Root zones and soil compaction
The root zone of a large tree extends well beyond the canopy edge: often much further. Foot traffic compacts the soil, reducing the air and water movement that fine roots depend on. Even a single busy season can cause measurable change.
Stay on marked paths where they exist. If there are none, move deliberately: do not gather in a group directly under the trunk. Avoid disturbing the duff layer, the loose decomposing material on the soil surface that insulates roots and retains moisture.
Around ancient or champion trees in particular, keep concentrated activity away from the immediate root zone. The trees are resilient over decades, but repeated disturbance accumulates.