Plant Species
You'll probably never see a healthy, thriving tree growing all by itself. There will always be associates. Shrubs, groundcover, vines, fungi, other trees...these will be plants that often work together with the tree you're focusing on.
Lots of observations of healthy, vigorous trees have been done over the centuries. It's been noted that pioneer trees help create the conditions for succession trees to thrive. Nitrogen fixing plants feed the soil. Deep taproot plants pull moisture and minerals to the surface to feed fungi and roots of other plants. Billions of microscopic wiggly things in the soil process chemicals and minerals that plants and trees use to thrive. Some plants loosen the soil. Some plants tie the soil up tight with their roots to prevent erosion.
What I'm trying to say is that rarely (perhaps never) will you see a tree existing by itself.
Therefore, if Old Men are going to plant trees, they should consider all the associated components that go toward building a healthy forest. Every tree planted should be part of a multi-layer system of biomass.
That's the point of this section. In the region where Old Men Who Plant Trees is currently headquartered, there is a long and growing list of plants that would be great components of a healthy and thriving forest. The list was gathered from an exercise to qualify to receive a Permaculture Designer's Certificate.
The Canopy layer is where the largest, oldest trees will end up.
The Understory layer will contain trees that can exist beneath the Canopy layer.
The Shrub layer fills in lots of gaps between the canopy and understory trees.
Bare soil is a sin in the forest. The Groundcover layer ensures that bare soil doesn't exist.
The Herbal layer is all those plants that turn brown and decay in fall and winter.
The Root layer pushes soil around and loosens it up.
The Vine layer crawls up trees, bushes, and anything else it can grab onto.
If there's water in the area, and there should be, it's loaded with aquatic plants.
We really want to, with every planting of trees, include as many of these layers as we can.
I've included local examples of species in the following lists:
Aquatic