Effects of elevated COb2s on sweetgum ecophysiology



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Texas Tech University


The photosynthetic responses of sweetgum {Liquidambar styraciflua L.) trees growing in a forest stand to exposure to an elevated-C02 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 (Agrowth) of mature upper-canopy leaves were found to be 44% higher in trees grown in elevated CO2 (-553 µmol mo-1) compared with ambient CO2 (-364 µmol mol''). There were no significant CO2 treatment effects on the CO2- and light-saturated rate of photosynthesis (Amax), maximum rubisco carboxylation rate (Vcmax) or the rate of ribulose bisphosphate (RuBP) regeneration via electron transport (Jmax), 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 Agrowth 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 Agrowth by 123% in young leaves and 31% in mature leaves, and by 45% and 52% in mature upper- and middle-canopy leaves, respectively. There were no CO2 effects on Amax, Vcmax or Jmax 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; NM) 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 {NA), and per unit leaf volume {Nv). These results provide evidence that leaves sampled throughout the sweetgum canopy exhibit enhanced photosynthesis in a C02-enriched atmosphere, and that the indeterminate growth exhibited by this tree species provides sufficient sink strength so that photosynthetic capacity is not downregulated.



Forest canopies, Carbon dioxide (CO2) -- Tennessee -- Environmental aspects, Sweetgum, Photosynthesis, Trees -- Tennessee -- Ecophysiology, Atmospheric carbon dioxide, Carbon dioxide (CO2) -- Environmental aspects, Ecophysiology, Carbon dioxide (CO2) -- Physiological effect, Trees -- Tennessee -- Effect of atmospheric carbon