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    STUDIA GEOLOGIA - Issue no. 1 / 2007  
         
  Article:   MODELING SEASONAL VARIATIONS OF CARBON ISOTOPE COMPOSITIONS (δ13C) IN TREE-RINGS AND CO2-RESPIRED BY THE TRUNK.

Authors:  THOMAS EGLIN, CHRISTOPHE FRANCOIS, CLAIRE DAMESIN, FLORENCE MAUNOURY, ERIC DUFRENE, NICOLAS DELPIERRE. 		 
       
         
  Abstract:  Carbon isotope composition (δ13C) of plant material and respired CO2 are widely used to assess the effect of climatic conditions on plant ecophysiology. First of all, these are largely affected by the atmospheric CO2 signature (about -8‰) and by fractionation processes occurring within the plant, in particular during photosynthesis (Farquhar et al., 1989) and respiration (Tcherkez et al., 2003). δ 13C is usually interpreted using a model of photosynthetic discrimination described in Farquhar et al. (1989). However, this model does not fully explain δ 13C seasonal variations in tree-rings (Porté & Loustau, 2001) and CO2 respired by the trunk (Maunoury et al., 2007). Carbon allocation and compartmentalization among tree organs have a role in this seasonal variability (Damesin & Lelarge, 2003). For example, starch is generally 13C - enriched compared to soluble sugars. Its synthesis, mobilization for growth and turnover directly influence the δ 13C of the other compounds. A model integrating known C fractionation processes, carbon allocation and C stock compartmentalization among tree organs has been developed. This model is coupled with CASTANEA (Dufrêne et al., 2005), a forest ecosystem model simulating carbon balance (photosynthesis, growth and respiration) and drought effects on canopy stomatal conductance from climatic data at a half-hourly time-scale. Five organs are described: leaves, branches, trunk, coarse roots and fine roots. C stocks are split into starch and soluble sugars within each organ. Simulated isotope compositions are compared to measurements carried out in an oak forest (Barbeau, France) during the year 2003. These measurements consist of the δ 13C of total organic matter, soluble sugars and starch in leaves and branches, the δ 13C of total organic matter in treerings and the v13C of CO2 respired by the trunk.  
         
     
         
         
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