What was done
Open-top chambers erected on a nutrient-poor calcareous soil in NW Switzerland have been receiving atmospheric CO2 concentrations of 360 and 600 ppm for six years to study the effects of elevated CO2 on the growth and development of plants in this naturally occurring temperate grassland.  In this paper, the authors report on how elevated CO2 affected leaf carbohydrate concentrations and leaf freezing resistance in the five most abundant species of the community.

What was learned
Averaged across the five tested species, elevated CO2 increased the foliar concentrations of sugar, starch and total nonstructural carbohydrates by 25, 53 and 40%, respectively.  Contrary to expectations, however, these enhanced carbohydrate concentrations did not increase the freezing protection of leaves exposed to low air temperatures.  In fact, the average temperatures at which freezing damage occurred on 50 and 100% of the tested leaf areas actually rose by 0.7 and 1.1°C, respectively, with atmospheric CO2 enrichment.

What it means
As the CO2 content of the air increases, temperate grassland species will undoubtedly display enhanced rates of photosynthesis that will likely lead to greater concentrations of foliar carbohydrates.  In turn, greater concentrations of foliar carbohydrates should lower the effective concentration of water in leaves, thus lowering the air temperature at which freezing damage occurs, as was previously reported for dormant buds in birch seedlings (Wayne et al., 1998).  In the present study, however, atmospheric CO2 enrichment did not confer any significant degree of freezing resistance upon individual tested leaves in these grassland species.

One reason for this unexpected result may be that the leaves were excised from plants prior to testing, so that leaf carbohydrate concentrations (although high) may still have been too low to offer increased freezing resistance in the CO2-enriched plants.  If the tests had been done on intact plants, individuals may have responded to reduced air temperatures by mobilizing carbohydrates out of their roots and into their leaves to produce high enough foliar carbohydrate concentrations to effectively convey increased resistance to freezing.  Clearly, more research needs to be conducted within this area to develop a better and more conclusive picture of the likely effects of the ongoing rise in the air's CO2 content on the freezing point of plant leaves.

A final point for all climate alarmists to remember in this regard is that in the face of global warming of the magnitude they continue to predict for the degree of atmospheric CO2 enrichment employed in this study, the slight elevation of the freezing point of leaves would not pose the slightest problem to the plants, for the air temperature of their environment would have risen even more.

Reference
Wayne, P.M., Reekie, E.G. and Bazzaz, F.A.  1998.  Elevated CO2 ameliorates birch response to high temperature and frost stress: implications for modeling climate-induced geographic range shifts.  Oecologia 114: 335-342.