Time to chill: Effects of simulated global change on leaf ice nucleation temperatures of subarctic vegetation

We investigated the effects of long-term (7-yr) in situ CO2 enrichment (600 mu mol/mol) and increased exposure to UV-B radiation, the latter an import ant component of global change at high latitudes, on the ice nucleation tem peratures of leaves of several evergreen and deciduous woody ericaceous shr ubs in the subarctic (68 degrees N). Three (Vaccinium uliginosum, V. vitis- idaea, and Empetrum hermaphroditum) of the four species of shrubs studied s howed significantly higher ice nucleation temperatures throughout the 1999 growing season in response to CO2 enrichment and increased exposure to UV-B radiation relative to the controls. The same species also showed a strong interactive effect when both treatments were applied together In all cases, leaves cooled to below their ice nucleation temperatures failed to survive the damage resulting from intracellular ice formation. Our results strongl y suggest that future global change on a decadal time scale (atmospheric CO 2 increases and polar stratospheric O-3 destruction) will lend to increased foliage damage of subarctic vegetation by severe late spring or early autu mnal frosting events. Indeed, in support of our experimental findings, ther e is now some evidence that increases in atmospheric CO2 concentration over the past three to four decades may already have acted in this manner on hi gh-elevation arboreal plants in the Swedish Scandes. The implications for v egetation modeling in a future "greenhouse" world and palaeoclimate estimat es from high-latitude plant fossils dating to the high-CO2 environment of t he Mesozoic are discussed.