By Kathleen Berger, Executive Producer for Science and Technology
The splendor of autumn leaves is embraced by those who love the fall season. The Danforth Campus of Washington University in St. Louis is an accredited arboretum with a diverse collection of more than 5,000 trees. Many tree species on campus predictably display their beautiful colors in the fall months.
“So, all the trees in the northern hemisphere, such as the trees here on campus have evolved for thousands of years,” said Susanne Renner, honorary professor of biology at Washington University in St. Louis.
Renner is especially fond of a row of Ginkgo trees on campus, which are native to China.
“You see here, I don’t know, 20,000 leaves and they rain down simultaneously, all at the same time,” she said. “We can predict when they will fall because the tree forms a special abscission zone, a layer (of cells) that is a place (area of stem) where it will break off, a break off place, a morphological area of cells – six days before dropping the leaves. So, they are all just hanging there, and a little bit of wind, and then they all fall down.”
The changes that occur in autumn are in preparation for winter. It’s about survival.
“And they do this by getting rid of their leaves, which would be impossible to maintain,” Renner explained. “Leaves evaporate water. And you can’t draw up water from frozen soil, so the leaves would constantly lose water and the tree would die of drought.”
Renner’s love for trees and all their benefits led to years of research about climate change.
“The general thinking is that with the nice warm conditions, the trees could well hang onto their leaves a little longer.”
But Renner and her colleagues discovered that’s not true! A warmer fall does not mean leaves change colors later.
“Because the signal that the trees use to prepare for winter is not the temperature, it’s the day length. Once the days are becoming shorter, all they want to do is get rid of those leaves because winter is coming.”
As one of the lead investigators, Renner and her team proved the timing of leaves falling has nothing to do with fall temperatures. But a spring frost would cause leaves to change colors later in the fall. That’s because the age of the newer leaves is important. Renner and her team conducted lab experiments with trees.
“We damaged them by a frost, an experimental frost. So, they have to make new leaves. These new leaves, they stay on longer, they stay longer and they photosynthesize more. The moment when leaves are shed doesn’t depend on the temperature, they were all experiencing the same temperature. It depends on how much photosynthesis, or energy gain, the tree has done in the spring. If it couldn’t do any because the leaves were damaged by us, it had to hold onto its leaves longer.”
However, warmer fall months from climate change may still cause some visual changes, as vibrant colors of leaves are expected to become less brilliant.
“Experiments have shown that the red color in particular, anthocyanins, the concentration is influenced by temperature and light. So, with bright light and night frost, we have a higher concentration of the red anthocyanins. So, if there is no night frost (from climate warming in the fall), we will have less brilliant, less disposition of the red color.”
But Renner said not to worry about drastic changes.
“The Northern Hemisphere is not going to change drastically with climate change. The main signal is the shortening of the days, that won’t change with climate change.”