It was once said that a falling leaf was how Summer waves goodbye. The Sun takes a step back, the leaves lull themselves to sleep, and autumn is awakened. But how do tree leaves know when it’s time to turn brown and fall to the ground?

The wonderfully bizarre process of leaf senescence

Leaves of deciduous trees turn yellow and red in a process called leaf senescence. Leaf senescence is the process in which trees suspend their growth and withdraw all the nutrients from the leaves back into the tree limbs. Once the tree has prepared itself for Winter, it drops its leaves to the ground (actually, without the nutrients, the leaves die and fall off). This marks the end of the tree’s productive period where it absorbs CO2 through the process of photosynthesis.

How trees know when it’s winter time

There are a few ways trees know when it’s time to settle down for Winter. First, lowering temperatures are a dead giveaway. When the temperature drops and the tree’s sap thickens, it knows Winter is near.

Secondly, trees know Winter is approaching by the length of the day – or the amount of sunlight they receive each day. Because of the Earth’s tilt on its axis and rotation around the Sun, days are longer in the Summer and shorter in the Winter. When the tree begins receiving less sunlight in the Fall, it knows Winter is approaching.

How global warming has impacted tree’s dormant period

With global warming, trees have begun turning greener about two weeks earlier than they did 100 years ago. However, autumn senescence is only occurring about 6 days later. If the temperatures are higher, you would surmise that trees would continue photosynthesis longer and drop their leaves much later. Scientists only recently discovered why global warming is not impacting leaf senescence as much as expected.

The third way trees know Winter is approaching

Trees have a sort of limit to how much CO2 they can absorb. Once they reach the limit, the photosynthesis process slows down. With global warming and a planet-wide increase in CO2, trees are completing their photosynthesis much earlier than they did 100 years ago. This leads to earlier senescence which results in earlier leaf fall in autumn.

However, there is a catch. The influence of higher CO2 levels on earlier senescence is countered by the later senescence triggered by increasing temperatures. In other words, warmer temperatures encourage the tree to hold its leaves longer. But the earlier completion of photosynthesis encourages the tree to drop its leaves sooner. The two opposing factors cancel each other out leaving leaf senescence about the same as it was 100 years ago.