Effects of chronic N fertilization on foliar membranes, cold tolerance, and carbon storage in montane red spruce

We evaluated the influence of protracted low-level nitrogen (N) fertilization on foliar membrane-associated calcium (mCa), sugar and starch concentrations, membrane stability, winter cold tolerance, and freezing injury of red spruce (Picea rubens Sarg.) trees growing in six experimental plots on Mount Ascutney, Vermont. For 12 consecutive years before this evaluation, each plot received one of three treatments: 0, 15.7, or 31.4 kg N(.)ha-1(.)year-1 supplied as NH4Cl. In comparison with trees from control plots, the current-year foliage of trees from N-addition plots had lower mCa concentrations, higher levels of electrolyte leakage, reduced cold tolerance, and greater freezing injury. Levels of mCa, membrane stability, and cold tolerance did not differ between N treatments, but trees in high-N treated plots experienced greater freezing injury. Although no differences in carbohydrate nutrition were detected in September, foliar sugar and starch concentrations from trees in N-treated plots were higher than control plot trees in January. We propose that foliar mCa deficiencies reduced cell membrane stability, decreased cold tolerance, and increased freezing injury for trees in N addition plots relative to controls. Declines in mCa may also help account for increases in respiration previously measured. Because soil, root, and mycorryhizal conditions were not evaluated, it is unknown how treatment-induced changes in these compartments may have influenced the alterations in foliar mCa and physiological parameters measured in this study.