What a Street Tree Actually Does Between Two Rainstorms
For three summers we walked the same eleven blocks with a handheld moisture probe, a cheap thermal camera, and a notebook that kept getting damp. The goal was modest: find out whether the trees on the north side of the avenue behave differently from the ones on the south side, and whether anybody would notice if they stopped.
They do, and nobody would — at least not for a while. That gap between what a canopy does and what a neighbourhood perceives is where most of this journal lives.
The two-hour lag
Surface temperature under the south-side canopy peaked nearly two hours after the open pavement did. That lag is not a curiosity; it is the whole argument for planting density. A single tree cools its own shadow. A row of trees cools the air that moves between them, and that air travels.
Shade is a local effect. Cool air is a neighbourhood effect. Most planting budgets pay for the first and hope for the second.
What the soil said
Moisture at 10 cm stayed within four percentage points across the whole strip, even after nine dry days. At 30 cm the variation doubled. Whatever the roots were doing, they were doing it shallow, and they were doing it consistently — a quiet kind of infrastructure.
We are not the first to measure this, and we will not be the last. The point of repeating it in eleven blocks instead of one is that repetition is the only thing that turns an anecdote into a planning input.
Notes for the next round
Next season we want to add a sonic measurement of canopy density and a simple traffic counter, because the loudest blocks were also the driest and we do not yet know why. If you are running something similar in your own city, the method sheets are open and the notebook is, regrettably, still damp.