The short version. Drought and fire risk aren't two separate stories — they're one chain. A dry summer stresses the trees and dries out the fuel on the ground, and the fire risk climbs, often before anything shows at eye level. Here's the mechanism, what the Swedish authorities actually know about it, and what a satellite can see while it happens.
What drought does to a forest
A tree breathes and moves water through the stomata in its needles and leaves. When the soil dries out, it closes them so it doesn't lose more water than it can take up — and growth slows. SLU puts a rough threshold on it: more than three weeks above 25–30 °C without rain, and trees begin to show reduced transpiration and reduced growth. On the tree, it reads as needle loss, yellowing needles, thinning crowns, and over time a general loss of vitality.
Not all of it suffers equally. Spruce is shallower-rooted and shows the greatest drought stress and the steepest decline in growth; pine is more drought-tolerant but still takes damage on poor soils. In numbers: Swedish forest growth fell by about 15 percent between 2012 and 2018, most of it in Götaland, according to the National Forest Inventory.
Source: SLU — researchers answer questions about drought; National Forest Inventory (Riksskogstaxeringen).
The well-known sequel — that drought-stressed spruce struggles to defend itself against the spruce bark beetle, because drought drains the tree's resin — is the ecological chain that follows the drought. Worth knowing, but what Mai follows is the drought stress itself, not the insect: the moisture in the canopy, not the beetle under the bark.
Why dry forest burns
A wildfire needs three things: fuel on the ground, fuel that's dry enough, and an ignition. The fuel that matters isn't logs but moss and litter — and it has to fall below roughly 25 percent moisture content, which usually takes about a week of dry weather. Prolonged drought does more than that: it dries out the deeper, compact humus layers, giving stubborn smouldering fires that are hard to put out completely, and it dries up mires and wet forest so the natural firebreaks that would otherwise slow a fire disappear.
That slow drying is exactly what SMHI's fire-risk forecast captures. It's built on the Canadian FWI system, in which the deeper dryness codes build up over long dry spells. It runs six days ahead and is graded from 1 (very low) to 5E (extremely high).
How big it can get showed in 2018: around 22,000 hectares of forest burned — roughly ten times a normal year. Two details are worth remembering: about nine in ten Swedish wildfires are human-caused, and much of the destruction in 2018 came from reignitions of fires that were never fully put out.
Source: SLU — summer, a high-risk period for forest fires; SMHI — fire-risk forecasts.
What a satellite actually sees
A satellite like Sentinel-2 doesn't measure "drought" or "fire" directly — it measures light at different wavelengths. From that you compute indices:
- A moisture index (NDMI) reflects the water content in the canopy. Under drought it moves first — before the greenness has fallen. That's the early warning.
- A vegetation index (NDVI) reflects greenness and vigour, and drops sharply when a stand is felled or dies.
- NBR reflects burn severity: after a fire, you map the burn and how severe it was by comparing the index before and after.
Spotting active fires is its own task: since 2022, MSB has run satellite-based fire detection tied into SOS Alarm, and the evaluation shows the great majority of alerts were real fires — 89 percent in 2022 and 95 percent in 2023.
An honest limit belongs here: the satellite flags a change. The likely cause only comes when you set that signal next to the local weather — the drought, the wind, the fire risk — and even then with some uncertainty.
That's why Mai watches the moisture index before anything else: it moves earliest when a forest begins to dry — often before the greenness, and therefore the naked eye, has reacted. That's the signal Mai is built to catch while it develops, not after.
Source: MSB — satellite detection of vegetation fire.
What it means for you as a forest owner
Sweden has around 308,000 individual forest owners, and the average holding is about 36 hectares. Many visit their forest a few times a year. A dry summer can change a great deal in that gap — first as falling moisture in the canopy, then as rising fire risk in the ground — without a single visit necessarily catching it. It's in the gap between visits that a look from above, every few days, makes the difference.
Source: Skogsstyrelsen — property and ownership structure.