Climate Change and the Northern Forests

June 1998

Introduction

The northern boreal forests comprise almost one third of the Earth's forest systems, covering 1.5 billion hectares. Along with the temperate forest of the mid-latitudes, and tropical forest near the equator, it is one of the three great forest ecosy stems of the world, supporting a rich diversity of wildlife, endangered species, and extremely valuable timber.

Unfortunately, over half of the existing boreal forest may disappear, due to the effects of climate change. Over the next 30 to 50 years, atmospheric levels of human-emitted greenhouse gases are expected to double, creating significant changes in the Earth's climate. Conditions may become too severe for boreal forest health and survival of its species, and up to 65 percent of the forest may be lost.

The Changing Climate

The United Nations' Intergovernmental Panel on Climate Change (IPCC), reflecting consensus of 2,500 of the world's leading scientists, recently conveyed its finding that human activities have begun to modify global climate. The global average surface temperature has increased 0.3 to 0.6 degrees C (approximately 1 degree F) over the past century, and IPCC projections indicate that the Earth will warm by 1 to 3.5 degrees C by the year 2100, with a best estimate of 2 degrees C. Even at the low end of these projections, the anticipated rate of warming over the coming years will be, according to the IPCC, "greater than any seen in the past 10,000 years."

A variety of human activities produce greenhouse gases, like carbon dioxide (CO2), nitrous oxide (N20), and hydrofluorocarbons (CFCs, HCFCs, and HFCs), methane (CH4), and sulfur hexafluoride (SF6), increasing their atmospheric concentrations. Acting as a blanket, these human-induced emissions trap the sun's heat and warm the planet. In the U.S., fossil fuel combustion is the most significant cause of greenhouse gas emissions, and has contributed to dramatic increases in CO2 levels. Since pre-indust rial times, atmospheric CO2 concentrations have gone up 30 percent, from 280 parts per million to more than 360 parts per million; in the last 35 years alone, CO2 levels increased over 12 percent. If this trend continues, emissions levels will double by the end of the next century. IPCC scientists clearly warn that humans will continue to drive future climate change, with potentially catastrophic and irreversible consequences, if action is not taken now to reduce greenhouse gases.

The Most Vulnerable Forest

There is general consensus that climatic changes will have the greatest impact on boreal forests; their unique adaptation makes them more sensitive to temperature fluctuations than temperate or even tropical forests. Indeed, fossil pollen and macrof ossil records demonstrate that North American boreal forests expanded and receded in response to temperature changes over the past 10,000 years. Even a slight increase in mean annual temperature is enough to affect many species' growth and regeneration.

Climate change will affect boreal ecosystems in a number of ways: air temperature will increase, rainfall and humidity will change and soils will become drier in some areas. As climate warms, conditions suitable for the growth of many boreal species a re likely to shift dramatically, and the trees' ecological niches may move northward ten times faster than the trees themselves can migrate. Overall, the boreal forest is likely to decrease in area, biomass, and carbon stock, with a significant disruptio n at its southern boundary. Taken all together, these impacts could add up to an alarming 65 percent loss of boreal forests.

Climate Change in the Arctic

To understand how climate change threatens northern boreal forests, it is important to understand where, and how, the impacts will be felt first. On a global scale, climate change will have its first and most severe impact at higher latitudes of the Northern Hemisphere, where tundra, boreal forest and polar desert zones will is projected face the greatest ecological change. The Arctic is expected to warm up by as much as 5 to 10 degrees C, causing widespread changes in evaporation rates, cloudiness,and precipitation.

According to projections, climate change is likely to create unsuitable conditions for many tree species, and boreal forests will be especially hard hit. Because clear-cutting already has devastated so much of our forested areas -- and other land use changes have altered what remains -- most forests now exist as patches and remnants. The few, extensive continuous tracts that remain lie primarily in the boreal region. And they are the most threatened.

Climate Change and Boreal Ecology

The northern boreal forest, also called the taiga, is a vast ecosystem encircling the Northern Hemisphere. The forest covers 11 percent of the world's land surface area, stretching from Siberia to Alaska and through most of Canada, Northern Europe and Northern Asia.

To the north, the boreal forest is bounded by the tundra, a mostly treeless expanse of shrugs, grass and lichen which stretches to the ice packs of the polar desert and the Arctic Ocean. At its southern edge are grassland and remnants of the surviving maple and oak temperate broadleaf forests.

For much of the year, the forest is locked in a cold winter freeze, down to minus 40 degrees C. The short boreal summer brings long, warm periods of daylight that lead to rapid boreal forest growth. Much of the forest consists of stand of evergreen conifers. In North America, the forest changes from pine to spruce and then to fir trees from west to east. Scots pine and Norway spruce are common in Scandinavia while the deciduous larch is dominant in much of the Russian boreal forests. Several less abundant, broad-leaved deciduous species also live in the boreal forest, including aspen, poplar birch, willow and alder.

Small fragments of the boreal forest also thrive in mountainous areas south of the northern system, as in America's Rocky Mountains and the alpine regions of Europe, where they are called "oroboreal," from the Greek word "oro," or mountain.

Forests have several major functions—most importantly they help to regulate the Earth's climate and they are home for millions of indigenous peoples, terrestrial animals and plants. Forests affect both the local climate (providing shade, shelter, prot ection from storm damage) and the global climate. Forests influence the global climate by taking in carbon dioxide and releasing it back into the atmosphere in a process which is not well understood. Under natural circumstances, the carbon stored in for est ecosystems is eventually returned to the atmosphere through decomposition—practices such as clear-cutting disturb this natural process threatening not only entire forest ecosystems, but also the earth's climate.

The boreal forest is a major carbon sink, absorbing an average of between 700 million and 1.3 billion metric tonnes of carbon each year between 1970-1990. This is the equivalent of the net addition of 750 million cubic meters of wood each year. This sink is 13-24 percent of the 5.5 billion tonnes of carbon released annually from the burning of fossil fuels during 1980-1989. By absorbing carbon dioxide the boreal forest is helping to slow the rate of climate change.

Unfortunately, the boreal forest (and carbon sink) is shrinking. The forest reached a maximum extent in 1950-1959 when the boreal forest had a net expansion of 1.25 billion cubic meters of wood each year. The decline since the 1950's is being driven by fire and insect disturbances as well as increased logging. These fire and insect disturbances may themselves be caused by global climate change, leading to a positive feedback loop—human induced climate change caused by the release of greenhouse gases causing more fire and insect outbreaks which releases even more carbon dioxide into the atmosphere.

Boreal Catastrophe: Fires, Insects and Forest Decline

Boreal forests require disturbances such as fires and insect outbreaks for natural reproduction and rejuvenation. However, climate change is likely to dramatically accelerate the intensity and frequency of fires and insect outbreaks, resulting in catas trophic disruption, loss of biodiversity and the release of more greenhouse gases into the atmosphere.

Fire contributes to the overall health and distribution of boreal forests by removing weakened trees and sparking seed release during reproduction. Fire, however, is more common during warm, dry weather, and as climate alters forest environments, the frequency of fire may increase, as well. This threat has already become a reality in North America. Canadian forest fires have been increasing in area burned, with the three highest recorded years this century being 1989, 1995, and 1994. Fire frequency has increased since 1975 in Alaska, as well. In the next century, Canadian researchers predict a 40 to 50 percent increase in area burned in Canada annually, under a doubling of carbon dioxide. There are similar projections for boreal forests in Russia over the next 50 years.

Insects also play a role in boreal ecology--they decompose litter, supply food for birds and small animals, and eliminate diseased trees. But insect attacks are likely to increase in frequency and intensity, as established forest stands succumb to the physiological stress associated with warmer, drier conditions. This could prove deadly to boreal species, because even under stable climate conditions, pest outbreaks usually occur when host species are under stress.

The Budworm

The eastern spruce (Choristoneura fumiferana), western spruce (Choristoneura occidentalis) and black-headed (Acleris gloverana Walsingham) budworms are all members of a family of needle-bud eating caterpillars that are the most dev astating insect pests of the North American boreal forest. Actually the larva of a moth whose favorite food is fir, the spruce budworm will also feed on spruce. The related black-headed budworm feeds on western hemlock and spruce.

Consecutive years of budworm defoliation may cause growth-loss, top kill, and, in severe cases, the death of the host tree. Major budworm outbreaks have occurred at thirty year intervals in eastern North America. The most recent major outbreak, which lasted from 1970 to the mid-1980s caused serious defoliation of 55 million hectares of forest - an area larger than France. The black-headed budworm, native to Alaska, had its last major outbreak there in 1993. Although cool, wet conditions led to a rapi d population crash in 1995, populations increased again in 1997, with important affected areas including the Tongass National Forest and Prince William Sound.

There are numerous reasons to believe that budworm activity would increase under the warmer, dryer conditions expected in a greenhouse future. This would be true even if, as most models project, there is more winter snow. Earlier springs would allow mo re time for winter snow to melt and evaporate, leading to drier summers and increased drought.

Sucrose content in drought-stressed balsam fir almost triples. Higher sucrose levels accelerate budworm growth.

The development rate of the larvae is closely tied to temperature. Feeding rarely occurs at all if the temperature is less than 10 degrees C. Maximum growth occurs at about 26.6 degrees C. One of the major restrictions on the budworm's range may be t hat cool temperatures in the northern boreal forest do not allow the larvae to develop into moths fast enough to escape the autumn freeze.

Female eastern spruce budworm moths can fly up to 600 kilometers to lay their eggs. Flights normally occur in the early evening in July and only occur if the evening temperature is above 18 degrees C. Cool evening temperatures discourage flights and lead to egg laying in a relatively small area of the forest. The hatched larvae may quickly deplete their food supply and the outbreak would then collapse. On the other hand, warm summer nights may cause the outbreak to spread the following year.

The number of eggs laid by a budworm moth is determined by the average temperature. At 25 degrees C. 50 percent more eggs are laid than at 15 degrees C.

The Bark Beetle

The North American spruce bark beetle, Dendroctonus rufipennis Kirby, is the most important insect pest of the Alaskan boreal forest. Bark beetles normally exist at low levels on the forest floor and usually attack weakened trees. Warmer tempera tures and increased debris from windstorms, forest fires, or logging operations can dramatically increase populations to the extent that they can overwhelm even healthy trees. Bark beetles have a symbiotic relationship with several species of blue stain fungus. The beetles carry this fungus under the tree bark, where it attacks the tree's defense systems. This allows the bark beetle to lay its eggs in channels carved in phloem, the tree tissue under the bark. Repeated attacks can kill the tree.

The worst bark beetle outbreak in recorded Alaskan history began in 1990, reached a peak of 445,000 hectares in 1996 and declined to 220,000 hectares 1997. Over the last ten years, between 800,000 and 1.2 million hectares of forest was affected. U.S. Forest Service scientists comment "The recent increase in bark beetle and defoliator activity may be correlated to climatic changes. There has been a significant warming trend throughout south-central and interior Alaska for at least 60 years. This, no doubt, has benefited spruce beetle populations by reducing the amounts (both duration and range) of unfavorable temperatures for brood development; increasing winter survival of the beetle brood; and allowing for a longer adult spruce beetle dispersal period. The sharp spike in spruce beetle activity seen in 1994-1995 can be partly attributed to the "mother" of all summers experienced throughout Alaska in 1993 which increased water stress on spruce trees during the spring dispersal of the beetles, and also aided beetle populations by halving beetle development times from a two-year to a one-year life cycle."

A key factor driving the recent outbreak is the growing number of beetles with one-year life cycles in Alaska. Bark beetles normally develop to egg-laying maturity in one year in northeastern North America and in coastal areas of the Pacific Northwest. However, two year life cycles are more common in colder regions such as the Rocky Mountains, British Columbia and Alaska. In 1978, one year life cycle beetles were found in the Chugach National Forest and around Fairbanks. Previous to this date, only two year life cycle beetles were reported in Alaska. Research has shown that larval development requires a minimum of 6.1 degrees C to occur at all, and that larval development accelerates with temperatures at least until 21.1 degrees C. A tree phloem temperature of 16.5 degrees C is required to initiate a one year life cycle. Beetles developing in one-year cycles can multiply twice as fast as two-year beetles.

The number of eggs laid by bark beetles is closely correlated with temperature. One study concluded that, at 15 degrees C, the average female bark beetle laid 54.2 eggs, but at 21.1 degrees C the female laid 98.35 eggs.

Bark beetles are not tolerant to freezing, and thus require the shelter of snow and debris to survive the winter. Climate change which cause warmer winters, a deeper snow pack, or more sheltering debris because of increased forest fires, insect mortality or wind damage, would increase the number of beetles successfully surviving the winter.

Rapid Climate Change and Forest Migration Rates

The rate of climate change -- and not the change itself -- is perhaps the biggest threat to the boreal forest. With rapid change, conditions may become unsuitable for trees to complete their life cycle. Seedlings are especially sensitive to short-te rm drought, saplings to varying levels of sunlight, and mature trees to soil moisture during the growing season. Thus, in a kind of "arrested development," healthy-looking tree populations may not ever mature to the point of reproduction. Entire remnant stands of forest may no longer sustain themselves, or their resident animal and plant communities. A temperature rise of only 2 degrees C could, for example, eliminate up to half of the animals currently inhabiting boreal mountain ranges from the Rocky Mountains to the Sierra Nevada.

Forests have historically migrated or shifted in response to past climate fluctuations: spruce generally migrated 240 to 1,500 feet per year in response to past climate variations, fir 60 to 900 feet, and pine 4,500 feet. These rates are fairly slow. By contrast, climate models indicate that the boreal forest range may have to shift 15,000 feet per year or more to follow suitable climate conditions -- more than 10 times the historic migration rates of most boreal species. As a result, boreal forests are likely to decline in their current ranges far more rapidly than they can expand into what are now tundra regions. This could drastically reduce boreal forest cover for several centuries and release significant amounts of carbon into the atmosphere, speeding up the rate of climate change.

Conclusion

The threats that global climate change present to northern boreal forests are numerous. Yet they reflect only a portion of a larger, bleaker picture of the Earth in a dramatically changing climate. These potentially drastic changes in the forest serve as a wake up call to governments and individuals alike to take action now. Greenpeace fully supports the IPCC's call for deep reductions in greenhouse gas emissions, and a shift away from a fossil fuel-based economy to one based on clean, renewable ene rgy sources. Solar and wind power and energy efficiency measures are available now. These alternatives have the potential to meet the world's energy needs, create jobs, encourage world trade in clean technologies, and reduce energy costs while protecting the planet. The barriers to these solutions are not technical, but political.