16.12. Mire plants and carbon dioxide dynamics under increased tropospheric ozone concentration and UV-B radiation

Public examination of a doctoral dissertation in the field of Ecosystem Ecology

Doctoral candidate: M.Sc. Jaana Haapala 

Date and venue: 16.12.2011, at 12 noon, N100, Natura, Joensuu campus 

Abstract

The background ozone concentration has more than doubled since the beginning of the 20th century in the northern hemisphere and this trend has been predicted to continue in the near future. At the same time, depletion of the stratospheric ozone layer has led to increased UV-B radiation levels in the polar regions. Increased tropospheric ozone concentrations and UV-B radiation are known to affect plant photosynthesis, growth and the soil microbial community. However, little is known about their long-term effects on ecosystem level carbon dioxide (CO2) dynamics. This thesis assesses the long-term effects of moderately elevated ozone concentration or supplemental UV-B radiation on the photosynthetic capacity of selected mire plant species and on ecosystem level CO2 exchange. The presented results were attained from two multi-year experiments; from a four-year free air ozone exposure study with fen microcosms and from a three-year UV-B exposure study on a natural fen. 

In the ozone experiment, only high ozone levels occurring in early summer affected the photosynthesis related parameters of the studied sedge, Eriophorum vaginatum. Furthermore, ecosystem level gross photosynthesis was decreased in the first growing season under supplemental ozone. However, during the following years, an opposite trend was detected due to the slight increase in the sedge leaf density in the ozone treatment. The effects of supplemental UV-B radiation on a natural fen were negligible. No significant effect on the plant cell fine structure or plant photosynthesis could be detected. At the ecosystem level, the instantaneous ecosystem dark respiration was slightly lower in the UV-B treatment. However, the change was statistically significant only in the third year, when a rapid drawdown of the water-table caused an anomalous respiration peak in all treatments.  

In conclusion, neither of the studied stressors had any drastic effect on the net ecosystem CO2 exchange over the long-term. Only the high springtime ozone concentrations transiently decreased the plant photosynthetic capacity and caused short-term ecosystem level changes. However, the long-term effects of both stressors differed greatly from the initial effects detected in the first study year and were very dependent on other prevailing environmental conditions. On the basis of these results it seems unlikely that the predicted increase in the background ozone concentration or the future levels of UV-B radiation would significantly decrease the net CO2 uptake of northern fen ecosystems. 

The doctoral dissertation of M.Sc. Jaana Haapala entitled “Mire plants and carbon dioxide dynamics under increased tropospheric ozone concentration and UV-B radiation” will be examined at the Faculty of Science and Forestry.The opponent in the public examination is Docent Sirkku Manninen, University of Helsinki and the custos is Professor Riitta Julkunen-Tiitto of the University of Eastern Finland.

Contact: Jaana Haapala,  jaana.haapala@uef.fi

Publishing year: 2011

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