped together. To date, there is no effective way to disaggregate them. Previous study always omitted one factor when studying the effect of the other. One major aim of my research is to decouple the effects of LUCC and climate change on terrestrial carbon sequestration based on ecological model. Related work, funded by National Natural Science Foundation of China, U.S. National Science Foundation, and Chinese Academy of Sciences, has been published in Journal of Geophysical Research (JGR)-Biogeosciences (2014).

Research Highlights

• LUCC has stronger impacts on NPP than climate change in Yangtze River Basin
• Forest restoration is an efficient carbon sequestration mechanism
• NPP models with static LULC could lead to increasingly large errors with time

Read More: Y. Zhang, C. Song, K. Zhang, X. Cheng, L. E. Band, Q. Zhang, 2014. Effects of land-use/land-cover and climate changes on terrestrial net primary productivity in the Yangtze River Basin, China, from 2001-2010. Journal of Geophysical Research: Biogeosciences. https://doi.org/10.1002/2014JG002616

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Spatial–temporal variability of terrestrial vegetation productivity in the Yangtze River Basin during 2000 to 2009

Research Highlights

• The spatial pattern of NPP in the YRB was predominantly determined by GPP and further modified by AR
• Increases in AR offset 62% of the total increased GPP, leading to a substantial decline of CUE, particularly in the warmer lower altitude regions
• NPP in the middle and lower sub-basins in the YRB is more sensitive to future climate warming

Read More: Y. Zhang, C. Song, K. Zhang, X. Cheng, Q. Zhang, 2014. Spatial-temporal variability of terrestrial net primary productivity in the Yangtze River Basin from 2000-2009. Journal of Plant Ecology. https://doi.org/10.1093/jpe/rtt025

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Dynamics and Sustainability of Coupled Human and Natural Systems

As a research scholar with international and multi-discipline backgrounds, I further expand my research interests to the dynamics and sustainability of Coupled Human and Natural Systems (CHANS). Understanding the complexity and interactions of Coupled Human and Natural Systems (CHANS) is central to the quest for both human well-being and global sustainability. By collaborating with interdisciplinary background researchers, I’ve further examined the complex interactions between humans and natural systems at diverse scales. Related works have been published in Biological Conservation (2013), Soil Biology & Biochemistry (2013), and Forest Policy and Economics (2014, 2015). I’ve also been involved to investigate the environmental impacts caused by large engineer projects such as the world largest dam Three Gorges Dam, and North Water Transfer Project by collaborating scientists from Chinese Academy of Sciences. Related works have been published in Acta Oecologica (2011), Journal of Hazardous Materials (2011), Ecological Engineering (2012) and Environmental Sciences and Pollution Research (2013). Among these collaborating researches, my expertise in vegetation modeling based on advanced statistics and remote sensing/GIS is a key component.