On 26 August at 10.15 Aigar Niglas will defend his PhD thesis "Effects of environmental factors on gas exchange in deciduous trees: focus on photosynthetic water-use efficiency" in the Faculty of Science and Technology in the University of Tartu.
Thesis supervisor: Associate Professor Arne Sellin, University of Tartu, Estonia
Opponent: Associate Professor Jürgen Burkhardt, University of Bonn, Germany
Environmental factors are steadily changing over time, while the atmospheric CO2 concentration and the Earth’s average temperature are predicted to increase during this century. Extreme weather events are also likely to become more frequent. On a regional scale, more frequent rainfall and higher air humidity are predicted for Northern Europe. At the same time, there is a lack of knowledge of how increasing air humidity affects forest ecosystems. To fill this gap, the Free Air Humidity Manipulation (FAHM) experimental facility has been built in Eastern Estonia in order to investigate the impact of increased air humidity on forest ecosystem functioning. In the present thesis, acclimation of leaf-level processes to higher air humidity and its ultimate effect on tree growth was studied in two deciduous tree species.
Our results reveal that effects of air humidity vary among the study years, depending on prevailing weather conditions. In rainy years, the high air humidity reduces soil nutrient acquisition and therefore tree growth rate declines. In drier years, high air humidity alleviates the effects of summer drought and the trees’ growth is rather stimulated. We also found that trees acclimated to higher air humidity show disproportionate changes in sensitivity of gaseous versus liquid-phase conductance to water deficit, which may impose a great risk of hydraulic dysfunction during weather extremes. Trees grown under increased air humidity are able more effectively to adjust their stomatal conductance and this should improve their capability to cope in adverse environmental conditions. On the other hand, the leaf acclimation to higher humidity have a negative impact on tree growth in humid years and the trees show a greater risk of hydraulic failure at high atmospheric evaporative demand. Therefore the positive effect on tree growth induced by the rising CO2 concentration and temperature may be smaller due to the negative effect of co-occurring increasing air humidity.