|About the Book|
Atmospheric [CO2] and soil N availability are critical resources for plant growth, both of which are increasing due to global climate change. Therefore, it is important to understand how additional resources in the form of elevated CO2 and increasedMoreAtmospheric [CO2] and soil N availability are critical resources for plant growth, both of which are increasing due to global climate change. Therefore, it is important to understand how additional resources in the form of elevated CO2 and increased N availability impact photosynthesis as the main driver of plant productivity. I conducted studies on pines, grasses and forbs, all grown under long-term Free-Air CO2 Enrichment (FACE) and nitrogen fertilization, to determine how these global change factors affect plant photosynthesis and nitrogen use.-Both forbs and pines showed down-regulation of photosynthetic capacity under elevated CO2 as changes in Vcmax of -23% and -17%, respectively. Grasses did not show significant photosynthetic down-regulation under elevated CO2 compared to ambient CO 2. Grasses showed the least reduction of Nmass in elevated CO2 (-7%), followed by pines (-12%) and forbs (-18%). When reductions in photosynthetic capacity occurred, as was observed in forbs, a smaller photosynthetic stimulation of 9% occurred under elevated CO2 than when no down-regulation was observed. Compared to forb species, CO2-induced photosynthetic stimulation was 31% to 57% for pines and grasses, respectively. A reduction in foliar N concurrent with down-regulation of photosynthetic capacity in elevated CO2 could indicate plant N redistribution where N is allocated away from photosynthetic components. This N redistribution in response to elevated CO2 may be a key response in adjusting plant growth to long-term elevated CO 2.-At the canopy scale, increased leaf area index (LAI) under elevated CO 2 due to photosynthetic enhancement could compensate for the effects of physiological down-regulation on canopy photosynthesis. Grasses had higher canopy photosynthesis than forbs under elevated CO2 due to large LAI increases in combination with no photosynthetic down-regulation. Both LAI and photosynthetic down-regulation are important in determining plant canopy productivity in elevated CO2. Given that few models include CO2-induced photosynthetic adjustments such as decreased foliar N or reduced photosynthetic capacity, I conclude that much of previous experimental work on CO2 enrichment has greatly overestimated photosynthetic enhancement in native ecosystems. Interacting effects of long-term elevated CO2 and N fertilization may ultimately determine the magnitude of C uptake from the atmosphere and overall plant productivity.