In situ Effects of Elevated Atmospheric CO2on Leaf Freezing Resistance and Carbohydrates in a Native Temperate Grassland

The objectives of this study were to quantify changes in leaffreezing resistance and carbohydrate concentrations caused bylong-term (6 years) exposure to elevated CO₂(ambient: 360 µll^-1, elevated: 600 µl l^-1) in five dominant plant speciesgrowing in situ in a native temperate grassland. Across allfive species tested from three functional groups, the mean temperatureat which all leaves were damaged (T₁₀₀) significantly (P = 0.016)increased from -9.6 to -8.5 °C under elevated CO₂ , anda similar marginally significant (P = 0.079) reduction was observedfor the mean temperature that caused 50% leaf damage (T₅₀),from -6.7 to -6.0 °C. The mean temperature at which initialleaf damage was observed (T₀) was not significantly influencedby elevated CO₂ . Although concentrations of soluble sugars(+25%,P = 0.042), starch (+53%, P < 0.001), and total non-structuralcarbohydrates (TNC, +40%, P < 0.001) were significantly higherunder elevated CO₂ , leaf freezing resistance actually decreasedunder elevated CO₂ . Concentrations of soluble sugars were positivelycorrelated with freezing resistance when viewed across all fivecommunity dominants, but within any individual species, no suchrelationships were found. We also found no evidence for ouroriginal hypothesis that increased concentrations of solublesugars increase freezing resistance. Thus, future atmosphericCO₂levels may instead increase the risk of late spring freezingdamage. Furthermore, the strong differences in freezing resistanceobserved among the species, along with decreased freezing resistance,may increase the risk of losing species that have inherentlyweak freezing resistances from the plant community. Copyright2001 Annals of Botany Company CO₂enrichment, frost hardiness, sugar, starch, total non-structural carbohydrates (TNC)