Field measurements of stem respiration in three clones of Populus tremuloides subjected to elevated carbon dioxide and ozone fumigation in a FACE experiment

Stem respiration measurements by carbon dioxide efflux were carried out in three aspen (Populus tremuloides Michx.) clones subjected to elevated carbon dioxide and ozone treatments, separately and in combination, using an automated multi-cuvette system at the USDA-FS Harshaw Experimental Forest research station near Rhinelander WI. In all three clones, mean nighttime respiration and daily total net carbon loss increased in stems subjected to CO₂ fumigation (1.64 umol CO₂/m²/s, 1.93 grams C/m²/d) compared to non-fumigated control stems (1.27 umol CO₂/m²/s, 1.45 grams C/m²/d). Stems subjected to O₃ alone showed the lowest dark respiration maxima (0.87 umol/m2/s) and the least respiration sensitivity (Q₁₀) to temperature (1.90). Adding CO₂ to the O₃ treatment (combined CO₂+O₃) increased both the nighttime respiration maxima (1.20 umol/m²/s) and the temperature sensitivity (2.47), compared to O₃ alone. The overall mean  temperature sensitivities of stem respiration, as indicated by Q₁₀ values, were variable, dependent on experimental treatment, and ranged from 1.8 to 3.0, similar to the range of values reported for other Populus species.