Late Snow Patches Are Habitats On Borrowed Time

Patches of snow that survive into late summer look like leftovers from winter. The ground beneath them supports a plant community found nowhere else on the hill.

The snow decides the growing season

Snow accumulates in hollows and on lee slopes where wind deposits it, and depth in these places can be many times the depth on open ground.

Deep drifts take months to melt, so the ground beneath is exposed for only a short window. In extreme hollows that window may be a few weeks.

A community adapted to that timing appears: low-growing mosses and small flowering plants able to complete their cycle quickly and tolerate being buried for most of the year.

Being covered is also protection

Snow insulates. Ground beneath a deep drift stays close to freezing rather than dropping to the air temperature above, and it is shielded from wind and desiccation.

Species in these hollows are often poorly adapted to frost precisely because they have never experienced much of it, having spent every winter under cover.

This is why an early melt is damaging twice over. It exposes plants to late frosts and removes the meltwater supply they depend on through summer.

Meltwater supplies the ground below

A snow patch releases water steadily through the warm months, keeping the slope beneath it wet long after rainfall alone would have dried it.

Distinctive flushes and mossy springs develop below patches for this reason, and they support their own invertebrates and plants dependent on continuous cold water.

Losing the patch dries the flush, so the effect of a change in snow cover extends well beyond the area the snow itself occupied.

The habitat cannot move upward

Most communities under pressure from warming can shift to higher ground. Snow patches sit in the highest hollows already, with nowhere further to go.

They are also small and widely separated, so recolonisation from elsewhere is unlikely once a site is lost. Each patch functions as an isolated island.

Patches that persisted through most years within living memory now clear in many summers, and the vegetation beneath them changes within a few seasons of that shift.

They are difficult to monitor and easy to damage

Recording depends on repeated visits at the right time of year to high, remote ground, which is why much of the long record comes from volunteers rather than institutions.

The vegetation itself is fragile, growing on thin wet soil with a very short season in which to repair damage. A single trampled line persists for years.

Walkers are drawn to these hollows because late snow is unusual and photogenic, which concentrates pressure exactly where the ground can least absorb it.