Senior Research Fellow Anu Sõber, University of Tartu
Sirkku Manninen, PhD, University of Helsinki
The consequences of human activities have rapidly increased the concentrations of the main greenhouse gases, atmospheric CO2 ([CO2]) and tropospheric ozone ([O3]). In addition, it is predicted, that the extreme weather conditions can be more frequent: both draught and water-logging can occur. Elevated [CO2] is generally beneficial for plants, because CO2 is a substrate for photosynthesis and causes increases in light-saturated net photosynthesis, Pn. Tropospheric ozone is known to have a negative effect on plant growth and productivity. As a strong oxidant, ozone causes damage to photosynthetic apparatus and decreases Pn. The value of Pn shows the amount of CO2, entering each unit of leaf area per unit of time and Pn, multiplied by leaf area, determines the carbon gain of foliage during time unit. The objective of this thesis was to find out how and why the long-term effects of elevated [CO2] and/or [O3] on photosynthetic responses vary in fast growing hardwood trees. Our findings demonstrate that the photosynthetic responses to increasing [CO2] and/or [O3] are changing in diurnal, seasonal and interannual scales and depend on environmental constraints such as drought and high temperature. Drought and high temperature stress alleviated the negative impact of ozone and increased the positive impact of CO2 on Pn. We found that the relative effects of elevated [CO2] and/or [O3] on Pn were generally more pronounced in autumn compared to summer. These results also provide novel evidence that the [CO2] effect has been increasing rather than decreasing in time, but the negative ozone effect has remained the same over the 11 years of the study. This study highlights the importance of multiple factors in determining the future responses of trees to climate change. The key conclusion of this study is that exposure to combined factors can influence trees even more than exposure to a single factor. As changes in photosynthesis (but also in stomatal conductance) are likely to affect both the ability of plants to sequester carbon, and plant water use, these changes can affect ecosystem carbon- and hydrological cycles. Consequently, interactions discovered in this thesis should be taken into account in models that predict changes in productivity of forest ecosystems and the feed-backs from these changes on climate.