Natural and NH3-induced variation in epicuticular needle wax morphology of Pseudotsuga menziesii (Mirb.) Franco

Changes in epicuticular wax morphology of Pseudotsuga menziesii needles were studied with scanning electron microscopy throughout the growing season in current-year and older needles in 20 trees from two sites, Kootwijk and Garderen ( The Netherlands). Fusion of crystalline wax rods leading to a reticulate structure and ultimately to degradation of the regular three-dimensional porous structure started several weeks after bud break and reached a high level at the end of the first growing season. The increase in amorphous ( solid) wax showed a similar, but slightly slower development. In 1- and 2-year-old needles the degradation of crystalline wax and increase in amorphous wax had progressed only slightly further. The rates of change in crystalline wax morphology were very similar for sun-exposed and shaded needles, for the two sites, and for the two tree vitality classes included in the samples. The development of epiphyllous fungi also appeared to be largely needle age dependent. The effects of NH3 on needle wax morphology were studied in young trees used in fumigation experiments. Short-term fumigation (approx. 5 weeks) did not affect wax morphology in current-year needles, but 1-year-old needles which had been exposed to different concentrations from bud break onwards showed a severe degradation of the crystalline wax, regardless of the NH3 concentration used. In 2-year-old needles the effect of NH3 could not be traced and was overshadowed by the natural ageing process. Ambient O-3, SO2 and NOx levels did not effect epicuticular wax morphology. It is suggested that the variation recorded for the two forest stands does not show effects of local pollution levels.