Effects of elevated carbon dioxide on sweetgum ecophysiology .

摘要

The photosynthetic responses of sweetgum (Liquidambar styraciflua L.) trees growing in a forest stand to exposure to an elevated-CO ₂ atmosphere were examined over three growing seasons at the Oak Ridge National Laboratory's Free-Air CO₂ Enrichment (FACE) facility in eastern Tennessee, USA. Net photosynthetic rates at the growth CO₂ concentration (A_growth) of mature upper-canopy leaves were found to be 44% higher in trees grown in elevated CO₂ (∼553 μmol mol−1) compared with ambient CO ₂ (∼364 μmol mol−1). There were no significant CO₂ treatment effects on the CO₂- and light-saturated rate of photosynthesis (A_max), maximum rubisco carboxylation rate (V_cmax) or the rate of ribulose bisphosphate (RuBP) regeneration via electron transport (J _max), indicating that exposure to 1.5 times ambient CO₂ did not limit the photosynthetic capacity of sweetgum leaves. In addition, a more-complete canopy profile was developed by examining the photosynthetic response of young leaves and leaves sampled from lower in the tree crown. The A _growth of leaves exposed to elevated CO₂ was enhanced by 76% in young and mature leaves growing in the upper canopy, and 48% in mature leaves collected in the upper and middle canopies. Within each leaf-age and canopy-position class, growth in elevated CO₂ enhanced A_growth by 123% in young leaves and 37% in mature leaves, and by 45% and 52% in mature upper- and middle-canopy leaves, respectively. There were no CO₂ effects on A_max, V_cmax, or J_max, of leaves in either of the age or canopy-position classes. A profile of the vertical distribution of nitrogen through the sweetgum canopy was developed to provide for the estimate of CO₂ effects on canopy- and stand-level carbon assimilation in sweetgum. Mean nitrogen concentration (mass-based; N_M) was 9% lower in the leaves of trees grown in ambient versus elevated CO₂ concentrations, with no CO₂ effects noted on foliar nitrogen when analyzed per unit leaf area (N _A), and per unit leaf volume (N_V). These results provide evidence that leaves sampled throughout the sweetgum canopy exhibit enhanced photosynthesis in a CO₂-enriched atmosphere, and that the indeterminate growth exhibited by this tree species provides sufficient sink strength so that photosynthetic capacity is not downregulated.