Elevated Growth Temperature Reduced the Physiological Plasticity of <em>Fagus sylvatica</em> Seedlings in Response to Drought and Shade

Climate change is increasing the growing temperature worldwide, adding a potential new stress factor to those already present during the regeneration phase of many tree species, particularly late successional species: water deficit and shade. We performed a multifactorial stress experiment by rising the growing temperature above optimum (25°C and +7.5°C) without elevating vapor pressure deficit (VPD), long-term water deficit or well-watered and high and low light intensity, and measured plant growth, leaf gas exchange and water relations and hydraulic traits on one year-old beech seedlings. Many functional traits showed synergies across treatments (e.g. stem growth and net photosynthesis, total leaf area and vessel area, leaf transpiration and hydraulic conductivity). Warming decreased seedling’s phenotypic plasticity. Plants under warmer growth temperatures showed lower midday water potential, water use efficiency, photosynthesis, leaf area and plant biomass but higher leaf transpiration, residual conductance and respiratory costs. When warming interacted with drought, gas exchange and the capacity to perform osmotic adjustment were severely affected, reducing the leaf safety margin and beech drought tolerance. Surprisingly, warming promoted a risky acquisitive strategy in shaded seedlings in well-watered plants, stimulating growth by increasing the number of less costly leaves. Global warming may compromise future regeneration of this shade-tolerant species under dry conditions.