ACID MIST AFFECTS DEHARDENING, BUDBURST, AND SHOOT GROWTH IN RED SPRUCE

Red spruce seedlings growing in open top chambers in a nutrient-poor m edium were exposed to mists containing one of five different combinati ons of H+, SO42-, NO3-, and NH4+ ions, three at pH 5.6 and two at pH 2 .5. The mists were applied twice weekly from May until November, and t he plants overwintered outdoors with no additional mist treatment. See dlings that received mists containing sulphate (1.6 mol m-3) the previ ous growing season were more frost-hardy (i.e., their dehardening was delayed) the following April than seedlings that received lower concen trations of sulphate or none. Neither ammonium nor nitrate in the appl ied mist had any effect on dehardening, but they did advance budburst when applied together. In the presence of sulphate there was no effect of ammonium-N on the time of budburst. Exposure to mists containing n itrogen significantly increased the dry weight and length of the leadi ng shoots that emerged from the buds and extended the following summer . There was no evidence that the stimulatory effects of nitrogen on gr owth made the seedlings less frost-hardy in spring.