An increased risk of frost is expected during the growing season, as climate warming increases spring temperatures in the Arctic. Because deciduous species have a growth season limited in length and also have generally larger conduit volumes, they are more likely than evergreens to be injured by freezethaw-induced cavitation during the growing season. To test whether growth at elevated temperature increases susceptibility to freezethaw damage, we grew a deciduous arctic shrub species (Salix pulchra Cham.) in simulated Alaskan summer temperatures and at 5 Cabove the ambient simulation (+5 C plants) in controlled environments. Stem specific hydraulic conductivity (ks) and leaf stomatal conductance (gs) were measured in plants grown at both temperatures before and after a freeze treatment simulating a mid-season frost. Before the freeze treatment, specific xylem conductivitywas 2.5 times higher and stomatal conductances were 1.3 times higher in +5 C plants than in ambient-grown plants. Reductions in hydraulic conductivity and stomatal conductance as a result of the freeze were 3.5 and 1.8 times greater respectively in +5 C plants than in ambient- grown plants. Many of the +5 C plants showed extensive leaf damage. Plants grown in the two treatments also differed in comparative xylem anatomy; +5 C plants had larger vessel diameters (25.4 versus 22.6 ?m) and higher vessel densities (71 versus 67.4 vesselsmm2) than ambient- grown plants. Our results suggest that higher growing season temperatures will increase the susceptibility of arctic deciduous shrubs to frost damage, which may offset their competitive growth advantage.
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