Friction Hitches 

Trees are bigger than the dinosaurs, they are the largest life ever on the planet.  For them to grow to such a huge size, and then maintain that size; they need to stake off and maintain a large territory.  Trees can't move to a better, richer territory, so they create one, and protect it.  The massive trunks that are their distinction protect the leafy green by raising it high as one territory.  The massive trunks also help protect by limiting trespassers across the sea of soil; a trees other feeding area.  This accessible territory needs protection; and receives it from the canopy, trunks and mulch. 

People's understanding of trees and their systems is so distorted.  We take all the mulch area away that trees desperately need.   Even though, wherever trees wander, that essential mulch territory; is the only thing they take with them; wherever they go.  Then almost in mockery,  we take a tree and territory over and may throw a small patch of mulch down against the trunk.  The only place; the mulch doesn't go, then; place it too deep!  The mulch is an essential territory of, barrier, protection, environment stabilizer, refuse disposal, garden etc. in a breathable layer for the roots and ground.  This familiar territory is essential to maintaining low stress in the magic of supporting and maintaining the longest lived, largest and tallest of any life born ever; for that is a real big job!  Many things done to trees; are really like we've caged them proudly, tightly into our world, rather than letting them live in theirs.  Keeping them starved for mulch, except too deep, the only place it doesn't go is just one example of that misunderstanding of their simple and essential mechanisms to be the wonder to live so long, and so large; but only in a land of plenty...

The soil is not this rich Soil Sea unless it is richly oxygenated, Permeable Ground; that is full of delicate life.  Every cup of the rich Sea of Soil, would have many miles of delicate fungi hydra etc. and is therefore in need of protection.  Soil should not be dead ground, but a rich spawning bed of miniature life mechanically and nutritionally conditioning the ground endlessly.  That is live ground, rich as a sea of soil!

If you peeled off the looser/ deader bark from part of the trunk , to expose the denser live inner bark and live wood below; a layer of the live bark and wood would have to die, to protect the rest of the inner live wood from the raw elements.  The denser wood would be harder to breathe through than the bark was for the remaining living tissues it seeks to protect.  There has been a loss of live tissue too as it had to 'scab' over and provide the essential protective skin to the more delicate life processes.   You need protection layers for the delicate life processes, but not at the cost of live tissue, or too much nutrients, as compromises etc..    So too the soil should be a denser live area, and have a deadish, looser covering over it, that allows gas exchange; yet protects the denser/ more delicate live area processes below (soil).

The best soil protection is natural mulch.  Moderating loading, temperature, humidity, competition, light etc. in a breathable layer; that also feeds the tree; while disposing of refuse of growth and injury.  The recycling, can help manage elements rarer to location; removing this particular mulch can cause a nutrient imbalance.


Load Bearing of Soil Sea Protections

Biologically: Bare soil is like no bark, some of the denser live layer (outer soil layer) must die off some to give 'skin' protectant.  This layer could be conducting feeding as live soil and is lost.  The denser layer of the smaller aggregate of soil compared to mulch as deadish covering is harder to breathe through, for the live soil underneath it to thrive. 

Mechanically: Bare soil takes the most direct unmoving weight, and dynamic weight drop forces too.  The forces are not spread out, the impacts are not buffered or redirected from dynamic (moving/impacting) forces.  This furthers damage, as forces are focused directly on the delicate live ground below.  the forces need spread out and buffered to give proper protection.  The 2nd picture shows the way impacting force is recieved to be passed onto how the medium transfers still weight in the first picture.  Covering both static/sitting and dynamic/moving force loading.

Grass, will give some skin covering to the ground, and a breathable layer; but also wishes to compete for gas exchange, water, nutrients.  It will also spread out static weight over a bigger footprint, for less loading per square inch.  Whatever, cushion/give the grass has, takes some of the impact shock of dynamic loading, before transferring the rest of the force, over a slightly larger footprint to the ground.  In impacting some of the force is absorbed, the rest passed on to the static weight drawing

Grass, also is a frail, easily starved (especially when managed at heights it can't shade itself) plant, conversely a tree is a large storage capacity giant.  Conditions like plenty of water for grass; will cause shallow rooting in trees etc.; they have different needs on some levels and compete on others.  People cause shallow rooting with overwatering, lawnmower damage, over fertilization etc. all by caring for grass inside of tree territory.

A non-poisonous 'Rock Mulch' can give a breathable layer of protection to the live ground. And not compete for resources.  The larger aggregate can lock to a wider spreading out to the ground of static/ sitting weight  on the the rocks.  Though the rocks don't have the give of the grass (so red lines are straight on impacting), the rocks can shift sideways, to dissipate some of the dynamic force of movement.  Also the stiffness of the rocks can place a wider footprint as mentioned, but also press that reduced force into the ground at an angle.  The weight of the mulch medium, restricts the depth of protectant that can be applied before compressing ground by mulch's own weight, overriding  protecting soil by absorbing and spreading out other compressive forces.  The inertia of the heavier aggregate would be more, so would move less initially or on momentary loading.
If, mulch is allowed to be the skin naturally, it most resembles the dead bark covering the live area of the trunk as skin.  It is a live breathing covering, that isn't neutral, doesn't take from the tree, actually gives to the tree, and the soil; recycling perhaps elements in scarcity for the area back to the tree.  It as the canopy, keeps down the competition of small plants.  Mulch will lock and spread out the weight sitting on it.  It will cushion impact and move to the side some.  Mulch will also maintain the median ground temperature and humidity better.  A tree is recycling it's own nutrient's, cheaper than it can get more, and using all the life and the heat that the process takes to it's betterment too.  These processes enrich and loosen the soil from below, and are very delicate, easily crushed/damaged.  The mulch will also encourage mychorhizae feeding to the tree, in community with other trees etc.  from the mulch, for a larger, more fortified system underground community, unseen.
The grey at the bottom is compacted to be impervious to oxygen to feed, room to move and grow.  The black soil is the alive layer allowing growth, life, oxygen etc.  it's depth is limited on the bottom by the grey as outside constraint; but also by breathability and weight of above layers; so the growth area is only about 18" deep anyway.  The mulch gives self regulating protection from pressure, impact, temperature, dryness etc. to soil biomass , as it also feeds, disposes of refuse etc.  The spars protect the biosphere like a sieve; limiting size, frequency and path of invaders, with mulch as second line of defense to protect soil.  

Protections of the Biosphere Cap

The tree's garden soil is also protected, by the tree's trunks sitting in woods limiting the size of animals to trample the soil sea's  covering, also the winds that would peel away the covering.  The tree's innovation of wooden stem also lifts the rich green tops out of reach of many animals; thus the tough trunks protect both rich food/energy areas.

The intertwining tops cuts down the competition by light starvation, keeps direct light off the soil biomass; cuts down on some foreign wind blown seed.  Perhaps gives some purity of local species, as the thick green cap could keep out competing wind blown pollen etc.(?)  As well as mistletoe, moss invasions etc.(?)

Trees Together

Biologically: Even as a single tree, the tree will eventually try to grow limbs to the ground, that will protect the soil garden.  As above if the branches (trunks) die, they still stand protecting the rest from invasion, by plant, beast, wind etc.!  The extended drip line low, carrying water to the outside of the canopy encourages more lateral root growth to secure tree, as well as deeper feeder root growth of the lateral roots underneath the canopy.  For water is scarcer in shallow areas underneath the canopy, by this strategy; both effects giving more leveraged (lateral roots) extension and more holding at that higher leverage (deeper roots).

Mechanically: The trees in woods will  touch or nearly touch, to support and or restrict movement at the tops; giving leveraged support far from the only connection at the ground.  Single trees can't avail themselves to this, so carry weight lower and wider, and eventually reach towards ground; when the reach of the branches would give meaningful leverage, in distance from the only connection to ground (trunk forming pivot).  Also protecting it's ground too.

Single Tree Application

 On the other end of the protection of the tough trunks are the leaves.  The lush growth caps the biosphere, sealing out competition for resources as well as preserving it's own strain purity, acclimating generations to that specific woods conditions quicker with 2 local parents.  The leaves appear green, for that is the only wavelength they don't take from the light energy source.  Leaving only light bands not great for plants lush growth, limiting competition that gets light to anemic, quickly dying, feeding the soil types mostly.  These plants can trap nutrients to feed the tree's mulch bed that might have just drained off.  The light allowed thru can also help dry the ground, keep down fungus etc.! 

The extremes leverages of the tops; reaching far before touching each other or the ground, giving more leveraged support at those points to the stump/roots, as well as shading more area of the ground; more leaf surface area too.  So, the light exists less dense in intensity, duration as well as broadness of wavelength spectrum, by the super competitor, trees setting up their territorial biosphere.

Biosphere Cap

  Animals give locomotion & mass wastes that bacteria and plants don't.  Bacteria fix nitrates etc. from plant, animal wastes as neither animal or plant generally can.  Plants get the basic life element carbon from the air (carbon dioxide).  All life depends on the carbon building block, but animals and bacteria must collect it from the food chain somehow from plants, as only plants can bring this building block of all life into the world.  Each of the 3: bacteria, animal, plants have a specialization that links it to the braid of whole life cycle in contribution and dependency.  Trees do this carbon mining in higher masses, and in removing the mulch we are forcing more dependency on 'costly' raw processing for such elements, more than recycling that which the tree can get easier; by the thievery!  We are also taking the feed for other life organisms that would condition the soil mechanically (by fracturing it to be more permeable constantly with growth etc.) and nutritionally as organisms returned the products to the tree!  The system is disturbed, running at a strain.
  Too much of anything is not good.  Too much light, too much water, too freely available.  Even refuse mulch is a system of balances that can be too deep, too wet, too fine, too slimy, too close to trunk etc.

The mulch should be no more than 3" deep, ideally spread to the unaltered 'drip line'; and not on the trunk or root crown; these areas are not root tissue.  They need light, air and dryness etc.