Abstract
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 one‐ and 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).
Highlights
Many ecosystems are currently subject to unprecedented shifts in environmental conditions on both regional and global scales [1]
This is true of forest ecosystems in particular, since trees are characterized by long life-cycles, and growth processes are mediated by the environment over centuries [2]
Beech saplings significantly increased their aboveground investments in N and nitrogen drought treatments (ND) treatments and followed the “resource optimization hypothesis” [36], according to which plants increase their aboveground biomass allocation as a result of improved nutrient supply
Summary
Many ecosystems are currently subject to unprecedented shifts in environmental conditions on both regional and global scales [1]. An increasing body of research has addressed ecosystem responses to environmental shifts by means of single-factor approaches, little is known about the interactive effects of co-occurring global change drivers and how these may affect ecosystem processes and services in the future [13,14] It is, for example, conceivable that tree growth responses to climate change could be strengthened by the deposition of reactive forms of N, probably due to fertilization effects on morphological traits such as shoot:root ratios (“plastic allometry” [15]). Plants would exhibit higher drought sensitivity than non‐fertilized plants; and (ii) one‐year‐old plants would be more sensitive to drought treatments than two‐year‐old plants
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