Document: UWE-3-66-9

Modeling autotrophic respiration for the DUKE forest: What the CO₂-induced alterations implicate.

GRUETERS, U.* ¹, J.KATTGE ² and Y.LUO ¹

University of Oklahoma, Norman, OK 73019, USA ¹
Justus Liebig Universitaet Giessen ²

Abstract:
At the Duke forest canopy-level responses to elevated CO₂ (E) were studied in an intact pine (Pinus taeda) plantation exposed to 200 ppm above ambient levels (A) using a free-air CO₂ enrichment technique. A model for the autotrophic respiration in the Duke forest was developed. The model separates respiration (R) into maintenance (R_M) and growth respiration (R_G). Biomass related data was available from the Duke forest (allometric relationships, nitrogen concentrations, leaf area, fine root mass + turnover). Parameterization of the respiration itself was done pine-specific, using published temperature and nitrogen/sapwood-volume functions for R_M and tissue construction cost for R_G, respectively. Since long-term respiration equals GPP minus NPP, model evaluation was performed by comparison with modeled GPP subtracted by measured NPP after some modification. In 1997, 900 (A) and 1326 gC m^-2 yr^-1 (E) were available for the autotrophic respiration, suggesting a 51% increase due to CO₂. Model results fitted into this range only when high N-concentrations of fine roots were applied exclusively to the class with diameter less than 1mm and when a sapwood-volume function instead of a nitrogen function was applied for modeling the bole respiration. In that case, model results amounted to 1009/1050 gC m^-2 yr^-1 for A/E treatments. None of the processes implemented so far in the respiration model allows respiration to increase to the extent that was predicted by R implied by GPP-NPP. These results have three possible implications: The CO₂-induced GPP increase was overestimated and/or a carbohydrate sink in the Duke forest is missing and/or the autotrophic respiration model needs to be revised to have a stronger influence of carbohydrate status.

Keywords: elevated CO@2@, FACE, Pinus taeda, model, autotrophic respiration

This abstract is being presented at: 12:00 PM in session:
Oral Session #40: Elevated CO₂ In Forest Systems.