Epsilon Open Archive

Abstract

This thesis is concerned with the possible effects of a further increase in atmospheric carbon dioxide on vegetation in a semi-natural grassland. It is based on an experimental field study focusing on production responses and changes in vegetation composition. A Swedish semi-natural grassland was exposed to ambient and twice ambient elevated CO2 in open-top chambers during six consecutive growing seasons. Plots without chambers constituted a control treatment. The field had been grazed previously, but when the experiment started this was replaced by a cutting regime with one cut (0 cm above the soil surface) each year in early August. Leaf-level photosynthesis and stomatal conductance, biomass production, and vegetation composition were studied. The first three growing seasons showed a 30 to 60 % increase in above-ground biomass at harvest under elevated CO2. In later years there was no difference in above-ground biomass between the treatments, but differences between treatments increased below ground. Throughout the experiment, elevated CO2 increased leaf-level photosynthesis and tended to decrease stomatal conductance. Nitrogen concentration in stems and leaves was lower at elevated CO2. CO2 effects on vegetation composition were small in comparison with effects of time and/or weather, but increased towards the end of the experiment. Legumes may benefit most from elevated CO2, which also tended to increase diversity (Shannon index) in summer, but decrease it in spring. Changes in the weather (especially precipitation) were highly directional over time. This means that weather effects are confounded with effects that are time-dependent (such as a possible succession after a change in management, or an increasing nutrient deficit). However, weather effects did appear to be decisive.