Fig. 1. Earlier onset of vegetation period: budding as early as March or April is becoming more common. Photo LFI/WSL
Climate change is fundamentally altering the growth dynamics of our forests. We might assume that a warmer climate that lengthens the vegetation period will lead to higher rates of wood production and carbon sequestration.
TreeNet - real-time data for forests
Fig. 2. Dendrometer in use on a spruce in Davos. High-precision point dendrometers measure trunk radii every 10 minutes. Photo: Roman Zweifel (WSL)
This assumption could not however be confirmed in a recently published study carried out by the TreeNet network using high-resolution measurements taken directly in the forest. Over a period of eleven years, the trunk radii of 228 trees of five species - silver fir, beech, spruce, Scots pine and oak - were continuously measured and recorded at 48 sites in Switzerland, and the data was evaluated.
High-precision point dendrometers (fig. 2) register changes in trunk thickness in the micrometre range every 10 minutes. This creates a unique picture of the growth of different tree species at different altitudes, in different climatic zones, and under different soil conditions.
Further information on the TreeNet network can be found in the article “Growth, water, warning signals - real-time data for forests under climate stress”
Results from eleven years of continuous measurements
As a result of global and regional warming, the growing season in Switzerland now begins much earlier and lasts longer than it did a decade ago. However, the extended time frame does not automatically lead to more growth - on the contrary, stem growth is declining in many stands (fig. 3). Silver fir, beech and spruce show a significant decrease in annual radial stem growth from 2012 to 2022. There is no clear trend for Scots pine and oak species, but they are considerably more sensitive to late summer heat.
It is therefore important to make a distinction between the vegetation period and the stem growth period: While the vegetation period is determined by budding and the onset of photosynthesis and lasts considerably longer, the stem growth period only includes the time during which the stem actually grows. Here too, the start is earlier, but the growth is lower overall. It is precisely this aspect that the TreeNet measurements take into account.
Shift in the start of growth
Fig. 4. The start of the growing season has become much earlier for four of the five tree species analysed.
Source: Bose et al. 2025, modified.
The start of the stem growth period has shifted forward significantly within just 10 years (fig. 4). Whereas in the past the cambium activity of many species began between the end of April and the beginning of May, today it usually begins on the same sites as early as the beginning to the middle of April, and at lower altitudes in some cases as early as the end of March. Accordingly, the first increases in stem radius can be measured two to three weeks earlier in some years than at the beginning of the 2000s. Despite this shift, the annual tree growth is lower (compare fig. 3, right).
Impact of dry years
Fig. 5. Downy oak (Quercus pubescens) in Salgesch. Hot summers such as 2003 led to premature leaf discolouration in oaks and massive declines in stem growth for species like spruce and pine all over Switzerland. Photo: Roman Zweifel (WSL)
The declines in growth are particularly pronounced in dry years. In 2015, 2018, 2019 and 2022, the stem growth remains well below the long-standing values (see fig. 3). Such summers leave their mark (fig. 5). The trees' carbon reserves are emptied and are lacking in subsequent years. The negative effect extends over several growth seasons. Water shortage is thus not just a short-term stress factor, but a driver of long-term growth losses.
Time window of growth
We might expect a longer warm season to automatically mean a longer growing season for the trees. More weeks with favourable temperatures, more days with photosynthesis – and therefore more growth. But that is exactly what is not happening. The decisive factor is not the length of the vegetation period, but the number of days on which stem growth actually takes place. The cambium cells only divide if sufficient water is available and the turgor in the tissue remains high enough. This often happens at night, when transpiration decreases and the tree’s water storage fills up (fig. 6). During the day, water loss often predominates, preventing cell division.
Fig. 6. The stem often grows during the cooler hours of the night, when evaporation is low.
Photo: Martin Moritzi (WSL)
Heat and dry air lead to a drop in the internal pressure of the cell – and growth stops. Only when evaporation decreases in the evening do the growth processes resume. Over an entire year, this process adds up to approx. 40 to 110 effective growing days, depending on the species and year (tab. 1). In between, there are many days when the trunk radii may well fluctuate - shrinking and expanding again – but without new wood being formed. This makes it clear that it is not weeks or months that count, but individual hours. An earlier start in spring is of no benefit if the number of these growing hours decreases because of heat waves.
| Years | Silver fir Abies alba | Beech Fagus sylvatica | Norway spruce Picea abies | Scots pine Pinus sylvestris | Oak Quercus spp. |
|---|---|---|---|---|---|
| 2012 | 107 ± 17 | 105 ± 15 | 67 ± 13 | 54± 23 | 70± 35 |
| 2013 | 105 ± 15 | 113 ± 13 | 82 ± 10 | 27± 17 | 97± 22 |
| 2014 | 97 ± 22 | 113 ± 10 | 83 ± 13 | 49 ± 7 | 82± 15 |
| 2015 | 116 ± 19 | 112 ± 8 | 61 ± 8 | 56± 13 | 66± 13 |
| 2016 | 101 ± 25 | 107 ± 9 | 74 ± 8 | 51 ± 9 | 77± 11 |
| 2017 | 96 ± 27 | 81 ± 11 | 55 ± 8 | 39 ± 8 | 53 ± 11 |
| 2018 | 96 ± 17 | 102 ± 8 | 51 ± 6 | 42 ± 6 | 64± 9 |
| 2019 | 93 ± 20 | 107 ± 10 | 53 ± 8 | 51 ± 7 | 62± 14 |
| 2020 | 91 ± 17 | 108 ± 9 | 52 ± 9 | 45 ± 7 | 72± 14 |
| 2021 | 111 ± 19 | 112 ± 10 | 60 ± 9 | 54± 7 | 71± 12 |
| 2022 | 109 ± 13 | 104± 9 | 56 ± 8 | 42 ± 6 | 40± 8 |
| Mean | 102 ± 19 | 106 ± 10 | 63± 9 | 46 ± 10 | 69 ± 15 |
Consequences for carbon sequestration
Forests are a key element of the climate-vegetation system of our planet: they absorb CO₂ from the air, sequester it in wood, and thus slow down global warming. Annual growth rings in the tree trunks prove this process - the wider the rings, the more carbon is sequestered.
TreeNet data show the limits of this relationship, however. Although the growing season now starts earlier and lasts longer due to the higher temperatures, this does not bolster the role of forests as carbon sinks. On the contrary: rising temperatures in spring and summer slow cell formation down and reduce the number of days when growth takes place. This shortens the effective growth period, reducing carbon sequestration in the wood. Just a few hot days in the relatively short time window for stem growth can significantly reduce the annual increment. In dry years, the growth performance of the forests is also reduced significantly by the major impact of insufficient precipitation.
The expectation that a longer season could compensate for losses is thus not fulfilled: a premature start followed by hot, dry summers leads to less growth and therefore less biomass over the course of the year.
Consequences for practice and policy
Global warming is fundamentally altering forest dynamics. Although higher temperatures promote growth early in the year, they slow it down later on, and drought reduces it further - the growth balance of important forest tree species becomes negative. This reduces the ability of the forests to sequester carbon, and generally weakens sensitive tree species such as spruce, fir and beech. Against this background, climate protection measures, species composition and thus sustainable forest management must be reassessed.
In practice, this means that drought resistance is more important than ever as a criterion for the selection of tree species. Growth benefits from longer vegetation periods cannot be exploited if there is a lack of water. Climate models that calculate on the basis of forests being constantly growing over a vegetation period need to be adapted. Policy makers also have a role to play. Climate protection strategies must not overestimate the role of forests: Forests remain pivotal – but they are not infinitely resilient and they are not a solution for the CO2 produced by the burning of fossil fuels.
Practitioners and policy makers must increasingly take uncertainties into account and improve the adaptability of forests. Monitoring networks such as TreeNet are very helpful in enabling us to recognise at an early stage that tree growth is slowing down, drought stress is occurring or the forest is recovering. They allow us to reliably recognise changes in the forest and their effects and to document them.
Scientific publication
Bose A.K., Etzold S., Meusburger K., Gessler A., Baltensweiler A., Braun S., … Zweifel R. (2025) Decreasing stem growth in common European tree species despite earlier growth onset. Glob. Chang. Biol. 31(7), e70318 (17 pp.). https://doi.org/10.1111/gcb.70318
