Study reveals a counterintuitive way climate change is affecting fall

The shift from autumn to winter brings with it an iconic riot of colors — the golden, blood red, and fiery orange leaves of deciduous trees turn brown and start to fall to the ground.

But these falling leaves are not just a sign of the changing seasons any more. They are also a serious sign of climate change.

A new study published Thursday in Science reveals a counterintuitive way the warming climate is affecting Fall, breaking with our traditional assumptions of how a hotter world alters the plant life we depend upon for sustenance, air, and shelter. This is critical to know — the leaves which define fall are not only beautiful, they are also a marker of how trees are storing carbon.

"The presence of leaves on deciduous trees not only marks the changing of the seasons, but also defines the period of time in which trees store carbon from the air in leaves, wood, and roots," Christine R. Rollinson, a researcher at The Morton Arboretum, writes in an accompanying perspective.

A Leafy Experiment — Scientists previously believed warming temperatures would spur temperate trees to shed their leaves some 2-3 weeks later than usual over the 21st century. But the past evidence was based on how day length and rising global temperatures would affect the falling of leaves, also known as autumn leaf senescence. Warming temperatures lead to longer growing seasons, which leads to delayed leaf senescence.

Or at least, that was what scientists had thought. But this new study instead examines environmental phenomena — increased temperatures, light levels, and carbon dioxide — to determine when trees will shed their leaves. Specifically, they examined how photosynthesis affects growing season productivity, which occurs in the spring and summer.

What they discovered completely goes against the old assumption — instead, the researchers estimate autumn leaves will fall 3-6 days earlier by 2100.

Inverse contacted the researchers behind the new study for comment, but did not get an immediate response.

Unexpected Results — Using data on Central European trees from 1948 to 2015, the researchers developed photosynthesis influenced autumn phenology simulation models, which they tested against other models from existing research.

The old models assumed the length of growing seasons determined how much carbon the trees took up from the atmosphere. But this study flips the switch, looking instead at how changing carbon capture — resulting from climate change — affects growing season productivity.

In this study, the scientists assume growing seasons will be longer due to climate change, and they hypothesize greater growing season productivity will decrease the number of cool autumn days required for the leaves to fall.

Their model also offset the effects of warmer autumn temperatures by predicting increased productivity, or photosynthesis-induced growth, in the spring and summer.

Due to lower carbon uptake, the researchers predict that this extended growing season will shorten autumn senescence — the transition from summer through to fall and then winter — by 3-6 days by the end of the century.

As the authors state in the study:

"These new predictions lower our expectations of the extent to which longer growing seasons will increase seasonal carbon uptake in forests."

Contrary carbon — The reason why their findings differ so much from previous studies is to do with carbon capacity.

"These findings build on a growing body of literature focused on the demand side of the carbon cycle, in which growth and productivity are limited by the ability of different tissues, such as roots or stems, to use and store carbon (carbon “sinks”)," Rollinson explains.

"Increasing atmospheric carbon from anthropogenic emissions increases the carbon available for plant photosynthesis, making carbon movement into leaves more efficient," she writes.

Essentially, tree roots and wood can only use and store so much carbon, so they cease to need it at a certain point. From that point on, it's costly for the trees to retain their leaves, so they shed them. If there is more carbon in the atmosphere, trees will take up that carbon more efficiently, but they won't necessarily keep growing for longer periods of time because they only have so much capacity.

Global forests — The study is limited in that it only includes data from deciduous forests in Central Europe, but it has serious implications for forests around the globe.

Forest trees act as vital carbon sinks, absorbing more carbon than they emit. What happens to a tree's leaves as a result of climate change impacts the entire world. Rollinson points out the effect seen in this study is likely not the sole factor in when trees shed their leaves, particularly in forests located in different climates.

But there does need to be greater emphasis on the effects of photosynthesis on autumn senescence in future climate-change models, the researchers argue:

"These results highlight physiological constraints on growing-season lengths and plant productivity in a warming, CO2-enriched world, which has direct implications for future carbon-cycle and climate projections."

Understanding the role of trees' ability to capture carbon in a warming world will be critical to controlling global warming in the future, Rollinson writes.

"Trees and forests remain one solution for mitigating the impacts of climate change, [but] they cannot be the sole means of climate change response," Rollinson writes.

Abstract: Changes in the growing-season lengths of temperate trees greatly affect biotic interactions andglobal carbon balance. Yet future growing-seasontrajectories remain highly uncertain becausethe environmental drivers of autumn leaf senescence are poorly understood. Using experimentsand long-term observations, we show that increases in spring and summer productivity due toelevated carbon dioxide, temperature, or light levels drive earlier senescence. Accounting forthis effect improved the accuracy of senescencepredictions by 27 to 42% and reversed futurepredictions from a previously expected 2- to 3-week delay over the rest of the century to an advanceof 3 to 6 days. These findings demonstrate the critical role of sink limitation in governing the endof seasonal activity and reveal important constraints on future growing-season lengths andcarbon uptake of trees.