At this time of year, deciduous trees across Europe are losing the last of their leaves. When will they come into leaf again? A study just published in Nature Communications has shown that while year-to-year leafing date variations can be explained by heat accumulation, local climate is a key factor in geographical variations.

When will Europe’s deciduous trees put out new leaves to replace those they are currently shedding? How do they know when the time is right to leaf again after winter? An article just published in the journal Nature Communications shows that local climate has a bearing on whether deciduous trees leaf earlier or later in the year. Climatic factors such as light, temperature and aridity play an important role in determining when the season turns, which explains why trees of the same species – beeches, for example – come into leaf at different times according to their location in Europe. In other words, the leaves of beeches unfold at one time or another depending on each tree’s local climate.

A birch in Norway will begin to leaf as soon as conditions are suitable, as it knows the period of good weather in the summer will be short-lived.

A birch tree in Norway, for example, will begin to leaf as soon as conditions are suitable, as it knows the period of good weather in the summer will be short-lived and must be taken advantage of. A birch in the Mediterranean region, on the other hand, will come into leaf later in the year, even if the weather is suitable for it to do so earlier, as it will wait for any early-spring frosts that might affect the growth of its leaves to pass.

Scientists already knew that trees need to receive a certain number of hours of heat to begin leafing, and that year-on-year variations in the time taken to accumulate the necessary number of hours result in the date on which leaves appear varying too. Until now, the same hypothesis was also used to explain why Norwegian and Mediterranean trees leaf on different dates. Now, however, the study described in the previously mentioned article has proved that the heat accumulated in early spring accounts for year-to-year variations but not for geographical variations in leafing dates. “In our study, we found that looking at changes in location instead of between years shows climate to be one of the main drivers of spatial variation in spring phenology”, says Adrià Descals, a CREAF pre-doctoral researcher who is writing a thesis on this very subject. “Trees of the same species leaf at different times in different places, partly because they have adapted to their local climate”, he spells out.

Led by CREAF researchers Marc Peaucelle, Adrià Descals, Roberto Molowny and Josep Peñuelas (a CSIC researcher based at CREAF), the study used a global database to obtain information on phenological changes between 1970 and the present. Along with European climate maps, the information in question enabled the researchers to identify the variables that determine when phenological spring begins. The study looked at eight different species of trees, including the beech, the birch and two species of oak.

Referenced article

Peaucelle, M., Janssens, I. A., Stocker, B. D., Descals Ferrando, A., Fu, Y. H., Molowny-Horas, R., Ciais, P., Peñuelas, J. (2019). Spatial variance of spring phenology in temperate deciduous forests is constrained by background climatic conditions. Nature Communications, 10(1), 5388. https://doi.org/10.1038/s41467-019-13365-1