Photosynthetic response of European beech to atmospheric and soil drought

Physiological response of European beech under soil and atmospheric drought conditions was investigated in this study. A group of six beech trees was irrigated during the growing season 2012, while the second group of non-irrigated (control) beech trees was treated under natural soil drought. During the experiment, we observed more than 45-day long period when no precipitation fell on the soil surface. The relationship of PN (CO₂ assimilation rate) to gS (stomatal conductance) was very tight in both groups, which indicates that stomatal opening was the main factor limiting PN. The statistically significant differences in g_S between the groups of trees were revealed only on the last measuring day. The significant differences in PN were confirmed on the days when the differences in soil water potential (ΨS) appeared. On these measurement days, the PN values of irrigated individuals were approximately 1.9 or 3.3 times greater than the values of non-irrigated individuals. At the level of primary photosynthetic processes (chlorophyll fluorescence parameters) we did not observe lower values of the control individuals in comparison with the irrigated trees in any of the evaluated parameters. Long-term soil water deficit caused strong decrease of leaf water potential (Ψ_L) in the control trees, but Ψ_L values of the irrigated trees were also rather low due to diurnal dynamics in higher parts of crown. Close relationship between Ψ_L and g_S was confirmed for the control (non-irrigated) trees, but could not be confirmed for the irrigated trees. We revealed significant influence of VPD (vapour pressure deficit of the air) on g_S only in the control group. On the days when ΨS decreased, the stomata of the non-irrigated trees were closed in spite of the low VPD values. Almost complete stomatal closure in both groups of trees was caused by the increase of VPD to 1.2 kPa.