in all the answers I'm missing the hint for allelopathie,

Allelopathy is the opposite of plants wanting "company." Allelopathy is a
chemical process that a plant uses to keep other plants from growing too close
to it. The best known example in this country is black walnut, which is famous
for preventing many kinds of plants from growing near it.

explained by mycorrhizal effects

Mycorrhiza has nothing to do with it. Mycorrhiza is the symbiotic arrangement
of a fungus living on or in the roots of a particular plant. It may be a plant
growing alone or in a group. If the right fungus is available, the arrangement
will occur. Tropical orchids which are grown in pots do not need mycorrhiza and
rarely have them. There is some evidence that bonsai & other potted trees can
benefit from mycorrhiza, but it has nothing to do with how many other trees are
around.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)