Does long-term elevation of CO2 concentration increase photosynthesis in forest floor vegetation? Indiana strawberry in a Maryland forest

As the partial pressure of CO2 (pCO(2)) in the atmosphere rises, photorespiratory loss of carbon in C-3 photosynthesis will diminish and the net efficiency of light-limited photosynthetic carbon uptake should rise. We tested this expectation for Indiana strawberry (Duchesnea indica) growing on a Maryland forest floor. Open-top chambers were used to elevate the pCO(2) of a forest floor habitat to 67 Pa and were paired with control chambers providing an ambient pCO(2) of 38 Pa. After 3.5 years, D. indica leaves grown and measured in the elevated pCO(2) showed a significantly greater maximum quantum efficiency of net photosynthesis (by 22%) and a lower light compensation point (by 42%) than leaves grown and measured in the control chambers. The quantum efficiency to minimize photorespiration, measured in 1% O-2, was the same for controls and plants grown at elevated pCO(2). This showed that the maximum efficiency of light-energy transduction into assimilated carbon was not altered by acclimation and that the increase in light-limited photosynthesis at elevated pCO(2) was simply a function of the decrease in photorespiration. Acclimation did decrease the ribulose-1,5-bisphosphate carboxylase/oxygenase and light-harvesting chlorophyll protein content of the Leaf by more than 30%. These changes were associated with a decreased capacity for light-saturated, but not light-limited, photosynthesis. Even so, leaves of D. indica grown and measured at elevated pCO(2) showed greater light-saturated photosynthetic rates than leaves grown and measured at the current atmospheric pCO(2). In situ measurements under natural forest floor lighting showed large increases in leaf photosynthesis at elevated pCO(2), relative to controls, in both summer and fall. The increase in efficiency of light-limited photosynthesis with elevated pCO(2) allowed positive net photosynthetic carbon uptake on days and at locations on the forest floor that light fluxes were insufficient for positive net photosynthesis in the current atmospheric pCO(2).