Sun Shucun

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Sun
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Shucun
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  • Preprint
    Chlorophyll fluorescence tracks seasonal variations of photosynthesis from leaf to canopy in a temperate forest
    ( 2016-11) Yang, Hualei ; Yang, Xi ; Zhang, Yongguang ; Heskel, Mary ; Lu, Xiaoliang ; Munger, J. William ; Sun, Shucun ; Tang, Jianwu
    Accurate estimation of terrestrial gross primary productivity (GPP) remains a challenge despite its importance in the global carbon cycle. Chlorophyll fluorescence (ChlF) has been recently adopted to understand photosynthesis and its response to the environment, particularly with remote sensing data. However, it remains unclear how ChlF and photosynthesis are linked at different spatial scales across the growing season. We examined seasonal relationships between ChlF and photosynthesis at the leaf, canopy, and ecosystem scales, and explored how leaf-level ChlF was linked with canopy-scale solar induced chlorophyll fluorescence (SIF) in a temperate deciduous forest at Harvard Forest, Massachusetts, USA. Our results show that ChlF captured the seasonal variations of photosynthesis with significant linear relationships between ChlF and photosynthesis across the growing season over different spatial scales (R2=0.73, 0.77 and 0.86 at leaf, canopy and satellite scales, respectively; p<0.0001). We developed a model to estimate GPP from the tower-based measurement of SIF and leaf-level ChlF parameters. The estimation of GPP from this model agreed well with flux tower observations of GPP (R2=0.68; p<0.0001), demonstrating the potential of SIF for modeling GPP. At the leaf scale, we found that leaf Fq’/Fm’, the fraction of absorbed photons that are used for photochemistry for a light adapted measurement from a pulse amplitude modulation fluorometer, was the best leaf fluorescence parameter to correlate with canopy-SIF yield (SIF/APAR, R2=0.79; p<0.0001). We also found that canopy-SIF and SIF-derived GPP (GPPSIF) were strongly correlated to leaf-level biochemistry and canopy structure, including chlorophyll content (R2=0.65 for canopy-GPPSIF and chlorophyll content; p<0.0001), leaf area index (LAI) (R2=0.35 for canopy-GPPSIF and LAI; p<0.0001), and normalized difference vegetation index (NDVI) (R2=0.36 for canopy-GPPSIF and NDVI; p<0.0001). Our results suggest that ChlF can be a powerful tool to track photosynthetic rates at leaf, canopy, and ecosystem scales.
  • Article
    Seasonal variations of leaf and canopy properties tracked by ground-based NDVI imagery in a temperate forest
    (Nature Publishing Group, 2017-04-28) Yang, Hualei ; Yang, Xi ; Heskel, Mary ; Sun, Shucun ; Tang, Jianwu
    Changes in plant phenology affect the carbon flux of terrestrial forest ecosystems due to the link between the growing season length and vegetation productivity. Digital camera imagery, which can be acquired frequently, has been used to monitor seasonal and annual changes in forest canopy phenology and track critical phenological events. However, quantitative assessment of the structural and biochemical controls of the phenological patterns in camera images has rarely been done. In this study, we used an NDVI (Normalized Difference Vegetation Index) camera to monitor daily variations of vegetation reflectance at visible and near-infrared (NIR) bands with high spatial and temporal resolutions, and found that the infrared camera based NDVI (camera-NDVI) agreed well with the leaf expansion process that was measured by independent manual observations at Harvard Forest, Massachusetts, USA. We also measured the seasonality of canopy structural (leaf area index, LAI) and biochemical properties (leaf chlorophyll and nitrogen content). We found significant linear relationships between camera-NDVI and leaf chlorophyll concentration, and between camera-NDVI and leaf nitrogen content, though weaker relationships between camera-NDVI and LAI. Therefore, we recommend ground-based camera-NDVI as a powerful tool for long-term, near surface observations to monitor canopy development and to estimate leaf chlorophyll, nitrogen status, and LAI.