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

Abstract

Abstract 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 2 for 2.7 years and trees of white oak ( Quercus alba L.) grown at these same CO 2 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 −1 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 2 partial pressures. These CO 2 -induced differences in W G ranged from 3.7% in yellow-poplar roots to 2.1% in white oak leaves. Only in the case of yellow-poplar leaves, however, were differences in V G observed between CO 2 treatments. Leaf V G was 1.46 g glucose g −1 dry mass in ambient-grown trees compared with 1.41 g glucose g −1 dry mass for CO 2 -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 −2 for yellow-poplar trees grown at ambient CO 2 and 106 g glucose m −2 for trees grown at elevated CO 2 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 2 partial pressures. These differences were driven largely by CO 2 -induced changes in total biomass. We conclude that while construction costs were lower at elevated CO 2 partial pressures, the magnitude of this response argues against an increased efficiency of carbon use in the growth processes of trees exposed to CO 2 enrichment.