Trees are losing their leaves earlier because of climate change

Previous models had suggested that fall would be delayed as the season got warmer over the coming century.

(CNN)Trees will start to shed their leaves earlier as the planet warms, a new study has suggested, contradicting previous assumptions that warming temperatures are delaying the onset of fall.

Every year, in a process known as senescence, the leaves of deciduous trees turn yellow, orange and red as they suspend growth and extract nutrients from foliage, before falling from the tree ahead of winter. Leaf senescence also marks the end of the period during which plants absorb carbon dioxide through photosynthesis.
Global warming has resulted in longer growing seasons -- spring leaves are emerging in European trees about two weeks earlier, compared with 100 years ago, researchers said.
"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," ecosystem ecologist Constantin Zohner said.
However, Zohner and a team of researchers have said their findings reverse this prediction.
"We actually predict by the end of the century, leaves might even fall off three to six days earlier," Zohner, a corresponding author on the paper published Friday in the journal Science, added.
Using a combination of field observations, laboratory tests and modeling, experts studied data that tracked six European deciduous tree species -- European horse chestnut, silver birch, European beech, European larch, English oak and rowan -- over the last six decades.
Increases in spring and summer productivity that come as a result of elevated carbon dioxide, temperature and light levels actually drive trees to lose their leaves earlier, the experts found.
It had been assumed, Zohner said, that fall temperatures and day length were the main environmental factors that cause trees to lose their leaves. Now, researchers have identified a third factor -- a "self-constraining" productivity.
"What we now see is there is this third huge mechanism that is going on -- the (tree) productivity is self constraining itself. 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," he said.
"This is a mechanism we also see in humans -- if you start eating earlier, you will be full earlier," he said.
The findings, Zohner said, have shown that trees have productivity constraints.
"We cannot just put more and more CO2 in the atmosphere and (expect) trees will just do so much more -- there are limits," he said.