The structure of forest canopies is an important determinate of the interaction between forests and the aerial environment. Leaves at the top of the canopy have maximum exposure to light, but the utilization of absorbed light energy in photosynthesis also is dependent on the nitrogen content of leaves, which may be more expensive to maintain in high-light leaves. Tree productivity may be maximized when the combined distributions of leaf area and nitrogen in the canopy are optimized. The morphology and nitrogen concentration of leaves of trees grown in a CO2-enriched atmosphere are well described, but assessment of leaf and nitrogen distributions in forest canopies is far more challenging. The completion of the 12-year free-air CO2 enrichment (FACE) experiment in a closed-canopy sweetgum (Liquidambar styraciflua) stand presented the opportunity for destructive harvest of experimental trees, including complete deconstruction of their crowns. In July, 2009, three to four trees were cut from each of five FACE plots (eight from ambient CO2 and eight from elevated CO2). Twenty leaves were collected from each 1-m increment of crown depth, and their area, thickness, dry mass, and nitrogen concentration were measured. All other leaves from each depth increment were collected, oven-dried, and weighed.
Leaves of trees grown in elevated CO2 were smaller, thicker, and denser than leaves of trees in ambient CO2. Both thickness and density contributed to the CO2 effect on leaf mass per unit leaf area (LMA) which was 12% greater in elevated CO2. LMA, thickness, and density all were greater at the top of the tree crowns, but the effect of CO2 was fairly consistent throughout the crowns. Trees in elevated CO2 had a larger fraction of their leaf area higher in their crowns (maximum at 2 m depth) compared to ambient-grown trees (maximum at 3 m depth). The distribution of nitrogen within the crowns was similar to that of leaf area and was similarly affected by CO2. Foliar nitrogen content per unit leaf area declined with depth in the crown and was 6% less in elevated CO2 throughout the crown. Reflecting the trend in LMA with crown depth, N content on a leaf mass basis increased with crown depth and was 18% less in elevated CO2. Foliar nitrogen concentration was very low, especially at the top of the crowns in CO2 enriched trees, consistent with a decade long trend of increasing nitrogen limitation in this forest stand.