Effects of CO 2 enrichment on whole-plant carbon budget of seedlings of Fagus grandifolia and Acer saccharum in low irradiance.

Carbon exchange rates (CER) and whole-plant carbon balances of beech (Fagus grandifolia) and sugar maple (Acer saccharum) were compared for seedlings grown under low irradiance to determine the effects of atmospheric CO ₂ enrichment on shade-tolerant seedlings of co-dominant species. Under contemporary atmospheric CO ₂ , photosynthetic rate per unit mass of beech was lower than for sugar maple, and atmospheric CO ₂ enrich ment enhanced photosynthesis for beech only. Aboveground respiration per unit mass decreased with CO ₂ enrichment for both species while root respiration per unitmass decreased for sugar maple only. Under contemporary atmoapheric CO ₂ , beech had lower C uptake per plant than sugar maple, while C losses per plant to nocturnal aboveground and root respiration were similar for both species. Under elevated CO ₂ , C uptake per plant was similar for both species, indicating a significant relative increase in whole-seedling CER with CO ₂ enrich ment for beech but not for sugar maple. Total C loss per plant to aboveground respiration was decreased for beech only because increase in sugar maple leaf mass counterbalanced a reduction in respiration rates. Carbon loss to root respiration per plant was not changed by CO ₂ enrichment for either species. However, changes in maintenance respiration cost and nitrogen level suggest changes in tissue composition with elevated CO ₂ . Beech had a greater net daily C gain with CO ₂ enrichment than did sugar maple in contrast to a lower one under contemporary CO ₂ . Elevated CO ₂ preferentially enhances the net C balance of beech by increasing photosynthesis and reducing respiration cost. In all cases, the greatest C lost was by roots, indicating the importance of belowground biomass in net C gain. Relative growth rate estimated from biomass accumulation was not affected by CO ₂ enrichment for either species possibly because of slow growth under low light. This study indicates the importance of direct effects of CO ₂ enrichment when predicting potential change in species distribution with global climate change.