To prevent severe loss in the economic and ecological value of Central European forestland in the context of climate change and therefore seasonal water shortage, a 'human aided migration' with suitable species is recommended. Mediterranean oaks might cope better with Climate Change and even become increasingly competitive in Central Europe compared to other tree species. Therefore, we compared the functional and ecological strategies to endure soil drought of 9-year-old half-deciduous Quercus pubescens and evergreen Quercus ilex and their competitiveness to the local deciduous Quercus robur, using a semi-controlled setup with the possibility to control the groundwater level (large lysimeter basins filled with a loamy soil). We analyzed responses to repetitive (2014, 2015) drought stress (DS) for approximately four months, monitoring relative leaf water content (LWC), predawn water potential (Psi(PD)), sap flow rates, the rooting depth (by deuterium-enriched water uptake) and a proxy for biomass increases by measuring the stem diameter 5 cm above the ground (D05), and compared the results to a well-watered control treatment (CO). In 2014, significant differences between the CO and DS treatments were only visible in Q. robur. In 2015, a longer drought period in combination with extremely high temperatures in summer led to a more profound drying out of the soil under DS. Therefore, all three species showed drought responses, with WpD levels dropping below -2 MPa and LWC values dropping below 80% LWC. Sap flow rates of DS plants were also reduced compared to CO treatments. Q. robur showed the strongest response with Psi(PD), below - 3 MPa and premature senescence. In contrast to these physiological measurements, we could not detect differences between CO and DS plants in the 005 increase and root depth for Q. ilex and Q. pubescens, only Q. robur DS displayed strong reductions in both values comparing to CO. The response for Q. robur has already been described, but the equal performance of CO and DS plants of Q. pubescens and Q. ilex contrasts previous analyses, which award evergreen oaks, like Q. ilex, a higher maximal stomatal conductance and less embolism under DS than deciduous oaks. Interestingly, both, Q. ilex and Q. pubescens, did not show a significant reduction of growth under drought (D05, rooting depth) in our lysimeter setup, despite their significant DS responses in LWC, sap flow and Psi(PD).
