Species-specific physiological responses of four sympatric Quercus species to water deficit during seedling establishment

Climate change projections for Southeast Europe indicate increasing temperatures and more frequent and severe summer droughts, raising concerns about the success of oak regeneration during early developmental stages. This study aimed to compare the drought responses of four sympatric oak species (Quercus petraea, Q. cerris, Q. frainetto, and Q. pubescens) during the early developmental stage after seedling establishment under severe water deficit. Seedlings were grown from acorns collected in Serbia and exposed to semi-controlled glasshouse conditions. Water deficit was induced rapidly by withholding irrigation for three weeks without a subsequent recovery period. Physiological responses were monitored through repeated measurements of stomatal conductance – g_sw and quantum efficiency in light – PhiPS2, while seedling vitality was evaluated at the end of the experiment. Linear mixed-effects models were used to evaluate the effects of water deficit duration, oak species, treatment, and time of day. The results demonstrated clear interspecific differences in physiological responses to water deficit. Across species, reductions in g_sw generally preceded declines in PhiPS2, indicating that stomatal regulation represented an earlier response to decreasing water availability than photochemical efficiency. Q. cerris exhibited the earliest phytological response and the greatest decline in vitality under the imposed experimental conditions. In contrast, Q. frainetto maintained higher PhiPS2 and delayed stress responses throughout the experiment. Q. petraea and Q. pubescens showed intermediate and similar physiological responses, maintaining relatively stable gsw during the first two weeks before a marked decline. These findings improve our understanding of species-specific drought strategies during the critical establishment phase and provide insights relevant for predicting natural regeneration dynamics of southeastern European oaks under future climate scenarios.