Can plants encourage each other?

[NYCorchidman]

As Dante points out, certain plants communicate and one good example is the emission of warning chemicals in the air that other plants of the same species pick up and ready themselves for the coming danger.

I was watching this documentary about a particular species of acasia tree in Africa. Many herbivores that feed on this acasia were found dead over relatively short period of time. Scientist went out to investigate the cause of this sudden mass death of the animals.
They looked and looked, but could not figure out what. Eventually, and very surprisingly, they found out that the cause was the asacia itself.
What happened was that there had been a drought in the region and animals in the number that was more than usual were feeding on the acasia. Acaisa is used to being eaten to some degree, but when it was being eaten too much, it started to produce toxins to kill off whatever was eating it too much. The trees then produced warning chemicals that was released in the air and travelled to other acasia trees. When animals came to eat the leaves of other acasia trees, the trees were already loaded with toxins.
All the animals that had bites died.
Amazing, isn't it?

Another example, also something I watched on TV, scientists studied one forest that is mainly populated by hundreds (or thousands even) coniferous trees. Some are very very old and some are very young. The root systems are quite extensive and many of the trees have their roots intertwined with one anothers.
They found that younger trees get the benefit of shared nutrients and other phytochemicals necessary for proper growths. So certain coniferous trees (at least the one that was studied in this program) enjoy living in one big family.

There are many other plants that are territorial, though. Their roots release toxins that will kill off others that are nearby, thus ensure water and nutrients all to themselves.

So, one cannot say plants in general benefit from being grouped. I'm not sure about orchids, but some will benefit and some may not.

Grouping may help with humidity but not too much. It will have to be quite a few plants to have any benefit regarding raised humidity.
Two, in case of pest infestation, grouping plants too close together can be bad.