What was done
The authors studied three perennial plants of contrasting morphology --Anthyllis vulneraria (a forb), Sanguisorba minor (a forb) and Bromopsis erecta (a grass)-- common to chalk grassland swards that exist in fragments scattered across much of Britain and continental Europe by growing them in plastic tubs that were inserted in the ground within established FACE plots.  These experimental swards were exposed to CO2 concentrations of 355 or 600 ppm for 14 months, after which various leaf gas exchange measurements were made to determine the effect of elevated CO2 on photosynthesis.  In addition, gas exchange measurements were also made following a simulated grazing event that altered source:sink balance of the plants.

What was learned
Prior to simulated grazing, all three species grown in elevated CO2 exhibited down regulation of photosynthesis, as indicated by an average 32% decrease in rubisco activity.  However, rates of net photosynthesis were still stimulated by approximately 28% in the FACE plots for all species except vulneraria, which exhibited rates not significantly different from those in the ambient control plots.  Two weeks after the simulated grazing event, however, the two forbs had reversed their decrease in rubisco activity and were not exhibiting any characteristics of down regulation, while the grass continued to display an approximate 20% reduction in rubisco activity.  Nevertheless, the grass at this time exhibited a 30% CO2-induced increase in net photosynthesis, while the forbs exhibited increases of 40%.

What it means
CO2-induced growth stimulation in these nutrient-poor chalk soils was moderate to negligible as long as the well being of the plants was not challenged.  Under such circumstances, the CO2-enriched plants "down regulated" their photosynthetic machinery to make more efficient use of scarce nitrogen reserves. When much of their foliage was removed in a simulated grazing event, however, the plants exposed to elevated CO2 directed greater quantities of this limited resource back to their remaining leaves, where they could make better use of it and hasten the production of new foliage.  This chain of events suggests that atmospheric CO2 enrichment may help plants better withstand, or even recover from, the debilitating effects of having their foliage eaten by pests and livestock, even under less-than-optimum conditions of soil fertility.