Atmospheric nitrogen deposition and canopy retention influences on photosynthetic performance at two high nitrogen deposition Swiss forests

Portable chlorophyll fluorometry measurements, providing plant photosynthetic efficiency (PE) data, were carried out at two contrasting Swiss forests experiencing high nitrogen (N) deposition. Fluorometry data were obtained in conjunction with controlled N treatment applications within forest canopies to more realistically simulate deposition of plant-available N species. At the high N deposition Novaggio oak forest, growing season canopy N applications caused increases in PE and other photosynthetic measures. Similar N applications at the Lägeren mixed beech and spruce forest site indicated a possible PE decrease in beech leaves and no effect on spruce needles. N is considered a growth-limiting nutrient in temperate environments where low to moderate N deposition can benefit forest growth; however, high N deposition can have negative effects on forest health and growth due to nutrient imbalances. We conclude that the growth effect dominates at both sites, thereby increasing the potential for carbon sequestration. We found clear evidence of direct leaf-level canopy N uptake in combination with increased PE at the Novaggio oak forest site and no definitive evidence of negative N effects at the Lägeren site. We conclude that PE measurements with chlorophyll fluorometry is a useful tool to quantify N and carbon exchange aspects of deciduous forest dynamics.