Quick answer: Mulch and Soil Temperature — A stump is the part of the problem that is still in the ground: it holds the roots that lift pavement, keeps a species alive if it re-sprouts, and occupies the spot where the next planting goes. Grinding removes it below the surface so the ground can be worked again, and the roots that matter are the ones under the driveway rather than the ones beside the trunk.
Field note. On this coast the stump is rarely the end of the job: Monterey pine and eucalyptus both keep enough energy in the roots to push new growth, so we are looking at the species before we quote the grind rather than after.
On the other hand, a stump left in a garden bed is sometimes the better answer — it decays, feeds the soil and costs nothing — so grinding is a choice, not a rule.
Air temperature is what the weather report gives you. Soil temperature at two inches is a different number, and in summer on bare ground it is much higher — high enough to damage the fine roots that live there.
The measurement
On a clear summer day, bare soil at two inches can run ten to twenty degrees Fahrenheit above the air temperature at the same moment. The surface itself, at half an inch, can be hotter still. The cause is direct solar radiation on a dark, dry surface with no evaporative cooling because there is no water left to evaporate.
Under a two-inch layer of coarse mulch, the same soil at the same depth typically peaks five to ten degrees above air temperature. The layer insulates, it shades, and it retains enough moisture that evaporative cooling continues.
That is a ten-degree difference at the depth where a tree’s fine roots are.
What heat does to roots
Fine roots are not adapted to high temperatures. Above roughly ninety degrees Fahrenheit at the root surface, root growth slows; above about a hundred, fine roots begin to die. On bare ground in full sun on this coast, those temperatures occur at the surface layer for several hours a day through the dry season.
The consequence is a root system that retreats downward and thins at the surface every summer, then has to rebuild in autumn. That cycle costs the tree carbohydrate it would otherwise spend on the crown.
The winter side, which is less obvious
Mulch also reduces the depth of frost in the soil, and it reduces the number of freeze-thaw cycles at the surface. Freeze-thaw heaves small plants out of the ground and breaks surface structure. On this coast the effect is modest, but it is the reason a mulch layer protects a newly planted shrub through its first winter.
The more relevant winter effect here is rain impact, which is the crust problem — infiltration rather than temperature.
Why the material matters for temperature
Light-coloured material reflects more radiation and stays cooler at the surface. Low-density material insulates better per inch because it traps more air. Straw is the best of the common materials on both counts; coarse bark is good; a thin layer of fine compost is nearly useless.
And darker material absorbs more heat at the surface — but because it still insulates, the soil beneath can be cooler than bare ground. The surface temperature and the soil temperature are two different readings, and the one that matters for roots is the second.
The depth effect
The insulation value rises with depth up to a point. Two inches of coarse material captures most of the benefit; four inches captures a little more and starts to have the gas-exchange problem. Past four inches, the temperature benefit stops improving and the oxygen problem grows.
What to do with the number
If a bed is in full sun on a south or west face, the temperature argument alone justifies a mulch layer, independent of water and weeds. That is the case for mulching a new planting even in a wet year.
And it is the argument for mulching around trees in a lawn that is otherwise well watered: the lawn’s irrigation keeps the soil moist, but the mown, bare surface still bakes at two inches, and the tree’s fine roots are in that layer.
The full practice is set out in the arborist ring.