Elevated CO2 concentration affects leaf photosynthesis–nitrogen relationships in Pinus taeda over nine years in FACE
Kristine Y. Crous (1, 2), Michael B. Walters (3) and David S. Ellsworth (4)
1. School of Natural Resources and Environment, University of Michigan, Ann Arbor, MI 48109-1115, USA / 2. Corresponding author () / 3. Department of Forestry, Michigan State University, East Lansing, MI 48824, USA / 4. Centre for Plant and Food Science, University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 1797, Australia / Received March 5, 2007; accepted July 16, 2007; published online February 1, 2008
Summary
To investigate whether long-term elevated carbon dioxide concentration ([CO2]) causes declines in photosynthetic enhancement and leaf nitrogen (N) owing to limited soil fertility, we measured photosynthesis,
carboxylation capacity and area-based leaf nitrogen concentration (Na) in Pinus taeda L. growing in a long-term free-air CO2 enrichment (FACE) facility at an N-limited site. We also determined how maximum rates of carboxylation (Vcmax) and electron transport (Jmax) varied with Na under elevated [CO2]. In trees exposed to elevated [CO2] for 5 to 9 years, the slope of the relationship between leaf photosynthetic capacity (Anet-Ca) and Na was significantly reduced by 37% in 1-year-old needles, whereas it was unaffected in current-year needles. The slope of the
relationships of both Vcmax and Jmax with Na decreased in 1-year-old needles after up to 9 years of growth in elevated [CO2], which was accompanied by a 15% reduction in N allocation to the carboxylating enzyme. Nitrogen fertilization (110 kg N
ha–1) in the ninth year of exposure to elevated [CO2] restored the slopes of the relationships of Vcmax and Jmax with Na to those of control trees (i.e., in ambient [CO2]). The Jmax:Vcmax ratio was unaffected by either [CO2] or N fertilization. Changes in the apparent allocation of N to photosynthetic components may be an important adjustment
in pines exposed to elevated [CO2] on low-fertility sites. We conclude that fundamental relationships between photosynthesis or its component processes with
Na may be altered in aging pine needles after more than 5 years of exposure to elevated atmospheric [CO2].
