Frost hardiness of Norway spruce grown under elevated atmospheric CO2 and increased nitrogen fertilizing

Frost hardiness was tested on needles from 6-years-old Norway spruces that had been cultivated for 2 years in artificial ecosystems at an atmospheric CO2 concentration of 280, 420 or 560 ppm and a nitrogen fertilization corresponding to 0, 30, and 90 kg N ha(-1) yr(-1) (NH4NO3) as well as combinations of these treatments. Samples of previous year's needles were taken after completion of frost hardening in autumn, and at three dares during the dehardening process in spring when potentially harmful late frost events naturally occur. The needles were exposed to freezing temperatures down to -35 degrees C. Samples were taken at 5 degrees C intervals and frost injury was estimated by the electrolyte leakage technique. After autumnal hardening, needles were injured only below -25 degrees C (LT50 < -30 degrees C), while after dehardening in spring the critical frost temperature (LT50) was around -10 degrees C. Frost hardiness was not influenced by the different CO2 and nitrogen treatments of the trees neither in autumn nor during the dehardening phase in spring.