Although the photosynthetic response of tree species to growth at an elevated CO2 concentration (CO2) has been widely investigated, very few studies have grown trees through to canopy closure. Critically, canopy closure increases competition for resources such as light, water and nutrients, all factors likely to modify the photosynthetic response to elevated [CO2]. This study took advantage of FACE technology to investigate the effect of elevated [CO2] on photosynthesis in three Populus clones, P. alba, P. x euramericana and P. nigra in an intensively managed agro-forestry system. Throughout the study, all three clones showed large increases in photosynthesis.
The stimulation of diurnal carbon sequestration ranged from 40% to 60%, while light saturated photosynthesis increased between 17% and 25%. To determine whether photosynthetic acclimation had occurred, the photosynthetic capacity was quantified by measuring the maximum velocity of carboxylation (Vc,max) and the maximum rate of RuBP regeneration (Jmax).
Throughout this study significant photosynthetic acclimation was infrequent, and did not increase in magnitude or occurrence during canopy closure in any of the three clones. Interestingly, some reduction in Rubisco protein appeared to occur in all three clones of poplar, however this was not important in decreasing photosynthesis at elevated [CO2]. A loss of photosynthetic capacity has been frequently correlated with an accumulation of carbohydrates. In keeping with this, no consistent increase in carbohydrate accumulation was evident.
In conclusion elevated [CO2] failed to decrease the photosynthetic capacity of any of the three Populus clones, which sustained a significant stimulation of photosynthesis throughout canopy closure.