Energetic Costs of Tissue Construction in Yellow-poplar and White Oak Trees Exposed to Long-term CO2Enrichment

Two methods were used to estimate construction costs for leaves, stems, branches and woody roots of yellow-poplar (Liriodendron tulipifera L.) trees grown at ambient (35 Pa) and elevated (65 Pa) CO # for 2n7 years and trees of white oak (Quercus alba L.) grown at these same CO # partial pressures for 4 years. Sample combustion in a bomb calorimeter combined with measurements of ash and nitrogen content provided the primary method of estimating tissue construction costs (W G ; g glucose g -" dry mass). These values were compared with a second, simpler method in which cost estimates were derived from tissue ash, carbon and nitrogen content (V G ). Estimates of W G were lower for leaves, branches and roots of yellow-poplar and for leaves of white oak grown at elevated compared with ambient CO # partial pressures. These CO # -induced differences in W G ranged from 3n7 % in yellow-poplar roots to 2n1% in white oak leaves. Only in the case of yellow-poplar leaves, however, were differences in V G observed between CO # treatments. Leaf V G was 1n46 g glucose g -" dry mass in ambient-grown trees compared with 1n41 g glucose g -" dry mass for CO # -enriched trees. Although paired-estimates of W G and V G clustered about a 1 : 1 line for leaves and branches, estimates of V G were consistently lower than W G for stems and roots. Construction costs per unit leaf area were 95 g glucose m -# for yellow-poplar trees grown at ambient CO # and 106 g glucose m -# for trees grown at elevated CO # partial pressures. No differences in area-based construction costs were observed for white oak. Whole-plant energy content was 1220 g glucose per tree in ambient-grown white oak compared with 2840 g glucose per tree for those grown at elevated CO # partial pressures. These differences were driven largely by CO # -induced changes in total biomass. We conclude that while construction costs were lower at elevated CO # partial pressures, the magnitude of this response argues against an increased efficiency of carbon use in the growth processes of trees exposed to CO # enrichment.