In situ effects of elevated atmospheric CO2 on leaf freezing resistance and carbohydrates in a native temperate grassland

The objectives of this study were to quantify changes in leaf freezing resi stance and carbohydrate concentrations caused by long-term (6 years) exposu re to elevated CO2 (ambient: 360 mul l(-1), elevated. 600 mul l(-1)) in fiv e dominant plant species growing in situ in a native temperate grassland. A cross all five species tested from three functional groups, the mean temper ature at which all leaves were damaged (T-100) significantly (P = 0.016) in creased from -9.6 to -8.5 degreesC under elevated CO2, and a similar margin ally significant (P = 0.079) reduction was observed for the mean temperatur e that caused 50% leaf damage (T-50), from -6.7 to -6.0 degreesC. The mean temperature at which initial leaf damage was observed (T-o) was not signifi cantly influenced by elevated CO2. Although concentrations of soluble sugar s (+25%, P = 0.042), starch (+53%, P < 0.001), and total non-structural car bohydrates (TNC, +40%, P < 0.001) were significantly higher under elevated CO2, leaf freezing resistance actually decreased under elevated CO2. Concen trations of soluble sugars were positively correlated with freezing resista nce when viewed across all five community dominants, but within any individ ual species, no such relationships were found. We also found no evidence fo r our original hypothesis that increased concentrations of soluble sugars i ncrease freezing resistance. Thus, future atmospheric CO2 levels may instea d increase the risk of late spring freezing damage. Furthermore, the strong differences in freezing resistance observed among the species, along with decreased freezing resistance, may increase the risk of losing species that have inherently weak freezing resistances from the plant community. (C) 20 01 Annals of Botany Company.