Next time you take a breath, be sure to thank a tree. From coast to coast, trees are a source of national pride for us Canadians. In 1982 the Japanese adopted a concept called Shinrin-yoku (“forest bathing”) that concentrates on breathing fresh air on a hike through forests. In its essence it means taking in the forest atmosphere, to “bathe” one’s mind and soul with the trees to improve health and overall well-being. And why not? Trees are true life-givers.
What is so likeable about trees? One possible answer is the multiplicity of benefits that trees provide. Trees provide cooling, shade and wind reduction, as well as playgrounds. They preserve water, absorb air pollution through carbon storage, and give us back oxygen in return. They buffet noise and filter dust. A healthy tree can reduce air-borne dust by as much as 7,000 particles per litre of air; a healthy tree is a free-standing air conditioner and purifier. Many trees also give us edible fruit. Trees attenuate storm water and provide food and shelter for wildlife. The annual fall of leaf litter enriches the soil and restores nutrients removed by the tree’s roots. Trees help stabilize the soil and restrain erosion. And even after trees die, their usefulness continues – as seed logs, host plants and habitat providers, returning nutrients to the soil and supporting fungi. Felled trees give us lumber, sheet board and paper.
There are also medicinal, health and economic values. In South America, locals call the forest their pharmacy. A recent urban tree study by TD Economics claims that the City of Toronto urban forests are worth an estimated $7 billion, or $700 per mature tree. One large tree provides a day’s worth of oxygen for four people. One ton of carbon dioxide is absorbed by 98 trees every year. The study also claims that for every dollar spent on tree maintenance the urban forests return several dollars worth of benefits. Considering that there are 116 tree and shrub species in the Toronto urban forest that covers 30% of the city, those figures are awesome. The point is, says Mark Cullen, that we need to learn to live with trees and that we need more of them. As a species we will not survive without trees. Remember Easter Island!
Trees are land plants with woody stems that have the power to grow steadily larger over the spans of years. The record height is held by a Montezuma Cypress in Mexico – 117 m high, with a 35.8 m girth at 1.50 m above ground level.
A tree has four lives: seed, living tree, dead tree (snag) and fallen tree. It starts life as a seedling and may end up as a century-old giant of the forest. In the great Redwoods of Oregon and California on the Pacific coast, the world’s tallest (up to 116 m) and oldest (up to 2,200 years) trees, more than 4,000 species live in or on a downed log, while 1,700 species of plants and animals depend on a single tree during its life span. These Redwoods are an example of a perfect recycling system. The soil contains few nutrients and all life-sustaining substances are in the trees themselves.
Trees do not bear flowers during their first few years of life as seedlings. Many kinds postpone flowering until they are more than 20 years old. On some trees (wild cherry and laburnum) the flowers follow the familiar pattern of common garden plants. Many trees are wind-pollinated. These typically bear catkins of male flowers set apart from female catkins. Usually both sexes of catkins grow on the same tree, but poplars and willows bear catkins of one sex only.
The secret of a tree’s continued growth lies partly in the winter-resting buds and the tips of twigs, which enable the shoots to grow longer each year, and partly in a sheath of tissue called the cambium. This sheath is only one cell thick, surrounds every woody stem, and extends to the underground roots. During the growing season it adds a fresh layer of woody tissue to the stem it surrounds by throwing off new cells on its inward side, increasing girth.
There are no fixed limits to the size or lifespan of a tree. Ages exceeding 100 years are frequent. Oaks can live 500 years, some yews 1,000 years, and some bristle cone pines in the Arizona desert are 4,000 years old. Unless a tree is felled for timber, its life is usually ended through internal decay caused by fungus slowly spreading through the trunk. Eventually the weakened stem collapses and the tree slowly rots away on the forest floor, still providing shelter for critters and nutrients for the soil. On exposed sites, even healthy trees are often blown over by strong winds in so-called blowdowns. Balsam Firs that have been attacked by Budworms are especially prone to blowdowns.
Like other plants, trees need a constant supply of water to live. This is drawn from the soil by a huge network of long roots. The root system of a tree is as wide as its canopy. Water flows up the woody zone of each stem as root sap, carrying with it the mineral salts from the soil that are essential to all plant life. When the root sap reaches the leaves it plays a part in the vital biochemical process called carbon fixation. The air penetrating the leaves holds a minute proportion of carbon dioxide gas. In the presence of energy-providing sunlight, the CO2 gas combines with water to form carbohydrates in a process called photosynthesis. This process occurs with the aid of that marvellous chemical substance called chlorophyll, which is present in all the green parts of the tree.
In their natural state, trees rarely stand alone. A forest is made up of individual trees, lots of them, as trees form natural plant communities. In most natural woodlands, trees of varied sizes and ages, as well as different species, commonly grow together and compete for living space. With a dense canopy, new saplings often don’t have enough sunlight to grow.
That is where forest fires and controlled burns play an important role in forest propagation. Indigenous peoples used controlled burns to help with agriculture and improve ease of travel. In grasslands, fire stimulates the growth of native grasses by targeting invasive plants and reducing the woody understory. This allows more sunlight to reach the plants at ground level. Jack Pine and White Birch are the first species to grow after a fire. A ring of birches growing around White Pine stumps are an example of natural forest succession. Fire kills White Pine trunks but not the roots. Jack Pine cones only open at fire temperatures. Some native species, such as Black Oak, are virtually unaffected by fire due to their thick bark. After a prescribed burn, many species return and become more prominent again.
Oaks are among the most impressive and beloved trees. They are the largest member of the beech tree family. The genus name Quercus means “a tree above all others.” There are more than 500 oak species worldwide, but only 11 are found in Canada. Red Oaks grow from Lake Superior to Nova Scotia. Their leaves have bristle-tipped lobes. White Oaks grow in deciduous forests of southern Ontario and Quebec and have leaves with rounded lobes. It takes two years for Red Oak acorns to mature and one year for acorns of White Oak. The acorns feed wildlife; deer often graze on young shoots when other food is scarce.
Oaks can live for several hundred years. Tall grass prairie and oak savannah were once found throughout east-central United States and in southern Ontario. Today, less than one percent of this ecosystem remains. There is still a small patch in the Don Valley and a few spots remain on the Oak Ridges Moraine. The Rice Lake Plains, located on the eastern flanks of the Oak Ridges Moraine, are home to the most promising remnants of oak savannah and tall grass prairie habitat in Canada.
There is our own boreal forest, one of the largest ecosystems mostly still left intact. It is inhabited by wolves, moose, lynx, martens, frogs and salamanders in numerous wetlands that filter millions of litres of water every day, and by insects and half of the birds of North America. The boreal forest covers one-third of Canada’s land mass – a treasure to be cherished.
With all this, what is there not to like about trees? Well, roots of large trees can damage buried conduits and cables. Tree crowns can be a hazard for overhead wires and obstruct sightlines. Trees are susceptible to diseases. Examples are the Dutch Elm disease and tree-killing pests like Pine Beetles, Budworms and the Emerald Ash Borer. The roots of walnut trees carry a toxic that prevents anything else from growing around the trunk. And then there are the invasive species. In Toronto we have 25 trees, 23 shrubs and 47 groundcover species that are non-indigenous. The worst offenders are Norway Maples, Trees of Heaven, Buckthorn, Japanese Knotweed, purple loosestrife, dog-strangling vine and garlic mustard.
Look for more marvellous facts about our native forests and trees – maples, oaks, walnut trees, etc. — in a future edition of Nature Notes. In the meantime you may want to look up a few photos and short stories about some of the most remarkable trees on our planet. Here is the link:
To sum up: forests are immensely calming. Spending time in the woods is not only pleasant and important, but also a necessary element of happiness and health. “The woods are a place to connect with nature” says Wendy Cridland, director of conservation, Ontario Region, for the Nature Conservancy of Canada. Fall is the best time for a walk in a forest as the colours of leaves change to brilliant yellow, orange and red; it is like stepping into a glorious painting. It is the time when everything bursts with its last beauty, as if nature had been saving up all year for the grand finale. Hug a tree on your next hike in the Rouge and enjoy, preserve and celebrate. S.G.
Sources: Trees (H.Edlin), The Green Book (S.Gahbauer), Canadian Wildlife Federation, Nature Conservancy of Canada, Urban Tree Study as quoted by Mark Cullen.