Developmental constraints mediate the reversal of temperature effects on the autumn phenology of European beech after the summer solstice

Abstract

Accurate projections of temperate tree growing seasons under climate change require representing developmental constraints that determine tree resource allocation. A phenological ‘switch point’ after the summer solstice (21 June) has been proposed, with pre-solstice warming advancing autumn phenology and post-solstice warming delaying it. We propose that this switch is flexible and occurs at the compensatory point between early-season development and late-season temperature effects. We performed trans-solstice climate manipulation experiments on potted European beech () saplings to test (i) how spring leaf-out timing and June-August temperatures influence end-of-season timing (bud set and leaf senescence) and (ii) whether daytime and nighttime temperatures before and after the solstice have distinct effects. Bud set and senescence were tightly coupled (=0.49), with stronger bud responses. Each day of delayed leaf-out postponed bud set by 0.24±0.06 days and senescence by 0.22±0.08 days. July full-day cooling delayed autumn phenology in late-leafing individuals (bud set +4.9±2.6 days; senescence +3.1± 2.8 days) but had a negligible impact on early-leafing trees. August full-day cooling advanced both stages. Pre-solstice daytime cooling had no effect, while post-soltice daytime cooling advanced autumn phenology. Nighttime cooling consistently delayed bud set. These findings support the Solstice-as-Phenology-Switch model and highlight the central role of developmental progression in constraining growing seasons. Faster early-season development – especially under nighttime warming – moves trees past the switch earlier, increasing sensitivity to late-season cooling and thereby triggering earlier autumn phenology. Phenology models should incorporate these developmentally-mediated and diel-specific temperature responses. eLife digest 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. plain-language-summary