Quick answer: Mulch and Water Infiltration — Mulch is the cheapest thing that happens to a tree: a flat layer over the root zone holds moisture through the dry season, keeps the soil temperature steady and feeds the soil life the tree depends on. Almost every mulch problem is a depth problem or a contact problem — too thick, or piled against the trunk where the bark has to stay dry.
Field note. What we see most on this coast is not too little mulch but too much, in the wrong place: the volcano against the trunk that keeps the bark wet through the fog season and invites rot where the tree cannot show it until the canopy thins.
On the other hand, a thin ring of the wrong material is worse than none at all — matted, water-repellent mulch sheds the rain it was meant to store.
The water-saving argument for mulch is usually made about evaporation. The larger effect is at the other end of the cycle: what happens in the first ten minutes of a storm.
What a crust is
When rain hits bare soil, the impact breaks the surface aggregates into finer particles. Those particles settle into the pores, the surface seals, and the result is a crust a few millimetres thick that is far less permeable than the soil beneath it. The next rain runs off it.
On a slope, that run-off concentrates and starts to erode. On flat ground it ponds and then evaporates. Either way the water is lost, and the loss is not small: on a crusted soil, a large share of a moderate storm can leave the site rather than entering it.
What mulch does about it
It intercepts the impact. The raindrop hits a chip rather than a soil aggregate, breaks up, and reaches the soil as a gentle drip from many points. The surface structure stays intact, the pores stay open, and the water enters.
That single mechanism is worth more than the evaporation saving, because it operates on the events that actually recharge the soil — the few big storms a year — rather than on the slow loss between them.
The numbers, roughly
On a crusted clay soil, the infiltration rate can fall to a few millimetres an hour. The same soil under a maintained mulch layer will often take ten times that. Which means a storm that would have produced run-off for its whole duration instead enters the profile, and the difference in stored water at the end of the rainy season is measured in inches of depth across the root zone.
That stored water is what a mature tree lives on through the summer. It is not the water anyone applies with a hose.
The structure that builds underneath
The second effect takes longer. Mulch breaks down, and its breakdown products are food for the soil biology that builds aggregates. Aggregated soil has larger, more stable pores, and stable pores infiltrate faster and hold more plant-available water than the same soil compacted.
This is why a bed that has been mulched for five years behaves differently from one mulched for one season, even at the same depth. The surface protection is immediate; the structural improvement is cumulative.
The failure modes
Fine material applied thick forms its own crust and can be less permeable than the soil it covers. That is the mat problem, and it is why particle size matters more than depth.
Mulch that has been walked on, driven on or stored materials on is compacted along with the soil beneath it, and no amount of chip on top will restore the pores.
And a mulch layer on a slope that is already eroding will slide, taking the surface with it. The intervention there is structural, not a surface treatment.
How to check your own ground
Do the ring test in two places: one under mulch, one on bare ground beside it. Time how long a litre takes to disappear. Repeat after the first autumn rain, because the crust re-forms and the comparison is most informative then.
If both are slow, the problem is deeper than the surface, which is soil and root zone remediation rather than mulching. If only the bare ground is slow, the mulch is doing exactly what it is for.