As soon as the days get shorter and the temperatures cooler, the foliage of our trees turns brightly colored and finally falls off. Up until now, it has been widely assumed that increasing average temperatures as a result of climate change enable trees to retain their leaves for longer, to carry out photosynthesis for longer and thus to bind more CO2. But this calculation only works to a limited extent. As researchers have now shown, the annual carbon storage capacity of deciduous trees is limited. If the reservoirs are full, the trees even shed their leaves earlier, although the temperatures would allow them to remain green for longer.
With the help of the green leaf pigment chlorophyll, plants convert carbon dioxide and water into sugar and oxygen during photosynthesis. In this way, the forests in Germany alone bind many millions of tons of CO2 each year and thus offset some of the greenhouse gas emissions. Observations over several decades have shown that as a result of global warming, trees in our latitudes sprout earlier and shed their leaves a little later. If this trend were to continue, it would mean that the trees can bind more and more CO2 per year because they have a longer green phase at their disposal. According to calculations, every hectare of forest that it later sheds could bind 98 kilograms of additional CO2 per day.
Observation data and own experiments
But is climate alone really responsible for how long trees keep their leaves? A team led by Deborah Zani from ETH Zurich has now checked. To this end, the researchers evaluated observational data from 3855 Central European locations from 1948 to 2015. It was recorded when six typical deciduous tree species – common horse chestnut, silver birch, red beech, European larch, English oak and rowan tree – sprouted and when they lost their leaves. In addition, the researchers carried out their own experiments in which they exposed trees to different temperatures, CO2 concentrations and solar radiation under controlled conditions.
The result: It is true that trees can keep their leaves longer in warmer temperatures. But this effect is limited. After a certain point, it is no longer worthwhile for the trees to continue photosynthesis because their annual growth potential has been exhausted and their carbon stores are full. Due to other limited resources – such as nitrogen – and due to the necessary maturation processes of the plant tissue, they cannot grow indefinitely in a year. While the climate has often been the limiting factor in the past, as temperatures rise, the limited carbon storage capacity could determine when the trees shed their leaves.
Higher photosynthesis activity causes leaves to age earlier
A role is played by the fact that the trees sprout earlier in spring and therefore start photosynthesis earlier in the year. Stronger solar radiation in summer and a higher CO2 content in the air also help the tree to fill its carbon stores faster. “Higher photosynthetic activity means that the seasonal cycle of the plants accelerates. Ultimately, this ensures that the leaves begin to age earlier, ”the authors explain. In one of their experiments they showed that trees that are in the shade and are therefore less able to photosynthesize, shed their leaves on average 8 to 13 days later than specimens of the same species in the sun. Overall, the trees in the shade and in the sun bound a similar amount of CO2 throughout the year.
On the basis of their findings, the researchers developed a computer modeling which, in addition to climatic conditions, also includes the photosynthesis rate and the trees’ limited carbon stores. They tested their model on historical observation data since 1948. And indeed: with the help of this model, Zani and her team were able to determine the time of autumn leaves fall up to 42 percent more precisely than previous models. “The high predictive power of our model underlines how important plant activity in spring and summer is for leaf fall in autumn,” the researchers write.
Will the leaves fall earlier in the future?
Earlier models, which focused on climatic conditions, assumed that autumn leaf fall would be postponed by an average of two to three weeks by the end of the century. The new model from Zani’s team, however, predicts that the leaves could fall three to six days earlier by the end of the century. If they were sprouted earlier in spring, the annual green phase would nevertheless be extended – but only by eight to eleven days and not by around a month as previously assumed. However, the researchers limit the fact that it is still unclear to what extent the observations on the six selected tree species can be transferred to other trees, such as those that can fix nitrogen.
“The study shows that the carbon storage capacity of forests is limited in a way that we do not yet fully understand,” writes Christine Rollinson of the Center for Tree Science in Illinois in a commentary on the study published in the journal Science. “Trees and forests remain a solution to mitigating the effects of climate change, but they cannot be the only means of responding to climate change. In order to mitigate the carbon emissions for which humans are responsible, a diverse portfolio of climate protection measures is important, including reducing emissions and maintaining and replanting trees. “
Source: Deborah Zani (ETH Zurich) et al., Science, doi: 10.1126 / science.abd8911