Epsilon Open Archive

Abstract

The aim of this thesis was to investigate how nitrogen (N) enrichment influences ecophysiological processes involved in driving changes in understory species composition in Swedish boreal forests. Studies were performed in a long-term N experiment started in 1996 including the following treatments: control, N additions (12.5 and 50 kg N ha-1 yr-1) and recovery (50 kg N ha-1 yr-1 for five years and then no N addition). Firstly, I studied plant-available N forms deposited with throughfall precipitation, and estimated uptake by mosses and lichens of this N. Regardless of the N treatments, rainwater contributed c. 2 kg N ha-1 yr-1 and snowmelt c. 0.3 kg N ha-1 yr-1 to the vegetation. The ground-living bryophyte Hylocomium splendens and the epiphytic lichen Platismatia glauca took up both organic (glycine) and inorganic (NH4+ and NO3-) N from the precipitation. The uptake did not significantly differ between the N treatment plots. On the 50 kg N ha-1 yr-1 plots the abundance of H. splendens decreased by 81% following eight years of N additions. The ecophysiological response of H. splendens to this N treatment included accumulation of arginine, but no significant changes in its soluble carbohydrate or chlorophyll contents were detected. Secondly, I studied N competition between Vaccinium myrtillus and Deschampsia flexuosa. I found no significant effects of the long-term N treatments on plant uptake of four different N forms (NH4+, NO3-, glycine and peptides). Both plants acquired N from NH4+, NO3- and glycine, but no substantial uptake from peptides was found. When N uptake of the two species was related to the plant biomass, D. flexuosa acquired all N forms more efficiently than V. myrtillus, but the difference between the species in this respect was greatest for NO3-. Finally, results of long-term (12 years) monitoring of the understory vegetation on control, 12.5 and 50.0 kg N ha-1 yr-1 plots demonstrated that two natural enemies (the fungal pathogen Valdensia heterodoxa and the herbivorous larval form of Operophtera spp.) exerted strong control over the abundance of the dominant plant, V. myrtillus. The study highlights the need for long-term studies to fully capture biotic interactions that influence vegetation dynamics. In summary, changes in N supply may have profound effects on quantitative and qualitative aspects of plant N availability, plant N uptake, plant biochemistry as well as interactions between plants and their natural enemies.