Mapping Vaia windthrow when salvage was a race against the timber's own value

On the evening of 29 October 2018 a scirocco gale came over the Dolomite ridges from a direction those valleys had never seen and flattened 42,525 hectares of forest. Eight and a half million cubic metres of spruce hit a market that could absorb a fraction of it. Stumpage prices fell 69 per cent.

A deep Mediterranean low had forced a south easterly wind over the Alpine barrier, which is why stands with no history of windthrow failed. Regional stations recorded gusts of 192 km/h on Monte Cesen and 190 at Passo Manghen, with 715 mm of rain in seventy two hours at Soffranco.

What fell was overwhelmingly Norway spruce: shallow rooted, even aged, dense and monospecific, favoured over centuries. That is also why the event did not end in 2018. The fallen timber bred bark beetle, and the attacks on standing trees came not in the first year but in the third. By 2024 the beetle had taken roughly as much forest as the storm.

The analysis covers the Val di Fiemme, the Val di Fassa and Paneveggio, about 32 by 33 kilometres of Trentino holding several of the worst hit valleys.

The comparison is an exact anniversary, 27 August 2018 against 27 August 2019: same day of year, same solar geometry, same phenological stage, both scenes under 5 per cent cloud. In terrain this steep that matters more than usual, because a month's difference in sun elevation deepens shadow in narrow valleys enough to move a vegetation index on its own.

Scoring runs only over closed forest canopy, ground whose vegetation index stood above 0.6 before the storm. That removes alpine meadow, scree and rock, which change far more between June and August than a spruce stand does, and without it a control in this terrain means nothing.

The control is a second pre-storm pair, 23 June against 27 August 2018, spanning a whole alpine growing season in which nothing happened. It is the harder of the two comparisons to pass, and every figure below is quoted against it.

The published research on this event converges on the same window. A 2021 study using continuous change detection across six regions found reliable detection only from seven to twelve months after the storm, for the reasons the emergency service struggled with: snow, shadow and cloud make anything earlier unreliable.

What this measures is canopy that is no longer there. By the summer of 2019 Trentino had already moved 2.1 million cubic metres to market, so cleared ground and flattened ground both count in the 9,937 hectares. For replanting, road sequencing and protection forest assessment that is the right quantity, and it is not a volume: cubic metres depend on stand age, density and species, and the published volume figures for Vaia are themselves area multiplied by regional standing stock tables, a derivation their own authors have criticised in print.

For scale, the harmonised 2024 stocktake credits the whole of Trento province with about 11,500 hectares. Our box covers several of the worst hit valleys rather than the province, so the two are context for each other rather than a scored comparison. No official hectare figure exists for the Val di Fiemme alone.

The scene that answered this was in the archive ten months after the storm, while the salvage programme still had two years to run and six years before the beetle outbreak it seeded was declared over. A forest service sequencing 1,600 kilometres of road repair against a depreciating asset needs to know which stands are worth reaching first, at patch level, across the whole damaged area rather than the tenth of it an emergency activation covered. That map existed in August 2019, and the same anniversary comparison run each summer since would have tracked the beetle outbreak that followed, which by 2024 had taken roughly as much forest again.