Sanasilva-Bulletin 2026 ¶
The state of Switzerland’s forests ¶
Stefan Hunziker, Christian Hug, Michael Plüss, Volodymyr Trotsiuk, Flurin Sutter, Marcus Schaub, Arthur Gessler
23 September 2026
The 2026 Sanasilva inventory shows only minor changes in conifers, but a marked deterioration in the condition of deciduous tree crowns. Their average overall defoliation rose to 29.7 per cent, and the increase among the trees surveyed again was 6.3 percentage points compared with the previous year – the second-highest since observations began. The consequences of the extremely hot and dry summer of 2026 are already evident in the current surveys in the form of exceptionally early browning.
Methodology of the Sanasilva Inventory
The Sanasilva inventory has been recording the condition of Swiss forests annually since 1985. Surveys are carried out at every intersection of a 16×16 km grid covering forested areas. This creates a sample grid that is representative of Swiss forests. The inventory forms part of the European forest monitoring programme ICP Forests, whose extensive network is based on the same grid.
Surveys are carried out every year using the same methodology and by experienced field teams. Internal courses and European training programmes organised within the framework of ICP Forests ensure the quality and continuity of the measurements. This is the only way to ensure that the observations remain comparable over decades.
The focus is on crown condition, one of the most important indicators of a tree’s vitality. This is described in terms of crown openness and recorded in 5 per cent increments: 0 per cent represents a fully leafy tree crown, whilst 100 per cent represents a dead tree with no living leaves or needles. In addition, further characteristics are recorded for each individual tree, including its social status within the stand, trunk diameter, dead parts of the crown, pest infestation and seed production. This makes it possible to track the development of each tree studied over the years and to gain a comprehensive picture of the condition of the forest.
Current situation ¶
In 2026, the average total defoliation for all tree species rose from 23.6 per cent in the previous year to 25.1 per cent. Total defoliation is the estimated proportion of leaves or needles missing from a crown compared with a completely healthy tree. The increase is almost entirely attributable to deciduous trees: their defoliation rose from 24.8 % in 2025 to 29.7 % (Fig. 1). This brings it back to the levels seen in 2022 and 2023, which followed the hot and dry summer of 2022. The trend is even more evident in the annual change in defoliation. It takes into account only trees that were recorded in both the current year and the previous year, and is therefore independent of whether trees have been removed from the sample network (mortality, felling) or newly added (new growth). For deciduous trees, this increase amounted to 6.3 percentage points, the second-largest rise since records of total canopy cover began in 1990 (Fig. 5). This deterioration follows immediately on from a recovery in 2024 and 2025. In the case of conifers, however, the condition of the crowns has hardly changed, as in previous years.
The increase in deciduous trees is primarily due to the fact that many crowns which had previously received very little light have now moved into areas with higher light levels (Fig. 2, top). The proportion of deciduous trees in the lowest light exposure class fell from around 30 per cent to approximately 12 per cent. During the recovery phase of the previous two years, the proportion of these very vigorous trees had actually increased significantly.
The findings regarding mortality are even more pronounced than those for light penetration. In 2026, 5.0 per cent of deciduous trees and 1.9 per cent of all trees died – in both cases, a higher figure than in any other year in the measurement series. The lower part of Fig. 2 shows that it was predominantly deciduous trees that were affected; these were trees whose crowns had already been significantly damaged the previous year and whose long-term probability of survival was therefore extremely low (Hunziker et al., 2025). However, individual trees with moderate light exposure were also affected. The proportion of trees in the highest light exposure class (80–95 per cent), in which most trees died, did not decrease, however, because trees from lower classes moved up into this class.
The time at which the data is recorded is crucial for interpreting these figures. The Sanasilva inventory is deliberately carried out at a time when the trees are still as fully leafed as possible. This allows the development of vitality to be reliably interpreted and continuously monitored. Over the course of the summer, a more or less pronounced reduction in foliage is normal, for example as a result of storms or insect damage. The direct effects of summer drought do not usually become apparent in the current inventory, but only in the following year in the form of reduced new leaf growth.
In 2026, the third-warmest spring was followed by the hottest summer since records began in 1864 (MeteoSchweiz 2026). Combined with the exceptional drought, this left clear marks on the forest at an unusually early stage. The browning of the crowns and leaf fall began so early in some regions that they occurred during the survey period; the same had already been the case in 2022. This is particularly evident in the high-resolution 8×8 km grid for the Jura and the Mittelland (Fig. 3), where an area showing an extreme increase in crown thinning stands out (coloured dark red).
This area comprises a stand of lime trees near Twann at the southern foot of the Jura. The region was one of the first in which widespread browning and leaf fall were observed. Fig. 4 shows the situation on 2 July 2026: the crowns had already largely lost their leaves, and the ground was covered with dry, partly still green foliage.
Apart from this extreme example, the map of changes from 2025 to 2026 shows an increase in light exposure across almost all areas, whilst the same areas had predominantly shown signs of recovery the previous year (Fig. 3). This is consistent with the nationwide analysis based on the 16×16 km grid. In addition to early browning, other factors are likely to have played a role. For instance, seed production in 2026 was significantly higher than in the two preceding years: 37 per cent of deciduous trees bore seeds, compared with 21 per cent in 2025 and 19 per cent in 2024; the increase was similar for conifers. Heavy seed production is associated with sparser foliage and increases the estimated light penetration. Added to this are the conditions during last summer, one of the warmest on record, with heatwaves in June and August as well as very little rainfall in some regions over an extended period (MeteoSchweiz 2025). Such events, when severe enough, often lead to increased crown light penetration in the following year. Other causes, such as insect or fungal infestation, are unlikely, however, to have had any significant impact this year (Fig. 1).
The observed deterioration in deciduous trees is therefore at least partly attributable to the direct effects of the summer of 2026. The extent to which the drought actually impairs the trees’ vitality will only be better assessed with the 2027 inventory. Furthermore, the consequences may persist over several years, for example because weakened trees become more susceptible to secondary damage such as insect infestation. In addition to possible direct consequences such as the dieback of parts of the crown due to embolisms in the water transport system, widespread early leaf fall places an additional strain on the trees: the period available for photosynthesis is shortened, and the trees lose important reserve substances which they would normally retrieve from the leaves in autumn. The latter is particularly problematic for beech trees (Gessler et al. 2026).
Long-term trends ¶
Over the entire observation period, both total defoliation and defoliation of unknown cause increased significantly (Fig. 5). The linear trend in total defoliation rose by 4.0 percentage points across all tree species between 1990 and 2026. For deciduous trees, this trend is twice as steep at 7.9 percentage points, whilst for conifers it is significantly flatter at 2.3 percentage points. This occurred despite natural and silvicultural adjustments to the stand structure.
The trend over time is characterised by significant fluctuations. The sharpest annual increases usually follow an extreme event: in 2000 following Storm Lothar in December 1999, which mainly damaged conifers, as well as in 2004 following the hot summer of 2003 and in 2019 following the hot summer of 2018, both of which mainly affected deciduous trees. From around 2009 onwards, there was a period lasting several years during which the condition of the tree canopies deteriorated year on year. This led to a significant increase in average canopy thinning, and since then this has fluctuated at this elevated level.
Some trends are evident exclusively among deciduous trees. Their mortality rate has increased significantly, and the proportion of severely damaged trees with 65 to 95 per cent canopy loss has risen markedly since the early 2010s. Added to this is an increased proportion of dead crown sections, which has been noticeable in recent years and particularly since 2022.
When comparing conifers and deciduous trees, it should be noted that the two groups are distributed very differently across the landscape. The conifers in the inventory are located on average at around 1,300 metres above sea level, whilst the deciduous trees are at around 700 metres. The average annual change in total canopy cover depends significantly on altitude and is considerably greater at lower altitudes than at higher ones: below 1,500 metres, the increase is, on a long-term average, around four times as high as above that altitude. Lower altitudes are more severely affected by increasing heat and drought, which has a corresponding impact on the tree species growing there.
Bibliography ¶
MeteoSchweiz (2025) Klimabulletin Sommer 2025.Zurich.
MeteoSchweiz, 2026: Klimabulletin: Frühling and Sommer 2026. Zurich.