Photosynthetic and morphological responses of oak species to temperature and [CO2] increased to levels predicted for 2050

Urban forests are environmentally, climatically, socially and economically important. An understanding of the response of urban trees to future climate change is crucial to the maintenance of urban forests and the ecosystem services they support. We conducted a controlled environment pot-experiment on four-year-old Mediterranean oak species: the evergreen Quercus ilex and the deciduous Quercus cerris, to investigate the combined impact of elevated CO2 and temperature on growth and leaf physiology to levels predicted for 2050 in urban areas of central Italy. Quercus cerris initially increased net-photosynthesis (PN) under elevated 2050 conditions (EC) compared to present ambient conditions (AC), before PN declined, possibly indicative of down-regulation of photosynthetic physiology. Quercus ilex PN was not influenced by EC throughout the 70 days duration of the study. Levels of PN and stomatal conductance (Gs) were generally lower in Q. ilex than Q. cerris. Quercus ilex also reduced Gs during growth at EC. This reduced transpirative water-loss caused a significant increase in the water use efficiency (WUE) of Q. ilex. This reduction in Gs may have been associated with the observed reduction in stomatal density in Q. ilex grown under EC, while the number of stomata on leaves developed under the experimental conditions were unaffected by the EC treatment in Q. cerris. Over the course of the experiment, above (stem dry weight: SDW) and below-ground biomass (root dry weight: RDW) and foliar starch increased in Q. cerris (in both EC and AC equally) but not Q. ilex. Chlorophyll a fluorescence (ChlF); Prompt Fluorescence (PF), Delayed Fluorescence (DF) and Modulated Reflectance (MR) also indicated that a greater resilience of photochemistry to growth under EC was more apparent in Q. ilex than Q. cerris. In particular, the reduction of the quantum yield efficiency (FV/FM) in Q. ilex may also be considered functional to maintain constant PN levels in elevated temperature and [CO2]. The results of this study suggest that Q. ilex exhibits greater plasticity and adaptation to EC, and may therefore perform more favourably under future 2050 climatic conditions.