Unusual leaf colors signal stress

A single glance at certain trees is enough to understand: this autumn is unlike any other. On the same branch, green leaves are found alongside brown leaves, sometimes already fallen to the ground. This contrast is not due to a disease or chance. It is explained by an exceptional meteorological sequence, inherited from a summer and early autumn marked by an unusual heat and drought.

Prolonged water stress causes leaf drop

Understanding this mechanism allows anticipating what awaits our forests and gardens in the coming weeks and adapting some simple actions as soon as possible to preserve the vegetation. To remember: the contrast of green and brown foliage on the same tree results from prolonged water stress, the tree sacrificing some leaves to limit evapotranspiration. Sunlight, wind and water stress act unequally on different branches, causing different yellowing rhythms on the same subject.

Heat and drought hit specific tree species

The summer of 2026 was the hottest since 1900 in France, particularly affecting the ash tree (25% of trees affected) and the chestnut (22%) according to the IGN. One tree, two faces: how to explain this disturbing phenomenon. In many regions, hikers observe an unusual spectacle: the same oak or ash tree has both still green foliage and entire branches already browned, even bare. This is not a seasonal illusion, but the direct consequence of prolonged water stress.

Trees use survival mechanisms

As early as mid-July, the DRAAF Occitanie had already noted wilting and early yellowing on the driest soils. These discolorations do not automatically indicate decline: in deciduous trees, they mainly represent a survival mechanism aimed at limiting evapotranspiration. The tree closes its pores, reduces water loss and sacrifices some leaves to preserve the essential. In short, the tree is literally living through two autumns at the same time.

Sunlight, wind, and water stress affect branches differently

Sunlight, wind and water stress: the three factors that confuse the picture. This color shift on the same subject is explained by three parameters that act simultaneously, but unequally depending on the part of the tree concerned. Sunlight accelerates chlorophyll degradation on the most exposed branches to the south, causing faster yellowing than on the shaded sides. Wind, on the other hand, dries out the most exposed branches more, increasing the local sensation of water stress and hastening leaf drop.

Water stress impacts roots unevenly

Finally, water stress itself never affects a tree uniformly: shallow roots suffer more quickly than deep roots, which explains why some branches drop before others. In October 2026, this unusual combination means that sunlight, wind and water deficit can change the color of a tree's branches at completely different rates. However, a point of vigilance is necessary: in conifers, a strong reddening is often a much more worrying sign, potentially indicating irreversible decline, unlike deciduous trees which are better able to withstand this shock.

Extreme summer conditions weaken trees

What does this mid-October weather reveal about the coming autumn of 2026? This phenomenon is not an isolated anomaly, but the direct result of an unusual climatic year. Météo-France has confirmed that the summer of 2026 was the hottest ever recorded in France since 1900, with an average temperature of 24.0 degrees, 3.6 degrees above the norm. The extreme peaks, such as the 43.8 degrees recorded at Palluau, Saintes and Pissos, or the 40.3 degrees recorded in Paris, have left a lasting mark on the soil.

Soil dryness impacts tree health

Result: in September, the average temperature exceeded the norm by 3.1 degrees, and soil humidity showed a level of dryness never observed at this time of year. This accumulation of heat and water shortage permanently weakens certain species. The national forest inventory published by the IGN confirms this: the ash tree is the most affected species, with 25% of trees affected, followed by the chestnut at 22%, the common fir at 11% and the pedunculate oak at 10%.

A signal for future vigilance

These are signals that suggest an autumn of 2026 marked by strong contrasts, early leaf falls in some areas, and increased vigilance to adopt for the health of trees in gardens and forests. This color disparity within a single tree is therefore not a simple aesthetic detail. It reflects an increasing sensitivity of nature to local climatic variations, a signal to monitor closely in the coming weeks and seasons.

Everyone can help preserve tree health

Observing these branches with contrasting colors is also becoming aware of a fragile balance, which everyone can help preserve by adapting watering and the maintenance of their own trees.