Canopy profiles of photosynthetic parameters under elevated CO2 and N fertilization in a poplar plantation

A poplar plantation has been exposed to an elevated CO2 concentration for 5 years using the free air CO2 enrichment (FACE) technique. Even after such a long period of exposure, leaves of Populus x euramericana have not shown clear signs of photosynthetic acclimation. Only at the end of the growing season for shade leaves was a decrease of maximum velocity of carboxylation (V-cmax) observed. Maximum electron transport rate (J(max)) was increased by FACE treatment in July. Assimilation rates at CO2 partial pressure of 400 (A(400)) and 600 (A(600)) mu mol mol(-1) were not significantly different under FACE treatment. Most notably FACE significantly decreased stomatal conductance (g(s)) both on upper and lower canopy leaves. N fertilization increased N content in the leaves on mass basis (N-m) and specific leaf area (SLA) in both CO2 treatments but did not influence the photosynthetic parameters. These data show that in poplar plantations the long-term effects of elevated CO2 on photosynthesis do not differ considerably from the short-term ones even with N deposition. (c) 2005 Elsevier Ltd. All rights reserved.