Effects of long‐term exposure to elevated CO2 and N fertilization on the development of photosynthetic capacity and biomass accumulation in Quercus suber L.
The effects of long-term (4 year) CO 2 enrichment (70 Pa versus 35 Pa) and nitrogen nutrition (8 m M versus 1 m M NO 3 ‐ ) on biomass accumulation and the development of photosynthetic capacity in leaves of cork oak ( Quercus suber L., a Mediterranean evergreen tree) were studied. The evolution of photosynthetic parameters with leaf development was estimated by fitting the biochemical model of Farquhar et al . ( Planta 149, 78‐90, 1980) with modifications by Sharkey ( Botanical Review 78, 71‐75, 1985) to A ‐ C i response curves. CO 2 enrichment had a small reduction effect on the development of the maximum CO 2 fixation capacity by Rubisco ( V Cmax ), and no effect over maximum electron transport capacity ( J max ), day-time respiration ( R d ) and Triose-P utilization (TPU). However, there was a statistically significant effect of N fertilization and the interaction CO 2 × N over the evolution of V Cmax , J max and TPU. Relative stomatal limitation (estimated from A ‐ C i curves) was higher (+20%) for plants grown under ambient CO 2 than for plants grown under elevated CO 2 . There was a significant effect of CO 2 and N fertilization over total biomass accumulation as well as leaf area. Plants grown at elevated CO 2 had 27% more biomass than plants grown at ambient CO 2 when given high N. However, for plants grown under low N there was no significant effect of CO 2 enrichment on biomass accumulation. Plants grown under low N also had significantly higher root : shoot ratios whereas there were no differences between CO 2 treatments. The larger biomass accumulation of Q. suber under elevated CO 2 is attributable to a higher availability of CO 2 coupled to a larger leaf area, with no significant decrease in photosynthetic capacity under CO 2 enrichment and elevated N fertilization. For low N fertilization, the effects of CO 2 enrichment over leaf area and biomass accumulation are lost, suggesting that in native ecosystems with low N availability, the effects of CO 2 enrichment may be insignificant.