Fall leaves. Source: hurtinva.com.
As suggested by the findings of a study conducted in the fall of 2020, trees will begin to shed their leaves earlier as carbon dioxide levels spike, pushing temperatures to unprecedented highs and directly contradicting the commonly held misconception that a warming biosphere will lead to a later onset of fall. Each year, changes in temperature and nutrient consumption cause chlorophyll in plant cells to break down and suspend the growth cycle. As a result, the leaves of deciduous trees take on orange, red, and yellow tints before eventually falling from the tree to blanket the surrounding undergrowth. This process, known as leaf senescence, marks the end of the period during which plants absorb carbon dioxide through the chemical reaction photosynthesis.
Researchers had previously claimed that global warming will result in longer growing seasons, an overall boon to human progress as populations spike. This assumption was fueled by researchers finding that European trees are regrowing leaves in the spring about two weeks earlier than they were 100 years ago. Speaking to this ill-founded postulation, ecosystem ecologist Constantin Zohner said that “[previous] models assumed that, because autumns will get warmer and warmer over the coming century, autumn will get delayed—growing seasons will overall get longer, and autumn will get delayed by two to three weeks.” Zohner and his team of researchers claim that their recent findings reverse this prediction.
“We actually predict by the end of the century, leaves might even fall off three to six days earlier,” Zohner, an author on the paper published in the journal Science, contributed. Over the last six decades, experts have poured over data tracking six deciduous European tree species: European horse chestnut, silver birch, European beech, European larch, English oak, and rowan.
These experts found that, contrary to popular belief, spikes in spring and summer photosynthetic production owing to elevated levels of carbon dioxide, increased temperature, and higher light levels cause trees to lose their leaves earlier. Zohner stated that the assumption at the time was that fall temperatures and reduced exposure to sunlight were the main environmental factors that cause trees to lose their leaves, but researchers have now identified a third contributory factor—a “self-constraining” productivity.
“If you have more going on already in spring and summer—if the plant absorbs more CO2 in synthesis through spring and summer, they lose their leaves earlier,” Zohner said. Easier explained in anthropomorphic terms, if a human were to start eating earlier, they would have eaten their fill sooner and have to stop eating earlier.
This self-constraining productivity is the exact problem with which we are presented today as a new generation: “We cannot just put more and more CO2 in the atmosphere and [expect] trees will just do so much more—there are limits,” Zohner explained. Trees have an intrinsic productivity constraint, so they will not be able to convert an unprecedented outpouring of CO2 into O2. The human race cannot subscribe to the previously held train of thought that natural foliage can be 100 percent effective in the removal of carbon dioxide from the biosphere; human behavioral changes must take place before any ground is won in the battle against global warming.
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