Janssens Ivan A.

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Janssens
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Ivan A.
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  • Preprint
    Steeper declines in forest photosynthesis than respiration explain age-driven decreases in forest growth
    ( 2014-04) Tang, Jianwu ; Luyssaert, Sebastiaan ; Richardson, Andrew D. ; Kutsch, Werner ; Janssens, Ivan A.
    The traditional view of forest dynamics originated by Kira, Shidei, and Odum suggests a decline in net primary productivity (NPP) in ageing forests due to stabilized gross primary productivity (GPP) and continuously increased autotrophic respiration (Ra). The validity of these trends in GPP and Ra is, however, very difficult to test because of the lack of long-term ecosystem-scale field observations of both GPP and Ra. Ryan and colleagues have proposed an alternative hypothesis drawn from site-specific results that aboveground respiration and belowground allocation decreased in ageing forests. Here we analyzed data from a recently assembled global database of carbon fluxes and show that the classical view of the mechanisms underlying the age-driven decline in forest NPP is incorrect and thus support Ryan’s alternative hypothesis. Our results substantiate the age-driven decline in NPP, but in contrast to the traditional view, both GPP and Ra decline in ageing boreal and temperate forests. We find that the decline in NPP in ageing forests is primarily driven by GPP, which decreases more rapidly with increasing age than Ra does, but the ratio of NPP/GPP remains approximately constant within a biome. Our analytical models describing forest succession suggest that dynamic forest ecosystem models that follow the traditional paradigm need to be revisited.
  • Article
    Soil respiration at mean annual temperature predicts annual total across vegetation types and biomes
    (Copernicus Publications on behalf of the European Geosciences Union, 2010-07-09) Bahn, Michael ; Reichstein, M. ; Davidson, Eric A. ; Grunzweig, J. ; Jung, M. ; Carbone, M. S. ; Epron, D. ; Misson, L. ; Nouvellon, Y. ; Roupsard, O. ; Savage, K. ; Trumbore, Susan E. ; Gimeno, C. ; Curiel Yuste, J. ; Tang, Jianwu ; Vargas, Rodrigo ; Janssens, Ivan A.
    Soil respiration (SR) constitutes the largest flux of CO2 from terrestrial ecosystems to the atmosphere. However, there still exist considerable uncertainties as to its actual magnitude, as well as its spatial and interannual variability. Based on a reanalysis and synthesis of 80 site-years for 57 forests, plantations, savannas, shrublands and grasslands from boreal to tropical climates we present evidence that total annual SR is closely related to SR at mean annual soil temperature (SRMAT), irrespective of the type of ecosystem and biome. This is theoretically expected for non water-limited ecosystems within most of the globally occurring range of annual temperature variability and sensitivity (Q10). We further show that for seasonally dry sites where annual precipitation (P) is lower than potential evapotranspiration (PET), annual SR can be predicted from wet season SRMAT corrected for a factor related to P/PET. Our finding indicates that it can be sufficient to measure SRMAT for obtaining a well constrained estimate of its annual total. This should substantially increase our capacity for assessing the spatial distribution of soil CO2 emissions across ecosystems, landscapes and regions, and thereby contribute to improving the spatial resolution of a major component of the global carbon cycle.