This year, the autumn coloring of trees seems to arrive earlier than usual in certain places. This can be due to many internal and external factors - be it wind and heat, ozone exposure, spring growth or the amount of seed produced. However, closer examination shows that many trees have turned brown without going through the usual stages of fall leaf discoloration. This controlled process normally allows the tree to reabsorb nutrients and store them as reserves during autumn.
The current browning occurred especially after the recent heat wave at the end of August. The species most affected appear to be beech and hornbeam in the North of the Alps, already weakened by the droughts and extreme temperatures of the summers 2018 and 2022. South of the Alps, especially lime trees but also birch and other deciduous tree species are affected.
It is notable that the trees most exposed to wind and strong sunlight, i.e. on southern slopes and forest edges, show the most marked signs of this phenomenon. Wind accelerates transpiration, increasing the risk of both soil and leaf desiccation, while intense sunlight and warmer, drier, more ozone-charged air can contribute to accelerated leaf senescence.
Legacy of droughts in 2018 and 2022
It is imperative to remember that north of the Alps, beech trees suffered greatly from drought and heat in 2018 and 2022, also showing premature coloration. The effects of such extreme events could persist for several years. Similarly, most deciduous trees wilted since late August 2022 in the Mendrisio region (southern Ticino). Such longer lasting effects occur especially when air bubbles (embolisms) build up in the water transporting vessels, which occurs when the loss of water via the leaves to the dry hot air exceeds the amount that can be taken up from the dry soil. This blocks the water supply to the leaves leading to leaf desiccation and also restricts water transport in the following years. A careful look at our forests shows many remnants of the last drought events: dead stems, branches and twigs.
Other factors that could contribute to this premature coloration is massive seed production, a phenomenon called masting, and ozone concentration in the air. Trees invest a large amount of their sugar resources in seed production, which makes them more vulnerable to drought. This partly explains why many hornbeams in the north and lime trees in the south, which have produced an impressive quantity of seeds this year, are already turning brown or have already lost their leaves. Besides, every year, the ozone dose to which vegetation is exposed exceeds tolerable limit values. Particularly in sunny summers - but with more or less regular amounts of precipitation, as in 2023 - natural vegetation ends up developing a range of characteristic symptoms, including reddening (various species of dogwood or viburnum), yellowing and browning (e.g. in beech or hazel). These symptoms can be observed throughout Switzerland in 2023.
Summer droughts likely to become more frequent
However, the phenomenon of premature discoloration is not new, and occurs whenever a severe summer drought is combined with a heat wave, as in 1947, 1976, 2003, 2015, 2018 and 2022. Nevertheless, this phenomenon is likely to become more frequent in the future, as ever-rising temperatures make each new drought more harmful to trees, by increasing evapotranspiration and thus reducing water reserves in the soil.
It is still too early to say whether this year's leaf coloring is exceptionally early across the country. It is certainly early in some places, such as in the Jura or Western central Plateau where precipitations during early summer were below the average. However, it is possible that the current warm temperatures of early September are delaying the coloring process of trees that have not been weakened by past heatwaves and droughts by slowing down pigment degradation. This could result this year in a great heterogeneity in leaf coloration among trees and regions which could then reflect the trees' state of stress.
Monitoring trees over the seasons
To keep a close eye on this phenomenon, monitoring networks of tree phenology (leaf coloration but also leaf emergence in spring) have been set up across Switzerland, coordinated by MeteoSuisse (citizen science network), WSL (WSL PhenoWald Project, annual inventories of ozone symptoms), Swiss rangers (SNF-WSL PhenoRangers Project), and Globe via the participatory science platform phaenonet.ch. These long-term monitoring networks will help to determine whether early leaf discoloration in beech and other tree species is significantly widespread this year, or limited to specific regions and whether we observe a trend over recent years in connection to climate warming.