1. Greenhouse gas (CO2, CH4, and N2O) emissions and carbon sequestration (blue carbon) in response to climate change and human management in coastal wetlands.
We uses the mobile gas flux system in combination with an eddy flux measurement system to examine the effects of nitrogen loading and wetland restoration on CO2, CH4, and N2O emissions and the mechanisms that control blue carbon stocks in coastal wetlands. Our initial results indicated that the restoration of coastal tidal wetlands could dramatically decrease carbon loss by decreasing CH4 emissions compared with freshwater wetlands and upland ecosystems.
2. Arctic ecotypes and climate change
We explore how arctic plant populations of Eriophorum vaginatum, or cottongrass, respond differently to climate change along a latitudinal gradient. These locally adapted populations, or ecotypes, differ from each other on the genetic level. We use field experiments (transplanting and warming experiments) and genetic analyses to explore the ecosystem function and its response to the changing environment.
3. Phenology, fluorescence, and carbon cycling
We explore phenology and its effect on ecosystem carbon balance by linking ground-based and satellite measurements with laboratory analysis. We developed a novel method to combine near-surface cameras, a field-based spectroradiometer, and an automated fluorescence spectroscopy system to monitor seasonality of leaf growth, color, structure, and chlorophyll and nitrogen contents. We found that the start of growing season has advanced 0.143 day per year in New England during the past 40 years (Yang et al. 2012). We further found that the increasingly-used camera-based phenology observation has to be corrected by field-based spectral measurement to reveal the physiological aspect of phenology (Yang et al. 2014; Liu et al. 2015) instead of the simple color pattern. We measured in situ fluorescence signals emitted from forest canopy and generated a first-ever year-around fluorescence dataset at a minute-frequency (Yang et al. 2015), which could be used to derive canopy GPP and incorporated into satellite-bases sensors for direct measurement of global pattern GPP.
4. Abiotic and biotic controls of soil respiration and soil carbon dynamics.
We develop an automated chamber system to measure CO2 gas fluxes from soils and trees at Harvard Forest. We found that the carbohydrate flow from tree leaves down to stems and to roots drove the respiration components, a process that has not been incorporated into our current knowledge in ecosystem respiration and the modeling effort.
In the lab, we test CO2 diffusivity and water evaporation from soil samples collected from the National Ecological Observatory Network (NEON) sites all over the U.S. and aim to develop a universal CO2 diffusivity model that can be applied to intact soils. The model will be used to calculate surface CO2 fluxes measured from belowground CO2 profile systems (Tang et al. 2003, 2005).
Watch Jim Tang’s talk on video for the AmeriFlux/NACP meeting 2011 (scroll down to Monday, January 31, 2:30 PM and click video).
5. Greenhouse gas emissions from agro-ecosystems and their responses to fertilization and management.
We developed an in situ greenhouse gas measurement system that uses advanced cavity ring-down spectroscopy and cavity enhanced absorption techniques. By measuring greenhouse gas emissions in cropland with various treatments, we have found that compost and biochar additions significantly decreased greenhouse gas emissions while not compromising crop yields (Ryals et al in preparation). Water, nitrogen availability, and soil carbon contents are important drivers for N2O emissions (Gelfand et al. 2015).
6. Synthesis/synergistic activities
- We synthesized global data on photosynthesis and respiration across biomes and forest age. Our results confirmed the age-driven decline in net primary production (NPP) as initiated by Odum’s theory in ecosystem dynamics; but in contrast to the traditional view, we found that the decline in NPP in aging forests is primarily driven by gross primary production (GPP), which decreases more rapidly with age than autotrophic respiration (Tang et al. 2014, PNAS).
- We (lead by Jim Tang, Pamela Templer, Kevin Kroeger, and Joanna Carey) synthesized warming experiments worldwide to advance our understanding of ecosystem responses to climate change. We held two synthesis meetings in 2014 and 2015 in the USGS Powell Center for Analysis and Synthesis in Fort Collin, Colorado. We expect to generate important publications that will address the response, sensitivity, and adaptation of ecosystems to warming.
- We (lead by Kevin Kroeger, Lisamarie Windham-Myers, Emily Pidgeon, Jennifer Howard, and Jim Tang) propose to develop a “Global Science and Data Network for Coastal Blue Carbon” (SBC). Supported by the Carbon Cycle Interagency Working Group (CCIWG), within the US Carbon Cycle Program, and by the Commission for Environmental Cooperation, the first workshop is held on January 12-14, 2016 at the USGS Western Region Headquarters in Menlo Park, California.