Response of Quercus robur and two potential climate change winners – Quercus pubescens and Quercus ilex – to two years summer drought in a semi-controlled competition study: II — Photosynthetic efficiency

Abstract

Decreasing precipitation as a result of climate change may result in new forest structures in large parts of Europe. North-east shifts of the geographical ranges are to be expected, which may even result in an increase of Mediterranean tree species in Western and Central Europe. In the present study, large basins with groundwater monitoring and precipitation shield were used to realize a competition experiment with Mediterranean and Central European oaks (Quercus ilex, Q. pubescens, Q. robur) under semi-controlled conditions over two summers. Gas exchange parameters (pn and gs) and instantaneous water use efficiency (Wt), SPAD values (as a proxy for the chlorophyll a + b content per leaf area) and chlorophyll a fluorescence (ChlF) induction curves were used as indicators of drought effects on the photosynthetic apparatus. The three species revealed significantly different reactions to long-term drought stress (DS). Q. pubescens showed the least negative effects of drought over all measurements. Most ChlF parameters revealed less negative effects of DS in Q. pubescens than in the other two species. Q. ilex showed less reduction of pn and gs upon declining pre-dawn water potential (ΨPD) than the two deciduous species. However, all three species showed very similar values for Wt under decreasing ΨPD values. Comparing the two consecutive summer drought periods, it could be shown that a late start of water withholding (2014) affected all steps of photosynthetic electron transport until the electron acceptors of PSI negatively. The efficiency with which an electron can move from the reduced intersystem electron acceptor to PSI end-acceptors was more affected by the earlier DS (i.e. before leaf development finished, 2015).The relative chlorophyll content, approximated as SPAD values, detected no or only small differences between the species and drought vs. control treatments. Only Q. robur showed strong significant reductions in SPAD values under DS. The previously reported higher maximum stomatal conductance in Q. ilex could be confirmed, but Wt did not benefit from this, suggesting that Wt under DS was more related to mesophyll conductance than to stomatal conductance.