The roots that cause over yielding
The over yielding effect is not a general rule, but it is highly dependent on the kind of mixture. The following figures will show under which circumstances an over yielding effect happens.
Visualization
As the plot above is complex it is also confusing. Therefore the histograms below are chosen as examples taken out of the plot above to exemplify why over yielding is happening.
While the miniMyDiv project only covered European broadleaves in monoculture and two species mixture, the IDENT project went further. In the following I discovered the influences of mixture on biomass production for 2 species, 4 species and 6 species mixtures. A significant over yielding effect can only be found in the mixtures of sycamore maple / silver birch and silver birch/ oak.
Mixtures of 4 and 6 species
The mixture of 4 species (figure 16) does not produce any overyielding effect in any mixture. I put that down to the fact, that some species like silver birch, paper birch, maple and larch are growing too fast in juvenile stages. Mixtures will always produce less biomass, if light demanding species are planted with shade tolerant species.
The mixture of 6 species(figure 17)
The mixture of 6 species(figure 17)
Statistics
ANOVA demonstrates significant differences between monocultures and all mixtures and within monoculture and all mixture. Cld/lsmeans tables below display a ranking. As expected, mixtures of light demanding and fast growing species are leading. Silver birch in monoculture performs in the best way of biomass production.
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