Beech trees in sunlight. Image credit: @ Richard Loader (CC0)
Many plants adjust their routines with the seasons. In regions with warm summers and cold winters, trees must balance growing for as long as possible while avoiding frost damage. Deciduous trees – those that shed their leaves each year – prepare for winter by stopping growth, strengthening protective tissues and eventually dropping their leaves.
The timing of these changes is called autumn phenology, which refers to plant processes that occur at the end of the growing season. Two key stages mark this process: bud set, when future leaf buds stop growing, and leaf senescence, when leaves break down nutrients, change color, and are eventually shed.
To time these changes correctly, trees rely on a combination of external and internal signals. External cues include temperature and day length, while internal cues reflect the tree’s developmental state, such as leaf age or the number of life-cycle events already completed during the year.
Recent work has shown that the summer solstice on 21 June acts as a phenological ‘switch point’, with pre-solstice warming advancing autumn phenology and post-solstice warming delaying it.
To understand why autumn phenology in temperate trees responds differently to temperature changes before and after the summer solstice, Rebindaine et al. tested how trees respond to cooling at different times during the season.
They studied young European beech trees, an important forest species in Europe. In spring, they cooled half of the trees while keeping the others under normal conditions, creating groups that developed at different rates. They then exposed both groups to cooler temperatures in July and August. Cooling in July delayed autumn timing only in the slower-developing trees, showing that a tree’s developmental stage strongly influences its response. In contrast, cooling in August caused all trees to prepare for autumn earlier, regardless of developmental differences, indicating that late-summer temperature cues override internal variation.
Improving our understanding of how temperate deciduous trees control their phenology enables scientists to better predict how trees interpret environmental signals and how forests will respond to climate change. Incorporating this knowledge into ecological models will provide more accurate forecasts of growing-season length and carbon uptake. This, in turn, could help guide decisions about which tree species to plant and how forests should be managed sustainably. Further studies across additional species will be needed to determine how broadly these mechanisms apply.
