Climate of the Boreal Forest 


The climate of the boreal forest is characterized by strong seasonal variation with short, moderately warm and moist summers and long, extremely cold and dry winters.

Temperature range is extreme, especially in the mid-continental areas, where seasonal fluctuations can be as great as 100°C.

The traditional calendar seasons, as defined by the path of the sun in the sky and the passage of the equinoxes and solstices must be reconsidered in the boreal forest. Many northern cultures identify six seasons rather than four. Each period poses distinctly different situations for people and wildlife. Break-up and freeze-up of waterways are seasons in themselves, added to spring, summer, autumn and winter.

The extent of the boreal forest from south to north is determined mostly by climate particularly the position of Arctic and other air masses throughout the year as well as by the net amount of solar energy received at the surface.

The boreal forest or taiga is one of the four major biomes of the world. A biome is a major , regional ecological community characterized by its dominant plant community - the location and nature of which is determined by the components of climate.

Temperatures
Since the Boreal forest is such a wide band there is a great variation in temperature from its southern to northern extremes.

On a hot summer day, typically in mid-July, temperatures in the low 30°s C can be reached but many consecutive days this warm is a rare occurrence. Summers are short. An average daytime high is around 15° to 20°C Spring and autumn temperatures range from overnight lows -5° to daytime highs of +10° C. This is typical for April ,May, September and October. The coldest month of the year is January. Temperatures are routinely around -20°C. In the depths of the long winter the thermometer can plunge to -40°C for days at a time.

Growing Season
The frost-free season is the period normally free of temperatures below freezing, starting with the last frost in Spring, ending with first frost in Fall.

At the southern edge of the boreal forest there are about 100 frost-free days but at the northern tip near Thompson, Manitoba, a period as short as 38 frost-free days has been recorded.

Another reflection of the climatic gradient from south to north is illustrated in the annual growth rings (radial growth) of Black Spruce and White Spruce. In central Canada the ring thickness sharply declines as the northern tree limit (hence the Arctic front) is approached. As well, the occurrence of Black Spruce communities is positively correlated with the frequency of the Arctic air mass and White Spruce communities show the converse relationship.

Some scientists suggesting that the relationship between air mass position and vegetation is significant only to the degree that the air mass characteristics determine the characteristics of the local energy budget during the growing season.

Energy At A Premium
In the boreal forest," the statement 'The sun is the source of all life' is no idle cliché but a basic ecological fact. The energy that reaches the surface in the boreal is far less than that which reaches the tropics. In travelling through a longer column of air, various wavelengths are filtered out and the total amount of energy reaching the earth's surface is reduced by reflection and refraction from water and dust.

As well, northern areas receive energy at a lower angle of incidence than equatorial or temperate areas. When the sun is near the horizon (8°) 50% of the energy is received by diffuse radiation as compared to 92% reception of direct radiation at 90°.

Another factor to consider is that the incoming energy in the boreal biome varies greatly from summer to winter as a consequence of the earth's is tilt on its axis.

In winter, when the sun is below the horizon for long periods, energy received flows back into space through radiative loss. A thin cover of cloud significantly reduces the loss but on cloudless nights, effective radiative temperature can fluctuate widely compared to diurnal air temperature fluctuations.

If you think not only in terms of air temperature but also of the amount "of heat flowing from a source to a receiver or accumulating body, then many aspects of boreal ecology are easier to understand."

Precipitation
Rainfall
Precipitation can differ considerably across this immense landscape. Generally, the climate is moister in the southern regions and becomes drier further north.

Most of the precipitation in the region falls as rain during the summer and is relatively light. In the northwest, where high mountain ranges restrict the inland penetration of moisture-laden air, annual rainfall is 18-38 cm. In central Canada, the annual rainfall increases to 38-51 cm. The eastern part of the boreal forest in Canada receives between 51 and 89 cm of precipitation as rain.

Snow
Snow links all parts of the boreal forest as much as does a shared vegetation type. For approximately half of the year, a blanket of snow covers this forest. Yet winter is the least studied and understood season of all.

The roles snow plays in the ecosystem are so diverse and intense that it has prompted one scientist to say,

....."one might accurately say that boreal ecology is the study of snow....it protects ground plants from low temperatures and from dessication; it accumulates on trees and may break them or influence their shape; it protects some animals from low temperatures and predation; it hinders other animals in their movements and food-procuring activities."

Snow crystals form around a condensation nuclei (possibly a dust particle) when water vapour is cooled below freezing. If the air contains an excess supply of water vapour (supersaturated air) the crystals begin to grow. When they reach a critical size they fall, adhering one another as they descend they form clusters of snowflakes. The shape of the snow crystals which reach the earth depends upon temperature, humidity and wind.

Once on the ground, snow undergoes a metamorphosis. Snow cover can be considered a mixture of ice and air, with the amount of air much greater than the amount of ice in most cases. Understanding the changes in these proportions and the changes in the structure of the snow cover is critical to understanding much of boreal ecology.

Snow crystals on the surface of the snow cover may be rolled by the wind. The fragile arms broken, the are crystals 'packed' in a snow drift, increasing density of the snowcover. Temperatures above zero or the stronger, late-winter sun may melt the snow surface. When the temperature drops a night the surface refreezes forming an ice-crust.

A more significant type of metamorphosis takes place throughout the winter at the bottom of the snow cover. Heat and moisture continue escaping from the ground rise and the crystals at the base are transformed to form a fine lattice of interconnected 'beams'. This is a 'constructive metamorphosis' which eventually alters the original crystals completely altered adding to the lattice. The result is a lattice-work frame of walls and ceiling around large spaces, all hidden beneath the layers of subsequent snowfall.

This new base layer of coarse snow near the ground is called "depth hoar" or "pukak". The term "pukak" is borrowed from an far northern aboriginal language. The structure of 'pukak' makes it easy for small mammals to excavate tunnels a the base of the snowpack. Once the snowcover has reached approximately 15cm it acts as an insulative blanket for the voles, mice, lemmings and shrews residing in this 'sub-nivian' (beneath the snow) environment. They are critical prey species for predators such as foxes and owls. Without the pukak layer to support small mammal populations the predator-prey ecology of the boreal forest would differ dramatically.

The predator and prey inhabit two distinctly separate worlds. While the fox trails the forest above the snow and copes with temperatures that fluctuate from freezing to -50°C, the vole lives in a dark, moist environment beneath knee-deep snow in which temperatures are consistently only a few degrees below freezing.

Northern peoples have many descriptive terms for snow, 'pukak' is only one. Scientists studying the boreal forest have borrowed from these languages. For instance 'qali' is the 'snow that alights and sits on the trees'. 'Siquq' is an 'ice crust'.

The insulating blanket of snow is the voles ally against the boreal deep freeze but its not enough to keep it from a fox's jaws throughout the winter. So keen is fox hearing, that even through the snowpack, it can locate and capture its prey.

In years when ice crusts and extremely deep snow prevail, the fox's hunting success is reduced. This can be reflected as smaller fox litters in the spring or even the death of adult foxes before winter's end. The fox-vole connection through the snow cover is another marvellous boreal forest pattern.

The barred owl's prey, such meadow voles, red­backed voles and red squirrels also seek refuge from the winter cold and their predators by living or escaping beneath the insulating blanket of snow.

Recently, it's been learned that the Barred Owl uses a hunting technique called 'plunging' when catching small mammals beneath the snow.The owl swoops down from a branch when it hear a vole under the snow and 'plunges' in for the catch. Snow plunging requires an unobstructed path from a perch to the snow. The more open forest floor found in mature forests would permit this hunting technique.