Interactions and competition processes among tree species in young experimental mixed forests, assessed with chlorophyll fluorescence and leaf morphology

Chlorophyll a fluorescence ( C hl F ) and leaf morphology were assessed in two sites in E urope ( K altenborn, G ermany, and S atakunta, F inland) within a forest diversity experiment. Trees at S atakunta, planted in 1999, form a stratified canopy, while in K altenborn the trees are 7 years old, with no apparent canopy connection among broadleaf species. The following C hl F parameters from measured OJIP transient curves were examined: F V / F M (a proxy for maximum quantum yield); Ψ Eo (a proxy for efficiency in transferring an electron from reduced QA to the electron transport chain); I ‐ P phase (a proxy for efficiency of reducing final acceptors beyond PSI ); and PI tot (total performance index for potential energy conservation from photons absorbed by PSII to reduction of PSI end acceptors). At S atakunta F V / F M and Ψ Eo in B etula pendula were higher in monocultures and lower in mixed plots, perhaps due to increasing light availability in mixed plots, which can induce photoinhibition. The opposite trend was observed in P icea abies , which was shaded in mixed plots. At K altenborn F V / F M decreased in F agus sylvatica and P . abie s in mixed plots due to competition both above‐ and belowground. At S atakunta LMA increased in B . pendula leaves with increasing species richness. Leaf area of ten leaves was reduced in F . sylvatica in mixed plots at K altenborn. By up‐scaling the overall fluorescence response to plot level ( PI tot_plot ), a significant positive correlation with tree diversity was found at K altenborn, but not at S atakunta. This could suggest that competition/facilitation processes in mixed stands play a significant role in the early stages of forest establishment, but then tend to be compensated in more mature stands.