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

Chlorophyll a fluorescence (ChlF) and leaf morphology parameters were assessed in two experimental sites in Europe (Kaltenborn, Germany and Satakunta, Finland), within a forest diversity experiment. The trees at Satakunta, planted in 1999, form a stratified canopy; while in Kaltenborn the trees are seven-years-old, with no apparent canopy connection on broadleaf species. The following ChlF parameters obtained from the measured OJIP transient curves were examined: the ratio FV/FM (a proxy of the maximum quantum yield; where Fv is the difference between the minimal (F0) and the maximal (FM) fluorescence in the dark-adapted state); J stepΨEo (a proxy of the efficiency to move an electron from reduced QA, the secondary PSII electron acceptor into the electron transport chain); the I-P phase (a proxy of the efficiency to reduce the final acceptors beyond PSI), and PItot (the total performance index for potential energy conservation from photons absorbed by PSII to the reduction of PSI end acceptors). At Satakunta FV/FM and J step ΨEo values in Betula pendula were higher in monocultures and lower in mixed plots, maybe because due to the increasing availability of light in mixed plots that can induce photoinhibition. The opposite trend was observed in Picea abies, which was shaded in mixed plots. At Kaltenborn FV/FM decreased in Fagus sylvatica and P. abies in mixed plots. This effect was attributed to competition processes both at aboveground and belowground level. At Satakunta LMA (Leaf Mass per Area) increased in B.pendula leaves with increasing species richness. LA (total leaf area) was reduced in F. sylvatica in the mixed plots at Kaltenborn. By upscaling the overall fluorescence response to plot level (PItot_plot), a significant positive correlation with tree diversity was found at Kaltenborn but not at Satakunta. This result may suggest that the competition/facilitation processes in mixed stands play a significant overall role in the first stages of forest establishment, but then tend to be compensated for in more mature stands.