How is climate change affecting trees? Do they lose their leaves earlier due to higher temperatures in autumn? Or could the warming also mean that the trees stay green longer in autumn and thus bind more CO2? A study now shows that the answer to this question depends on the timing of the heat: If high temperatures occur before the summer solstice, the trees lose their leaves earlier. If, on the other hand, it gets warmer later in the year, the fall of autumn leaves will be pushed back.
Trees play an important role with regard to climate change: During photosynthesis, they absorb a lot of CO2 and store it in biomass. But how does global warming affect this CO2 fixation? Some studies have suggested that warmer temperatures lengthen the growing season, meaning trees stay green longer in the fall, potentially giving them a longer opportunity to sequester CO2. Others, on the other hand, came to the conclusion that although heat causes the trees to sprout earlier in the spring, it also causes premature leaf fall in autumn, i.e. premature senescence - possibly because the annual growth potential is exhausted earlier and the carbon stores are full.
Opposite effects of temperature
A team led by Constantin Zohner from ETH Zurich has now got to the bottom of the phenomenon. To do this, the researchers first used satellite data from the years 2001 to 2018 to determine when the forests in the northern hemisphere sprouted in each year, when the leaves began to turn autumnal and when they fell. They combined this data with information about soil composition, carbon flux, climate and day length. They also conducted their own experiments by artificially exposing young beech trees to cooler or shadier conditions at different times of the year.
"Our data show that warming at the beginning and end of the season has opposite effects on leaf senescence, with a reversal occurring after the longest day of the year, the summer solstice," the Zohner and his team report. If high temperatures occurred before the summer solstice, the foliage began to change color earlier in the fall. Each degree more in spring and early summer pushes the onset of senescence forward by about two days. After the summer solstice, i.e. from the end of June, higher temperatures ensured that the trees kept their leaves longer - on average around 2.5 days more for each additional degree.
Leaves colored longer
In addition, the team observed that the time between the first change of color of the leaves and their fall also depends on the temperature: if it was cool in late summer and autumn, the trees shed their colorful leaves after a short time. However, if warm temperatures prevailed during this time, this phase lengthened: Although the leaves began to change color early in warm years, the process progressed slowly. Despite the earlier start, it often took longer for the last leaves to fall from the trees than in cooler years.
The current study brings together the results of previous, seemingly contradictory investigations: "It is now clear that warmer temperatures and increased vegetation activity before the summer solstice cause an earlier onset of senescence, while warmer temperatures after the solstice accelerate the progression of senescence, in agreement with previous studies slow down,” explains the team. "This means that senescence continues to start earlier but progresses more slowly."
Adaptation to severe winters?
According to the researchers, one reason why the trees react so differently to heat depending on the time could be the adaptation to seasonal climate zones with severe winters. "The observed response allows the trees to reliably anticipate the course of the seasons and prepare for hibernation long before temperatures begin to drop," write Zohner and his team. "The solstice switch in trees' physiological response to temperature calibrates their seasonal rhythms and mediates how they respond to warm or cool temperatures, now and in the future." the different effects of warming before and after the summer solstice, the researchers say.
Source: Constantin Zohner (ETH Zurich, Switzerland) et al., Science, doi: 10.1126/science.adf5098