Oct 182019
 

Still lots to look forward to in the parade of fall colours

From May’s gentle pastels and summer’s kaleidoscope of greens to early fall’s dazzling reds and oranges, each time of year has its signature colours. Now, as we move into the second half of October, yellow is taking over centre stage.

Once the red and sugar maples have shed their leaves, the main show belongs to the aspens, poplars, birch, tamaracks, and oaks. Only a matter of days ago, most of these species were simply part of the green blur, but they’ll soon stand out like yellow beacons on the landscape.

The trembling aspen is the most widely distributed tree in North America and one of my favourite species. The round, heart‑shaped leaves change from lime green in spring, to emerald in summer, and finally to lemon-yellow in fall. Not only are the leaves beautiful to look at, but they also produce their own soothing music. Their flattened leaf stem allows them to quiver at the slightest breeze, hence the name “trembling”. Why the aspen has evolved such flexible leaves is still a matter of speculation, but it may be to help protect the tree from strong winds by all allowing the wind’s energy to pass through the canopy more or less uninterrupted.

Although somewhat less common, the bigtooth aspen is an equally attractive tree. The leaves are larger than those of trembling aspen, with curved teeth on the margins. While most of the leaves do become bright yellow in the fall, some acquire rich shades of orange.

Balsam poplar, a tree of moist, low-lying habitats, also turns various hues of yellow in October. Almost as widespread as trembling aspen, balsam poplar has resinous, fragrant buds that perfume the spring air. It is the smell of May in the Kawarthas. The buds also possess medicinal qualities and exude a resin that is used to make balm of Gilead. Besides smelling wonderful, the balm is said to have anti-inflammatory and analgesic qualities.

By month’s end, tamaracks reach their colour zenith. Lime-green in early spring and smoky gold by late October, tamaracks are one of our most beautiful and interesting trees. They are the only conifer to lose all its needles in the fall. As the great American conservationist, Aldo Leopold, wrote so eloquently, the ground becomes “dusted with tamarack gold.” Our other conifers, such as pines and cedars, shed only a portion of their leaves each autumn. That is why some of their foliage becomes yellow or orange at this time of year before falling.

Preparation for winter

Colour change and the shedding of leaves are manifestations of a 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 no longer able to absorb water through their roots. Because leaves are continually releasing water vapour – think of the high humidity of a greenhouse – trees must get rid of their leaves in order to minimize water loss and avoid death through desiccation.

However, leaves are full of important, but scarce minerals, and it is to the tree’s advantage to salvage these nutrients first. These same minerals are used to produce chlorophyll, the green pigment that captures the sun’s energy and uses it in combination with water and carbon dioxide to produce the sugar‑based substances that make up all of the tree’s tissues – wood, bark, leaves, flowers, and fruit. As the amount of daylight decreases in late summer and fall, trees stop producing chlorophyll and begin to remove the minerals from the leaves in order to store them in the woody tissues until next spring. This same response occurs in times of drought.

As the chlorophyll disappears, colour change becomes apparent. Without green chlorophyll to mask the other colour pigments in the leaves – most of which have been there all along ‑ these pigments gradually become visible. The yellows and oranges come from carotene pigments, while anthocyanins give us the beautiful reds. The red pigments are created from excess sugars and seem to be brightest when there is lots of fall sunshine accompanied by cool nights such as this year.

The actual shedding of the leaves is achieved by the formation of a corky layer of cells at the base of each leaf stalk. 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 the leaves for years at a time. This is because the cork layer never had the time to form.

The story of October’s yellows would not be complete without mentioning our non-native trees. Norway maple, a species native to Eurasia, has become one of our most common urban trees. Its purple-leaved cultivar is especially popular. Of all the maples, the Norway is the last to change colour. The trees remain green until mid- to late October, before turning various shades of yellow. Some trees usually still have leaves on Remembrance Day.

In late October through early November, you get a real sense of just how ubiquitous Norway maples are. Along with other non-native trees like weeping willow and European buckthorn, they stand out conspicuously at a time when most native trees and shrubs have lost their leaves. To me, these trees stick out like soar thumbs in late fall and, not really belonging here, take away from our sense of place.

By November, the only native deciduous trees that still retain leaves are the oaks and some silver maples. Red oaks, usually dressed in brownish-orange leaves, stand out in particular. This makes it easy to see how common oaks are in many areas, especially on the Canadian Shield. Some oaks, along with young American beech, sugar maple, and ironwood (hop-hornbeam) retain a portion of their leaves all winter.

Seed and fruit

In addition to the colour parade of the fall leaves, there’s another interesting phenomenon happening this year. Anyone paying attention to our trees, shrubs and vines has no doubt noticed the abundance of seed, be it in the form of fruits, berries, nuts, or cones. Especially noticeable is the seed crop on sugar maples, American beech, mountain-ash, apple trees, white spruce, and wild grape.

Not surprisingly, the amount of seed produced in a given year is an adaptation to assure that the plant’s genes are successfully transferred to a new generation. We may not think of trees as having survival strategies, but millions of years of evolution have fine-tuned plants as much as animals to survive in a rough and tumble world.

In the boom or bust cycle of seed production, the prodigious quantities that we see some years is known as masting. The exact mechanisms are still unclear, but some researchers believe that trees may have biological clocks that are somehow synchronized and pre-programmed to mast at opportune times. It may also be that environmental cues such as wide-ranging climate conditions trigger the masting phenomenon.

Masting may also be a clever adaptation for survival. Let’s look at the example of oaks.  For several years in a row, oaks will produce no or very few acorns. This has the effect of greatly reducing the number of acorn-eating herbivores because, with little or no acorns, many will die, leave the area, or simply have fewer young. Then, with the herbivore population knocked down a few notches, something amazing happens: the trees suddenly produce a giant acorn crop. Those herbivores that are still around quickly become satiated and can eat no more. In this way, a significant number of acorns – or, depending on the species of tree, any other type of seed – will survive to grow into seedlings.

Masting has important ecological effects, too, as the food chain becomes distorted by so much food available. For example, the abundance of seeds resulting from a masting year in conifers allows seed-eating birds such as crossbills to lay more eggs than usual and to raise more young. However, in a low seed year following masting, the larger than usual number of seed-eating birds must migrate elsewhere to avoid starving.

As for this year, the abundance of wild grape will almost certainly mean that large numbers of robins will spend the winter in the Kawarthas. This is what happened just two years ago. Why migrate further south when all the food you need is here?

What’s happening this week

Golden eagles migrate south through the Kawarthas from mid-October to early November, along with large numbers of red-shouldered and red-tailed hawks. These raptors are best seen from a height of land where a large swath of sky is visible. Last year, a group of us saw several golden eagles and numerous hawks from a ridge just east of Petroglyph Provincial Park.

Climate Crisis News 

As we head to the polls on Monday, let’s remember the importance of keeping the climate crisis front and centre in our minds. A recent article in Maclean’s by Canadian climate scientist, Kathyrn Hayhoe, and economist, Andrew Leach, graded each party’s climate plan. They give the Liberal’s plan a B for ambition and an A for feasibility; the NDP’s plan an A for ambition and a D for feasibility; the Green’s plan an A+ for ambition and a C- for feasibility; and the Conservative’s plan a D for ambition and a F for feasibility. The high marks given by experts to the Liberal plan have made my decision much easier.