Oct 132016
 

In October, our attention is drawn to leaves like at no other time of year. They never cease to astound us with their blazing colours and wonderful, spicy smell as we rake them into piles. For the rest of the year, however, we mostly take leaves for granted. They just seem to “be there” doing nothing. But, like everything in nature, leaves are far more amazing than what initially meets the eye. Nothing about them is arbitrary or purposeless.

The characteristics of leaves only make sense when considered through the lens of evolution by natural selection – the process that favors the survival and reproduction of individuals that are best adapted to their environment. So, let’s consider leaves the way Charles Darwin would: By asking “why” questions. Let’s begin by exploring why trees have leaves in the first place. Simply put, leaves make food for the plant – be it a towering white pine or a lowly moss – so that it can grow. This happens through photosynthesis. “Photo” is the Greek word for “light,” and “synthesis” means “putting together.” That’s exactly what is happening. Leaves harness the energy of sunlight to make food in the form of sugars like glucose. Some of the glucose is immediately used for growth (e.g., the production of cellulose and lignin which makes up wood) while the rest is stored for later. Glucose is produced from two ingredients: carbon dioxide and water. Plants breathe in the carbon dioxide, an invisible gas, through tiny holes in the leaves. They use their roots to suck up water. Some of the water is released back into the atmosphere through the leaves by transpiration. At the same time as glucose is produced, oxygen is released as a waste product. Not only is photosynthesis responsible for the production and maintenance of most of the Earth’s oxygen, but it provides the organic compounds necessary for life on Earth. No small feat!

Sugar Maples - Cy Monkman

Sugar Maples – Cy Monkman

Photosynthesis is directly related to another why question: why are leaves green? Leaf cells house tiny structures called chloroplasts. Each chloroplast contains a green pigment (chemical) called chlorophyll, which absorbs the sun’s energy and carries out photosynthesis. As long as chlorophyll is present, the leaf remains green and oxygen and glucose are produced.

Colour change

This begs the question of why leaves change colour and why they are shed from the tree. Both of these phenomena are manifestations of the tree’s preparation for winter.  It is a coordinated undertaking on the part of the entire organism.  Since winter is a time of drought in which water is locked up in the form of ice, trees are less able to take up water through their roots – most of which are near the surface in soil that freezes.  In addition, leaves are continually releasing water vapour through transpiration – think of the high humidity of a greenhouse. Trees must therefore get rid of their leaves in order to minimize water loss and death through desiccation. Also, the leaves of most trees are far too delicate to withstand the rigours of winter.

Before shedding their leaves, however, trees have evolved to salvage the scarce but valuable minerals or nutrients in the leaves. These were originally obtained from the soil through the roots. They include magnesium (an essential component of chlorophyll), calcium, phosphorus, potassium and nitrogen, which is part of all proteins. Given their relative scarcity, a tree can’t afford to lose most of these nutrients each fall when the leaves are shed. It wouldn’t be able to reabsorb them in time or in sufficient quantity to be available for the next generation of leaves. Therefore, as daylight begins decreasing in mid-July, trees start removing these nutrients from their leaves and storing them in the woody tissues until next spring. By sometime in September, the leaves can no longer manufacture chlorophyll and begin to lose their green coloration. At this point, other pigments like the yellows and orange carotenoids gradually become visible. These pigments were present in the leaves all along but were masked by the chlorophyll.

A group of red or purple pigments known as anthocyanins are also abundant in some leaves like those of white ash and both red and sugar maples. They are produced in late summer from excess sugars and are brightest in years with lots of sunny days and cool nights. Scientists are not yet certain about the role of anthocyanins but they may protect the leaves from ultraviolet light.

Leaves of red maple changing colour - Photo by Drew Monkman

Leaves of red maple changing colour – Photo by Drew Monkman

The actual shedding of the leaves is achieved by the formation of a cork-like “abscission” layer of cells at the base of each petiole (leaf stem). Eventually, the leaf’s connection with the twig is broken, and it falls off in the wind, rain or simply from the warming effect of the morning sun.  You have probably noticed how squirrel nests, made up largely of leaf‑bearing twigs nipped off the tree during spring and summer, will hold their leaves for years at a time. This is because the cork layer never had the time to form so the leaves remain attached.

Other stories

Leaves have many other why questions to answer and stories to tell. To think like Darwin, let’s consider other challenges a leaf faces. These include being eaten, over-heating, drying out, being blown off the twig, receiving enough sunlight – to name a few.

You may, for example, wonder why so many different shapes and sizes have evolved. Scientists have discovered that toothed or lobed leaf margins (e.g., toothed in elms, lobed in oaks) are an adaptation that allow leaves to more quickly rid themselves of absorbed heat. If heat release is not a problem, as with plants like hostas that grow in shady habitats, the margins are “entire”, which means they are even and smooth all the way around. Almost all leaves, however, come to a sharp point – often at the tip – which is an adaptation to shedding water.

Darwin no doubt wondered why some leaves are “simple” like those of a maple or compound like those of a sumac or walnut. To tell if a leaf is compound, look at where the petiole (leaf stem) is attached to the twig (usually a different colour and woody). You should be able to see a bud. A simple leaf has a petiole and one blade. A compound leaf has an elongated petiole with three or more leaflets (blades) coming off it. Each leaflet looks like a separate leaf, but there is no bud at the base of the leaflet’s stem (petiolule) – only where the main petiole is attached to the twig.

Why would natural selection sometimes favour compound leaves? First, they provide lots of surface area for photosynthesis – sumacs can have 31 leaflets – but still allow wind and rain to largely pass through them. Imagine what would happen to a huge simple leaf in a storm! In addition, compound leaves don’t heat up so much because air circulates around the leaflets. These advantages may explain why compound leaves are so common in the tropics.

Compound leaf of ash (left) and simple leaf of sugar maple Note tiny bud where stem meets the twig - Photo by Drew Monkman

Compound leaf of ash (left) and simple leaf of sugar maple Note tiny bud where stem meets the twig – Photo by Drew Monkman

The overall size of leaves is not a matter of chance, either. Leaves tend to be largest on plants that grow in shaded areas – think of the size of Hosta leaves – and on the lower, more shaded branches of trees such as oaks. Leaves at the top of a tree tend to smaller. Larger leaves, of course, gather more light.

Leaf thickness, texture and hairiness are also interesting. Hairs and even spines on leaves have evolved to make them less appetizing to herbivores like caterpillars and deer. Hairs can also protect delicate growing parts from the cold. You often see them on early-spring species like hepatica and arugula. Thick and waxy leaves – think of conifer needles and the leaves of English holly – suffer less water loss, which means the tree doesn’t need to shed them in the winter. They are also common in hot, dry environments.

Take time to smell leaves and to ponder the question of why some leaves are so aromatic. Although certain leaves might smell good to us – wintergreen and bergamot, for example – it’s quite likely that the chemical compounds responsible for the smell are poisonous or taste bad to leaf-munching herbivores!

Leaf collection

Why not take some time this fall to really get to know the leaves of our common broad-leaved trees? One way is to make a collection, either by yourself or with your children, grandchildren or students. Place the leaves between sheets of newspaper with heavy books on top. Leave for a week or so. When the leaves have dried out, you may wish to place them between two sheets of clear, adhesive contact paper for greater protection. Using one or two sheets of Bristol board, group the leaves by colour, by genus (e.g., all the maples together) or by simple and compound. A basic collection for the Kawarthas would include simple leaves like sugar maple, red maple, silver maple, red oak, white birch, American elm, trembling aspen, American basswood, chokecherry and willow. As for compound leaves, try to find white or green ash, staghorn sumac, Manitoba maple, black walnut, Virginia creeper and black or honey locust.

Fall is a wonderful time to explore and celebrate nature, especially through the lens of why questions. Nothing in this Universe is more magical or awe-inspiring than reality!

Nov 062014
 

We don’t often think of November as a particularly interesting month. For many, it’s simply the grey, damp interval between the dazzling leaves of October and the swirl of lights and decorations of December. For me, however, late fall has always been a favourite time of year. I love the change of pace and the sense of nature slowing down. Most migratory birds have departed and insect activity has been reduced to a few hardy moths and dragonflies. Gone are the dramatic day-to-day changes in leaf colour that enlivened the landscape just a few short weeks ago. With the veil of foliage now lifted, what stands out are nature’s fundamentals – sky, water, soil, rock and tree

Common Raven - Wikimedia

Common Raven – Wikimedia

With these November thoughts in mind, I took advantage of last Sunday’s beautiful weather to go for a long walk. And what better place to explore the outdoors than the Shield country of Big Gull Lake, where my wife and I were spending the weekend. It’s such a pleasure at this time of year to walk freely, unencumbered by summer’s heat and insects or winter’s snow and ice. The cool, invigorating air infuses your every step with new-found energy. But rather than focus on exercise or species’ identification, I decided to concentrate on walking mindfully and being fully aware of the simple pleasures of sight, sound and smell.

Sound
Other than the rhythm of my own footsteps on the dirt road, I was surrounded by what seemed like total silence. There was no rattle or buzz of insects and no singing birds. Man-made sounds, too, were absent. No chain saws, no ATVs, and no outboard motors. But I soon realized that this November day did indeed have a voice; its voice was the wind. And it wasn’t one wind but many: the roar of high-altitude gusts that sent clouds scurrying across the sky, the murmur of softer breezes in the pine boughs overhead and especially the constant crackle and rustle of air rushing through the dry leaves. About the only other sounds that came my way were the croaking of a distant raven, the explosion of wings of a startled Ruffed Grouse and the loud scolding of a disgruntled Red Squirrel.

Oaks are among the last trees to lose their leaves in fall - Drew Monkman

Oaks are among the last trees to lose their leaves in fall – Drew Monkman

As much as I love the soundscape of the natural world, I also take pleasure in the absence of sound. The stillness of late fall somehow makes us more aware of the ancient rhythms of the land as nature settles down for winter. How rare it is anymore to ever enjoy real quiet, removed from the cacophony of man-made noise to which we’ve become so fully habituated. As Sigurd Olson, an American writer and conservationist once said: “In the end we turn to nature in a frenzied chaotic world to find silence, oneness, wholeness, and spiritual release.”

Sight
On such a spectacular sunny day, I couldn’t help but notice the special quality of the light. November light is different. It is not the harsh, mid-day glare of summer that blazes down upon us from straight above. November light comes to us aslant, casting long shadows in its course. A quick glance at the sun explains why. Even at noon, the late-fall sun is little more than half way between the horizon and the sky’s zenith.

 Cottage road in November. Note the long shadows, even at noon - D. Monkman


Cottage road in November. Note the long shadows, even at noon – D. Monkman

Even though I’ve walked this road countless times, late fall always surprises me with new vistas and natural features I’ve never noticed before. With most of the leaves now fallen, the outline of distant hills and valleys seemed somehow different. Even the roadside woods appeared bigger and invited exploration. Where only recently there was a curtain of greenery, I could now make out granite ridges, scattered erratics (huge stones left behind by the glaciers), newly fallen trees and even some woodland ponds that I’d never seen before. But more than anything, the forest interior was mostly a play of dark tree trunks, long shadows and fallen leaves. What really caught my attention, however, was how the angled November sunshine magnified the oak leaves’ natural glaze and made them sparkle like diamonds. Even the individual leaves on the road’s surface blazed like tiny flares.
Scattered about the forest floor, I also noticed numerous oases of greenery. Some were patches of green moss on a rock or log, but there were also clumps of evergreen ferns like Wood Fern and Rock Polypody, club-mosses such as Ground-pine and Ground-cedar, and evergreen wildflowers like Hepatica and Wintergreen. It was almost as if these plants were saying: “We don’t want anything to do with this change of season!”

True to its name, Wintergreen stays green all winter - Drew Monkman

True to its name, Wintergreen stays green all winter – Drew Monkman

I was also reminded of just how common oaks are in the Kaladar area. Since most of the oaks still retained some of their leaves, they stood out clearly against the other deciduous trees, most of which were bare. Conifers, too, were more conspicuous than usual. Looking towards the horizon, I could see numerous White Pines towering above the other trees and clearly displaying their iconic wing-like branches. Along the edges of wetlands, spire-shaped Balsam Firs, columnar White Spruce and smoky-gold Tamaracks were all easy to spot.
The roadside, too, provided items of interest, including a few remnants of fall colour. A small display of pinks, reds and burgundies came courtesy of the remaining leaves on the Maple-leaved Viburnums, Common Blackberries and the seedlings of Red Oak. I also noticed a handful of asters still clinging to their mauve petals. Much more common, though, was the huge variety of seeds such as those of the milkweed spilling from half-open pods and the fluffy grey seed heads of the goldenrods. The promise of spring wasn’t hard to find, either. The numerous young maples and cherries growing along the edge of the road were covered with fully-formed buds, just waiting for April rains and warmth to swell and open.

Smell

In much the same way as November is not a time of sounds, nor is it a time of smells. The cool temperatures of this particular morning had even neutralized the lovely fragrance of the fallen leaves. As I returned to the cottage, however, the faint smell of wood smoke immediately caught my attention and had me looking forward to the warmth of the fire and a cup of coffee.
When we arrived back in Peterborough in late afternoon, it was already getting dark. With the return to Standard time, dusk comes early at this time of year. But short days, too, are something I love. In my mind’s eye, I see the northern hemisphere tilted sharply away from the sun. I imagine the sun’s rays striking our part of the world diagonally, scattering themselves over a much larger area than in summer and creating far less warming. Short, cool and reduced to nature’s fundamentals, these November days are no less than a palpable reminder of the change of season – and seasonal change is what makes living in the Kawarthas so wonderful.

The smell of a woodstove

The smell of a woodstove