Phenotypic Plasticity Explains Response Patterns of European Beech (Fagus sylvatica L.) Saplings to Nitrogen Fertilization and Drought Events

Climate and atmospheric changes affect forest ecosystems worldwide, but little is known about the interactive effects of global change drivers on tree growth. In the present study, we analyzed single and combined effects of nitrogen (N) fertilization and drought events (D) on the growth of European beech (Fagus sylvatica L.) saplings in a greenhouse experiment. We quantified morphological and physiological responses to treatments for oneand two-year-old plants. N fertilization increased the saplings’ aboveground biomass investments, making them more susceptible to D treatments. This was reflected by the highest tissue dieback in combined N and D treatments and a significant N × D interaction for leaf δ13C signatures. Thus, atmospheric N deposition can strengthen the drought sensitivity of beech saplings. One-year-old plants reacted more sensitively to D treatments than two-year-old plants (indicated by D-induced shifts in leaf δ13C signatures of one-year-old and two-year-old plants by +0.5‰ and −0.2‰, respectively), attributable to their higher shoot:root-ratios (1.8 and 1.2, respectively). In summary, the saplings’ treatment responses were determined by their phenotypic plasticity (shifts in shoot:root-ratios), which in turn was a function of both the saplings’ age (effects of allometric growth trajectories = apparent plasticity) and environmental impacts (effects of N fertilization = plastic allometry).