Where Does The Green Go?

 by Kim Woolcock

It’s autumn where I am, and the leaves are turning. They look like they’re setting themselves on fire before they fall, going out in a blaze of glory. Crispy husks carpet the forest floor, ready to be turned into next year’s nutrients. 

 

Leaves of Acer palmatum subsp. matsumurae (Koidz) Ogata

Photo credit: 松岡明芳

I love it, but it also seems extravagant. Why don’t leaves just stay green until they fall?

It turns out the trees are being thrifty. The leaves are full of chloroplasts, which contain lots of chlorophyll, the green light-harvesting pigment that lets plants spin sunlight into sugar. Chlorophyll is expensive, nutrient-wise. It’s loaded with nitrogen, and so trees tuck it away for winter. They break down the chloroplasts, pack the nutrients for transport, and send them to the trunk and roots. When they’re done collecting what they need, they build a waxy wall between the branch and the leaf and then let the leaf drop.

Packing up the green pigment lets the yellow and orange pigments, carotenoids, shine through. They were there all along, helping the leaves capture light of different wavelengths, but they were masked by the green. As the chlorophyll is removed, the carotenoids pick up some of the slack, making as much energy as they can with the last rays of autumn.

Not all leaves turn yellow or orange—some turn flaming red, thanks to anthocyanins. These pigments aren’t there in the summer, but are made specially in the fall. They act as sunscreen, protecting other leaf molecules from sun damage after the chlorophyll’s gone. That’s why they’re brightest in areas where fall days are sunny. They’re also made from leftovers. As the days get shorter, leaves keep producing sugar and sending it to the roots. But when nights get too cold, sugar transport is slowed, and some sugar gets stuck in the leaves, where it’s made into anthocyanins instead.

 

The green is almost gone. Image credit: Sander van der Wel

It’s a big job, getting ready for winter. Consider a single aspen tree (Populus tremula). Researchers made a detailed calendar of fall events for this tree, tracking components such as pigments, metabolites, nutrients, and photosynthesis rate. The tree has several million leaves, each of which contains ~30 million cells. Each cell contains ~40 chloroplasts. So every autumn, the tree has to synchronize the dismantling and transport of 10¹⁵ chloroplasts (one quadrillion, or the total number of ants on earth, just for scale), all in about a month. That’s just one tree. No wonder they look like they’re on fire.

So that’s where the green goes. It’s stashed away in trunks and roots for the winter, waiting to be remobilized in spring. Winter has always seemed drab compared to fall, to me. But knowing this makes me look at winter tree trunks differently – they are actually full of secret green. 

 

Kim Ryall Woolcock is the co-author of Design Like Nature: Biomimicry for a Healthy Planet (Orca, 2021) with Megan Clendenan. Her next book Tough to be Tiny is coming out from Flying Eye Books in July 2022. You can find out more at www.kimwoolcock.com
 

Resources:

John King. 2011. “Reaching for the Sun: How Plants Work, second edition.” Cambridge University Press, Cambridge. 298 pp.

Johanna Keskitalo et al. 2005. “A Cellular Timetable of Autumn Senescence.” Plant Physiology, 139:4, 1635–1648. https://doi.org/10.1104/pp.105.066845

https://www.sciencedaily.com/releases/1997/10/971005050137.htm

 

 

Make Mine Maple

by Helen Mason

At the first signs of spring, when the days get longer, the snow begins to melt from around the roots of the maple trees, and the temperatures rise to slightly above freezing during the day but stay cold at night, the sweet sap in the maple trees begins to flow. That's when both professionals and hobbyists tap sugar maples for sweet sap that is boiled until most of the water has evaporated. What's left is an amber-coloured liquid called maple syrup.

Sugar bush in early spring

The sugar in this sap was produced the previous year when sunlight, reacting with the chlorophyll of the leaves in sugar maples, produced sugar. This sugar was stored in the tree and released when the spring warmth caused the sap to rise from the winter-frozen roots.

Sugar maple trees are unique to eastern North America. Their sweet sap was first harvested by Aboriginal tribes during the "maple moon."

French and English explorers wrote of the "sweet water" that North American Indians took from the trees and boiled down to a coarse dark brown sugar. In the 18th and 19th centuries maple sugar was an important food item and was used for trading. By the late 19th century, however, white cane sugar, which was cheaper and easier to get, replaced maple sugar as the sweetener.

This hobby bush is in a sugar maple stand near Parry Sound, ON.

Today, maple syrup and sugar are made both by hobbyists and professionals. Commercial bushes use pipeline to move the spring sap to an evaporating shed where it is boiled in large commercial evaporators. Many hobbyists tap a small number of trees and make just enough syrup for their family and friends.

In 1980, Mark Mason used a battery-powered drill to tap.

In small bushes, the process begins when the hobbyist drills a hole in the tree trunk to collect some of the sap as it rises. A cone-shaped funnel or "spile" is hammered into each tap and a bucket hung from the hook attached to each spile. As each day warms, sap plinks into the buckets, creating a springtime maple melody.

In most bushes, the sap is collected and poured into a large flat evaporating pan. The sap is boiled until the watery liquid gradually turns from pale yellow through gold, to a warm amber. This process uses up a lot of firewood, gas, or propane. In some commercial bushes, it's sped up by reverse osmosis.

Partially boiled sap is poured into a hydrometer cup. Is it maple syrup yet?

The work demands patience. About forty litres of sap produce only one litre of maple syrup. Making it demands hours of boiling, buckets of patience, and many maple trees.

As the temperature of the boiling sap begins to rise, the specific gravity is checked. The finished product has a specific gravity of 1.37 and boils at exactly 104 °C. Traditionalists use a glass column called a hydrometer that will show about two-thirds of its length when the correct specific gravity has been reached.

A felt sock filter purifies some dark syrup from late in the season.

Once the syrup reaches this stage, it's quickly removed from heat, filtered to remove any impurities, and bottled or canned, ready for you to enjoy over pancakes or in your favourite maple recipe.