Strange shapes and fascinating adaptations at the treeline
By James Raffan
AT THE SO-CALLED treeline, one doesn't walk out of the forest onto the open tundra, turn around and look back at a line of evergreens. For the treeline is not a line at all, but a zone.
Trees at the northern edge of our 2.5 million sq. km. boreal forest, or taiga, grow slower and smaller, eventually taking up the form of ground-hugging creepers or bushes. These fascinating adaptions to marginal conditions have caused scientists to debate the question, "When is a tree not a tree?"
Some argue that tundra begins where trees in "tree form" do not grow, while others contend that the treeline includes all evolutions of northern trees regardless of their form. So, when is a tree not a tree? "Tree-form" people would say a tree isn't a tree when it lies on the ground. "Tree-species" proponents claim a tree is always a tree no matter what its shape.
Regardless of the definition, the treeline is best considered as a transition zone, not a precise "line", between the forest and the tundra. At no point can you connect up individual trees and join them to form a line.
Why then can we look at almost any map of Canada and see a "treeline" drawn across the country? The answer is that it is an approximation. Give or take a few dozen kilometres, the northern edge of the forest follows a general course across the nation and, for that matter, around the world.
Why do trees change their form at the treeline, and why are they eventually supplanted by tundra plants? How is it that trees will grow at 68┬░ N near Inuvik in the Mackenzie Delta, but not beyond 59┬░ N near Hudson Bay? To answer these questions, we must look at northern growing conditions.
Arctic conditions are harsh. Winters are cold, dry and long. Storms cross the region but the cold air is usually too dry to form effective snow-producing clouds; as a result, snowfall is light.
Come May, when temperatures rise above freezing, only the top layer of the frozen ground thaws. This active layer, which varies in depth depending on the nature of the soil, is generally wet, poorly ventilated and acidic, far from ideal for growing. Most of the treeline zone occurs along the southern limit of continuous permafrost.
During the short summer, days are cool despite more hours of sunlight. Under these conditions growth is slow because, like all chemical processes, the rate of plant growth depends on temperature. Soil bacteria also suffer from inadequate heat and cannot break down organic matter as quickly as they do in warmer conditions. This is especially true of microorganisms that capture atmospheric nitrogen and make it useful to plants. In areas of the North where animal wastes add nitrogen and other essential nutrients to impoverished soil, healthier vegetative growth is seen, especially around nesting sites, and boulders used as owl perches or around human settlements.
As for moisture, the treeline is at the edge of a virtual desert. Much of Canada beyond 60┬░ N gets less than 25 cm. (10 in.) of precipitation annually; nearly all of it falls in July and August.
Because of the many factors which inhibit their growth -- cold temperatures, short growing season, poor soil, permafrost and water shortage -- black spruce, tamarack, willows and dwarf birches on the treeline have evolved special adaptations to survive.
To cope with strong drying winds, black spruce generally grow in clusters. A fine covering of hair on black spruce twigs -- a charcteristic that distinguishes it from its southern cousin, the white spruce -- provides a layer of dead air around the tree that reduces transpirational water loss. The needles of northern conifers are shorter and thicker than their counterparts elsewhere, another water-saving adaptation.
In particularly exposed places, the tree branches nearest the ground grow disproportionately larger than higher branches. Although wind-driven snow damages the higher tree branches (at -50┬░ C, ice has a hardness approaching that of quartz), there are other explanations for this vigorous growth near the ground. Because of the absorption of heat by vegetation, the temperature of surface soil and air surrounding the lower branches may be 10┬░-to-20┬░ C warmer than the air above. So, while air temperatures may fall below freezing, temperature near ground level may be warm enough for growth. In extreme cases a spindly trunk with a tiny green topknot protrudes from a dense thicket of lower branches -- appearing from the distance like a northern golf green complete with pin!
The expansive "golf green " mat of lower branches may be an adaptation to cope with wind and cold, but the branchless tree trunk reflects a zoological factor occurring at the treeline. It is here, at least in the eastern Northwest Territories, that the range of the muskox begins. In summer, when moulting hair must be shed, the animals crave scratching posts. Muskox often strip healthy spruce trees of all their branches with vigorous rubbing of their hindquarters, an exercise that leaves a well-trodden mat of branches on the ground and a naked trunk pointing to the sky.
Trees have also made internal adaptations to northern cold. Spruce can withstand occasional summer freezing without ill effect. Concentrations of sugar or resins in the sap appear to work as antifreeze in the watery juices of the tree, allowing growth to start earlier in the season and to continue later. It has also been shown that treeline species have lower optimum temperatures for photosynthesis -- the use of light, water and carbon dioxide to produce sugars, wood and oxygen -- and can use light more efficiently than southern species.
Another adaptation to northern living is a reproductive phenomenon called "layering". Normally, coniferous trees must complete a cycle of sexual reproduction where male pollen fertilizes the ovules in the female cones to produce seeds. When the growing season is too short or too cold to complete this cycle, it is an advantage for a species to be able to reproduce asexually -- that is, without seeding. In layering, the lower branches of the tree take root, becoming new trees without developing from seed. Sometimes this is the only way that the black spruce can reproduce, a characteristic that sets it apart from its boreal neighbours.
A variation of layering occurs when the trunk and crown of an older tree dies, leaving behind a well-established root system upon which new trees can grow from the old lower branches. This method gives the new stem the physical and nutritional support of the old tree.
There are other species -- notably tamarack -- that grow to the edge of the tundra. The tamarack has many of the same adaptations as the black spruce, as well as a deeper and more extensive root system. As a result, it may grow alone, without a windbreak from a stand of neighbours. Thanks to some other adaptive strategies, tamarack on or near the treeline often live three and four times as long as the average spruce; it is not unusual to find a tamarack 300 to 400 years old. Such a tree takes on an almost sacred quality with the realization that it was growing long before explorers such as Hearne, Mackenzie and Franklin first set foot on northern soil. Even more startling are reports of trailing stems of juniper and lapland rhododendron that are just as old and yet no thicker than a person's thumb.
With all their adaptations to the North, why do trees eventually give way to the tundra? And why are there trees at the Arctic Circle in the Yukon but not at the Circle near Repulse Bay in the eastern Arctic?
The answers to these questions are related to temperature and elevation. It gets colder as you climb a mountain and it gets colder as you go farther north, so the treeline trends with latitude and altitude temperatures. In fact, the 10┬░ C (50┬░ F) isotherm for July conforms to the treeline rather closely. (An isotherm is a line on a map connecting places having the same mean temperature.)
Continental land mass, topography, water, air masses and wind patterns affect temperature. And because of these influences, the isotherms extend north in the West, dip south around Hudson Bay, and are found north again at the head of Ungava Bay. Lacking more complete field data about the treeline, we are left to assume that, though there are other contributing factors, ultimately it is temperature that permits tree growth at the Arctic Circle in the West but not in the East.
The treeline is also a boundary for other kinds of life. To most land mammals, it represents the outer limit of environmental safety, with the exception of those whose adaptations to harsh tundra conditions allow them to live north of the treeline year-round. Of the summer migrants to the tundra, few can stay for long; even the barren-ground caribou returns south to the forest in winter for food and shelter.
As for humans, the treeline marks the limit beyond which special survival skills and equipment are required.
