Introduction:
Boreal forests represent the largest undisturbed eco-systems in the world, accounting for about a third of all forest cover, or 12 million square kilometers. About half of the Boreal Forest is undisturbed, primary forest. It is known for its vast expansive of conifers, (largely Spruce/Fir forests), but this biome also consists of bogs, fens and shallow lakes, which hold vast quantities of fresh water. Species that live in the Tiaga are specialized to withstand long winters, (for instance, the conical shape of Spruce trees, designed to shed snow and ice, or the snowshoe hare, which sheds its brown summer coat for a white winter one). Perhaps most impressive, however, is the role these forests play in the carbon cycle. Trees are known to uptake carbon through photosynthesis, and store it in their biomass as carbohydrates. The average carbon content is generally 50% of the tree’s total volume. Old growth forests, like those in the Boreal Forests, are capable of storing carbon for up to 800 years in the live mass of trees. Since Boreal Forests represent the largest portion of old growth forest, they are responsible for storing more carbon than any other biome at 703pg. The nearest biome is the Tropical Rainforest at 375pg. However, this living carbon sink is particularly vulnerable—first, because temperatures are rising faster in the adjacent arctic region, and second because a change of just 1.5 degrees Celsius would cause climate zones to migrate north at rate of 5 kilometers per year, far outpacing the migration rate of trees. Even if the biome could migrate quick enough, the carbon from dying trees in the southern extent of their range would release much of the carbon they have been responsible for storing, thus accelerating the greenhouse effect. This effect is consequential not just for the healthy functioning of the ecosystem, but also for the health of the entire planet.
I. The Boreal Biome:
The Boreal Forest is the northern most forest type, occurring just south of the Arctic Tundra, and transitioning from the northern hardwoods of the Temperate Broadleaf forests to the south. Boreal forests overlay areas formerly covered by glaciers and permafrost. The vegetative biodiversity is relatively limited, with most of the forest existing in patches of subclimax plant communities. The Boreal Forests of North America are primarily composed of Balsam and Douglas Fir, White and Black Spruce, Hemlock, Cedar and other conifers, but also include some populations of deciduous trees such as, Sugar Maple, Speckled Alder, Yellow and Paper Birch, White Ash and American Beech.
Winters are long in the Boreal Forest: typically up to six months of below freezing temperatures. The growing season is also very short: between only 50-100 days without frost. When the sun is near the horizon, during winter, the angle of incidence at which energy is received in the Boreal Forest is lower than it is in the tropics, causing more energy to be received by diffuse radiation as opposed to direct radiation. The species that live in this biome must develop adaptations in order to survive the strict economy of energy, during the winter. Many mammals are larger as a defense against the cold, and many hibernate through the coldest months to prevent starvation and freezing. While the nesting range of many bird species, like Cedar Waxwings, Red-winged Blackbirds, Hermit Thrushes, Boreal Chickadees and Common Loons, lie within the Boreal Forest, most migrate south for the winter. Conifers dominate the northern extent of the forest due to their adaptation to snow and ice loading. The broader shape of hardwoods make them more susceptible to ice loading, and the premature shift of sap from the roots to the trunk and branches in spring can cause the trees to crack in a late frost. However, conifers have capillaries that have evolved to be able to turn water movement on and off, depending on conditions. They also have stronger cell walls, and can better withstand ice expansion.
While the Boreal Forest may seem inhospitable, it is, in fact, a vital ecosystem. Between carbon capture and storage, water filtration, waste treatment, biodiversity maintenance, pest control and other services, the Boreal Forest provides ecosystem services estimated to be worth about $250 billion per year. The boreal forest has been described as “a giant carbon bank account.” Boreal Forests “store an estimated 67 billion (tons) of carbon in Canada alone – almost eight times the amount of carbon produced worldwide in year 2000.” Globally, the Boreal Forest contains about 1/3 of the world’s vegetation and soil carbon.
It is important to remember, however, that carbon uptake and carbon storage are vastly different, and the slow rate of primary production in the Boreal Forest acts as a limit to the forest’s ability remove carbon from the atmosphere at a rate that could combat our use of fossil fuels. That said, the destruction of the forest causes carbon that has been stored for hundreds of years to be released, acting as a positive feedback to global climate change. What is particularly disturbing about this fact is that the Boreal Forest may be in decline as a result of global climate change, as well as contributing to it.
II. The Effects of Climate Change on the Boreal Forest:
Boreal Forests are, by their very nature, extremely resilient. However, as the abstract to “Boreal Forest Health and Global Change” suggests, “(…) projected environmental changes of unprecedented speed and amplitude pose a substantial threat to their health.” Though the biome remains one of the largest on earth, “it faces the most severe expected temperature increases anywhere on Earth.” It is widely accepted that a warming of 1.5 to 2.5 degrees Celsius would increase the risk of major vegetation changes, including the loss of heat sensitive species in the Boreal Forest.
As this 2010 report from the U.S. Forest Service’s Northern Research Station suggests, “One of the big uncertainties of the global climate change phenomena is what will happen to the trees.” Many optimistic foresters predict that increased levels of atmospheric carbon dioxide will improve photosynthetic rates and thus accelerate tree growth. While there is evidence to support this prediction, there is also concern about what effect changes in temperature and precipitation patterns will have on forest and species distribution. Already, some research points to climate change contributing to advancing infestations of invasive and pest species such as the Hemlock and Balsam Wooly Adelgids, the Emerald Ash-borer, the Spruce Budworm, Mountain Pine-beetle and the Asian Longhorn Beetle. Many of these invasive insects cause forests to be more susceptible to pathogens, as Professor Tom Wessels notes in Reading the Forested Landscape of New England: “Beech-bark Scale Disease, Dutch Elm Disease, Chestnut Blight, and White Pine Blister Rust—are all caused by fungi, their spread often facilitated by insects.” In at least the case of the Hemlock Wooly Adelgid (HWA), cold winters can hamper the spread of the insect. It has been found that the HWA struggles when winter temperatures average less than negative 5 degrees Celsius, which has so far hampered their spread into the boreal forest. However, as Anna Szyniszewska writes on the Climate Institute’s website, “The impact of climate change and rising average world temperatures can have a profound influence on species’ geographical ranges that are often set primarily by climate…”
Professor Wessels also writes about the destructive potential of the HWA:
At the doorstep of central New England awaits another insect defoliator. Accidentally introduced from Asia and first discovered in Pennsylvania in the 1960s, the wooly adelgid has spread as far north as southern Massachusetts. Its tolerance to cold temperatures has researchers worried about the future of its host, the eastern hemlock.
It is largely expected that as climate changes, many damaging invasives will be able to expand their ranges, and this is expected to be very damaging to host trees in the Boreal Forest. As Roger Olsson notes in “Boreal Forests and Climate Change,”
The impact of insect damage in boreal forests is significant. In terms of area affected it exceeds that of fire. Spruce Budworm, for exampled, defoliated over 20 times the area burned in eastern Ontario between 1941 and 1996… Insect outbreaks are expected to increase in frequency and intensity with projected changes in global climate through direct effects of climate change on insect populations and through disruption of community interactions…
It is likely that both native and invasive pests will increase, and that the damage to forests will amount to billions of dollars, (in both crop loss in the forestry sector, and somewhat less directly in the loss of ecosystem services).
Invasives are not the only concern when considering the impacts of climate change on Boreal Forests, and animals are not the only species capable of expanding or contracting in range. As the U.S. Forest Service notes, “Tree ranges in ancient times certainly shifted according to changing climates, but the changes were relatively slow.” Northern Research Station scientist Christopher Woodall used existing data to analyze movement in the geographic distribution of current trees. His study found evidence that northern tree species “are exhibiting a northward migration,” and that, “Over 70 percent of this study’s northern species have mean locations of seedlings that are significantly farther north than their respective mean biomasses.” Woodall also recorded a number of species, which exhibited negative area changes, “that is the areas in which they thrived decreased.” Black Spruce, Bigtooth Aspen, Quaking Aspen, Balsam Fir, Paper Birch, Yellow Birch, Northern White Cedar, Striped Maple, Black Ash, Scarlet Oak, Eastern White Pine, Red Pine, Eastern Hemlock, Red Spruce, Sugar Maple, Sweet Birch, American Basswood, Hawthorn, Sourwood and Northern Red Oak, all lost area, while southern species “demonstrated no significant shift northward despite greater regeneration success in northern latitudes…” Woodall estimates that tree migration amongst northern species may accelerate to a rate of 100 km per century, which sounds like a small amount, but this rate will likely outstrip the rate at which southern species can take the place of northern species, potentially leaving a vast savannah, where once there was a Boreal Forest. As Dmitry Schepashenko, co-author of, “Boreal Forest Health and Global Change,” notes in an interview with the Canadian Broadcast Company, “The (southern) forests can’t go so far to the north. The speed at which forests can move forward is very slow, like 100 meters a decade.”
The decline in thriving habitat of many boreal species is likely due to temperature induced drought stress. As Roger Olsson suggests in “Boreal Forests and Climate Change,” “Some tree-growth declines are large and have been seen at different points across a wide area. Temperature induced drought stress has been identified as the cause in some areas.” He notes that studies of tree-rings have shown a negative correlation with temperature increases during the 20th century, and that growth decline occurred more often in the warmer areas of a species distribution, “suggesting that direct temperature stress might be a factor.” As temperatures increase, and drought becomes more common in the already arid biome, the destruction of habitat may become as widespread as Schepashenko suggests. To again quote Roger Olsson:
If global warming exceeds 2 degrees celsius the change of ecosystems in the boreal forest region may be even more far-reaching than outlined… Direct effects of warming on forest growth and distribution, combined with indirect effects of climate-induced changes in disturbance regimes may transform vast areas of boreal forest into open woodland or grassland… In regions where the boreal forest presently is succeed by continental grasslands in the south, a contraction of forest is projected due to increased impacts of droughts, insects and fires. With global warming of more than 2-3 degrees Celsius extensive forest and woodline decline in mid-to high latitudes is predicted.
Besides simply degrading forest aesthetics by replacing vast primary forest with anti-entropic, high growth, early successional habitat, there is one glaring problem that would result from the recession of an old growth carbon sink. With so much of the world’s untapped carbon reserves existing in the biomass of the Boreal Forest, it is daunting to think of the impact that releasing that carbon would have. If global warming negatively impacted the Boreal Forest, as it seems almost certain to, the carbon released would be a positive feedback into the carbon cycle, causing more warming, and thus more forest degradation.
The spectre of carbon cycle feedback was raised in a 2006 “Realclimate.org” article entitled, “Positive Feedbacks From the Carbon Cycle,” and suggested that warming could be accelerated between 25-75 percent. In Michael E. Mann’s summary of the IPCC’s findings entitled, Dire Predictions, he notes that current emissions due to deforestation amount to between 4.5 and 5.5 gigatons of carbon dioxide per year. The result may be that the forests we have known to be carbon sinks may no longer be providing us that service. Again, Roger Olsson suggests that, “Modelling results suggest that forest ecosystems in Canada shifted from a carbon sink to a carbon source around 1980… Projections for a hypothetical North American boreal forest landscape indicate that carbon losses from disturbances cannot be offset by increases in growth, if higher decomposition rates caused by altered disturbance regimes are taken into account.” Thus, even the successional habitat that is likely to replace the disturbed forest, even being anti-entropic (taking up exponentially more energy and nutrients, using energy to increase complexity over time), it will not be able to take up as much carbon as is currently stored in the Boreal Forest.
Conclusion:
Professor Tom Wessels of Antioch University writes extensively on the topic of forest disturbance. He argues that complex systems like a forest ecosystem are self-organizing, meaning they, “take in energy and use it to increase their level of complexity through time.” He notes that, “A clear cut forest is left in a simplified state. In time it grows back to a forest with complex structure and a wide variety of organisms.” However, he also suggest entropy, “a process where things naturally move from a state of order toward disorder,” effects complex systems. He notes that all energy conversion is inefficient and that some diffusion results from all complex systems. However, he defines systems as anti-entropic if they take in more energy than they release, as is the case with early successional habitat. The goal of a forest ecosystem is to reach the dynamic equilibrium of an old growth, which would be defined as taking in as much energy as is released, and where nutrients are recycled across trophic levels. However, when ecosystems die they become highly entropic and release both stored energy and carbon dioxide. This is what is behind the aforementioned feedback. The high level of entropy of dying forests is greater than the anti-entropic effect of the savannah habitat that will likely replace the forests.
Professor Wessels points out that Global Climate Change is largely an entropy imbalance, and that positive feedbacks, like dying Boreal Forests releasing carbon, eventually cause bifurcation. He writes:
In a system with positive feedback, the feedback amplifies the system’s behavior in a directional, accumulative way… With sustained positive feedback the impacts eventually may build up to such a degree as to throw the system into a totally new mode of behavior. The point at which a complex system jumps into a new behavioral patter is known as a bifurcation event…Although the positive feedback leading up to bifurcation may be gradual, the change in system behavior is abrupt.
Global warming is essentially a gradual building of positive feedbacks related to a high level of entropy. To get under the hood of how the positive feedback works, defoliation from pathogens, pests, increasing acidity, drought and other climate related causes reduce the forest’s ability to photosynthesize. Reduced canopy means more radiation reaches and warms the forest understory. Warming on the forest’s floor increases decomposition rates, “a process that releases nutrients.” As Professor Wessels notes, “When photosynthesis drops and decomposition increases, the loss of nutrients from the ecosystem is accelerated…” Nutrient availability becomes greater, but because of the reduced photosynthesis, less is taken up. Eventually trees start to dieback. When a tree species is lost, the reverberation is felt amongst all the species that associate with it, and if there is not enough niche redundancy this results in a loss of biodiversity. Forest ecosystems are intensely interconnected, and such declines can have a snowballing effect.
What will the bifurcation eventually look like in northern forests? Professor Wessels suggests that:
For species of trees that don’t grow south of New England… the warming climate will most likely translate into an inability to germinate successfully… Drought sensitive trees like sugar maple and white ash will experience more dieback… Southern trees will take centuries to complete a northern migration to the region… Coupling these reductions with those already created by introduced forest pathogens and potential declines from atmospheric deposition, we see a very bleak picture of our future forests.
The complexity of the relationship between global climate change and the largest biome on earth is immense. Yet, there is one factor that unites the entire issue: the ecological interconnectivity of each of these issues. This is why declines in on part of the forest result in ecosystem feedback. For centuries the Boreal Forest has served as the largest carbon sink in the world. However, this appears to be changing, and concern is rising that, a decline in productive biomass in this biome, defoliation and deforestation, are releasing carbon and energy as a carbon source. Since the decline of the forest creates positive feedback in the global system, once decline starts, it is likely to escalate, causing both localized and global destruction. The habitat that will likely result from the shrinking or loss of Boreal Forests, though anti-entropic, is neither likely to be able to absorb the release of carbon and energy, nor to replace the ecosystem services currently provided. If the Boreal Forests transition from sink to source, as some research suggests has already happened, not only will it accelerate global climate change, but we would likely lose the last, largely undisturbed, functioning forest ecosystem in the world.
Bibliography To Be Added Later
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