[Un article de The Conversation écrit par Andrew Hacket-Pain – Senior Lecturer, School of Environmental Sciences, University of Liverpool]
Thus, recent studies (including one of mine) suggest that trees can use the longest day of the year as a key marker of their growth and reproductive cycles. As if the solstice was an arboreal memory aid.
For example, trees that grow in cold regions slow down the creation of new wood cells around the solstice and focus their energy to finish already formed but still incomplete cells. This would make it possible to have time to complete the construction of cells before the arrival of winter – whose icy temperatures damage the incomplete cells, making them useless for the transport of water the following year.
Does the solstice initiate the browning of the leaves before fall?
It also seems that the trees take advantage of the solstice to prepare for autumn, with the “senescence” of their leaves. The senescence allows the tree to reabsorb the essential nutrients present in the leaves before they fall. This process must arrive at the right time: if senescence is too early, the loss of sheet reduces photosynthesis (and therefore the acquisition of energy which is used for the growth of the whole tree). If the senescence is too late, the autumn frosts destroy the still green leaves, which makes the tree lose the precious nutrients that had stored there.
Thus, satellite observations of forests and controlled experiences in greenhouses show that high temperatures just before the solstice tend to advance the browning of the leaves in the fall
Conversely, high temperatures just after the solstice seem to slow down the senescence process, which extends the transition period between the green leaves and the fully brown leaves. This fine adjustment would allow trees to extend the photosynthesis period on the years when temperatures remain higher and not to miss these favorable conditions.
These observations and their interpretations are however debated among scientists.
Indeed, from the point of view of evolution, the solstice may not be the best time marker of these transitions in the annual cycle of trees. For example, in the forests of the Far North, the leaves do not appear before the beginning of June, just days before the solstice, and the growth season can be extended until October. In these forests where certain trees have just started their growth for the year, using the solstice to initiate the process of progressive reduction in activities does not seem to have meaning.
On the other hand, there is a broader consensus on the use of the solstice by plants to synchronize their reproduction.
Solstice, tell me how many seeds produce this year
In many plants, in particular trees of temperate medium latitudes, the number of seeds produced varies considerably from year to year, what is called the lodge. For example, a large European beech can produce hundreds of thousands of seeds in an exceptional year (a ” Masting Year ) And completely give up reproduction of other years.
The reduction in the production of seeds by beech trees, which allows them to increase the effectiveness of their reproduction, is often done on a continental scale, and in stages.
A little butterfly, Cydia Fagiglandana (or the carpocapse of the faces, in French – the Faine is the fruit of beech), lays its eggs in beech flowers. When the larvae hatch, they eat and destroy development seeds. The alternation between fasting years in bells and years of scarcity contributes to protecting the populations of beech trees from these butterflies.
In the United Kingdom, for example, beech trees lose less than 5 % of their seeds because of Cydia. Indeed, the cycles samples butterflies and reduce their population, which awaits the splendid years. If the trees are desynchronized in these high and low fruit production cycles, the loss of seeds can reach more than 40 %.
We have known for decades that years of abundance occur after a particularly hot summer. Indeed, high temperatures increase the formation of floral buds, which generally leads to a greater seed harvest in the fall.
How do be beech trees know that the solstice is the longest day?
On the other hand, we do not know why or how the beech trees in all Europe seem to use the same seasonal window (late June-start July) to determine their production of seeds, whatever place they grow in Europe. How can a beech know the date?
In 2024, by studying dozens of forests across Europe, my team showed that these trees use the solstice as a seasonal marker: as soon as the days begin to shorten after the solstice, the beech trees of all Europe seem to perceive the temperature simultaneously.
Wherever temperatures are above average in the weeks following the solstice, we can expect a high production of faces the following year, while the weather conditions of the weeks preceding the solstice do not seem to have any importance.
As weather cards show, periods of heat and freshness tend to occur simultaneously on large areas.
This allows beech trees to maximize the synchronization of their reproduction, whether by investing in a high production (hot temperatures) or by renouncing reproduction for a year (low temperatures). The use of a fixed mark like the solstice is the key to this synchronization and the advantages that result from it.
From now on, we collaborate with a dozen other European groups to test this effect on different sites, by manipulating the temperature of beech branches before and after the solstice. The research in progress seems to indicate that the flowering genes are activated at the time of the summer solstice.
Finally, studies on the circadian rhythms of plants show that these have molecular mechanisms allowing to detect tiny changes over the duration of the day, and to respond-it would be the basis of this extraordinary synchronized reproduction scale.
If the weather remains hot during the coming month, the beech trees in your region will undoubtedly produce many stories next fall – and it is very likely that this is the case in the center and northern Europe.
With an unwavering passion for local news, Christopher leads our editorial team with integrity and dedication. With over 20 years’ experience, he is the backbone of Wouldsayso, ensuring that we stay true to our mission to inform.
