Autumn Glory: Why Trees Lose Their Leaves
Take a trip to upstate New York, Vermont or Quebec in September and you are in for a treat.
A short climb or drive up any hill with a vantage point will afford you a viewpoint that will literally bring a tear to your eye. Laid out before you, the hills undulating away to the horizon, a seemingly endless vista in a blaze of colour like nowhere else on earth.
It is breathtakingly glorious to see.
Every shade of pale to deep yellows, bright to dark ochre oranges, and brilliant ruby to scarlet to burgundy reds mixed up and laid out before you in a vast never-ending expanse of explosive colour. Just wonderful.
Ever wonder why?
It turns out autumnal leaf drop is a form of self-protection.
While evergreen plants in cold climates have thick waxes and resins to protect their leaves from freezing and fracturing, deciduous species generally have thin leaves that are susceptible to cold temperatures. Since water expands when frozen, the tender leaf cells would rupture during the winter, making them useless for photosynthesis.
Without dropping these leaves, such a tree would be stuck with thousands of unproductive appendages and no way to make food! As if that were not reason enough, the surface area of all those leaves would also pose a threat to the plant’s physical integrity.
Winter months are often windier than other seasons, and the wind against the broad leaves on a cold, brittle tree could cause major breakage. The same goes for the weight of snow collecting on all those leaves.
Finally, by the end of summer, many leaves are insect-eaten, diseased, or otherwise damaged. Dropping them gives the plant a fresh start in the spring, and the nutrients from the decaying leaves are recycled to help grow the next leafy generation.
Interestingly, autumn leaves are not simply blown off trees but are separated from the plants in a highly controlled process. As day length shortens and temperatures cool, hormones within the plant are activated to begin the abscission process.
Chlorophyll production stops and the pigment starts to degrade revealing showy reds and yellows that were masked by green. The vessels that carry water to the leaf and sugars to the rest of the plant are closed off, and a layer of cells, known as the abscission layer, starts to grow between the leaf stalk and the twig holding it. These cells serve to slowly cut the leaf from the plant without leaving an open wound. As the leaves fall, the plant enters dormancy, saving its energy for the great bud burst of spring.
Now you know.