Effects on High Latitude Forests from Permafrost Thawing Due to Climate Change

High latitude forests, also called boreal forests or Taiga, are characterized as an almost continuous belt of coniferous forests from 50 to 60 degrees latitude with a considerably cold climate and low annual precipitation. Winters are long with temperatures staying well below freezing for more than half the year. This area is also prone to huge seasonal temperature fluctuations, the most extreme recorded being in Verkhoyansk, Russia where the low was -68 degrees Celsius and the high was 32 degrees celsius, a 100 degree difference [1]. Permafrost is soil or rock that remains under 0 degrees celsius for more than two years and is a defining characteristic of arctic and subarctic ecosystem’s physical geology and chemistry. Nearly ⅓ of the boreal region is on permafrost soils. This ecosystem, and its permafrost, are sensitive to climate change and will undergo drastic changes that will affect nutrient cycling, economy, species diversity, and atmospheric carbon on a global scale as the temperature rises in the future [2][4][6].
Forests provide numerous ecosystem services and the boreal forest, accounting for about 30% of the world’s forest cover, is considered a valuable resource on a local and global level [2]. This vast landscape harbors only 85 species of mammals which are adapted to live in the very harsh yet specific climate of the northern latitudes. This is a relatively low diversity but some of these animals like the squirrel and caribou hold important trophic positions driving an intricate food web which includes humans. Another important mammal, the beaver, shapes landscapes creating microenvironments such as ponds, bogs, and marshes that are critical to to other species [1]. Nearly half of the north american birds depend on northern forests for migratory breeding and conifer trees and make up the majority of vertebrates living in the boreal forests [10]. 600 Indigenous communities of the boreal forest and other local human populations benefit from the hunting, fishing, and economic opportunities from this biome. About two thirds of the forest is managed for industrial wood and paper pulp with 33% and 25% of the worlds productions respectively [2]. Canada alone exports 31 billion kilograms of wood products which makes them 17.1 billion US dollars annually [11].
Boreal forests contain the world's largest stores of frozen and unfrozen freshwater [12]. Precious fresh water, which only makes up 2.5% of the water on earth, is stored in permafrost, lakes, ground water, wetlands, and bogs. The health of these water sources goes hand in hand with the health of the ecosystem. Boreal aquatic ecosystems house many endemic species of fish and wildlife that make up a large part of the diet of humans in the area. These headwaters also help to cool water that flows into the arctic seas stabilizing ocean currents [12].
Through the use of these freshwater systems and energy exchange with the atmosphere, the biome as a whole affects global climate and weather patterns. Due to the low photosynthetic rates, the rate of transpiration is also low [12]. This phenomena, paired with the high amounts of solar radiation create large sensible heat fluxes that create atmospheric boundaries all around the planet driving a dynamic, yet stable, weather pattern on the edges of the biome [12].
Arguably the most important ecosystem service provided by the high latitude subarctic forests is the large amounts of soil carbon pools locked up in frozen soils, particularly in the permafrost[6][8]. Only ⅓ of the boreal forest is in permafrost, however this is still about 5.6 million square kilometers of land. The carbon locked up in these reserves are currently not cycling which means it is not circulating in the atmosphere as a greenhouse gas increasing the global temperatures. There is anywhere from 200 to 500 gigatons frozen in these soils at high latitudes which would increase the the concentration of carbon in the atmosphere by 50% if it were to be released from climate change [8]. This is estimated to be about 32% of the worlds global terrestrial carbon [2]. Carbon is not the only nutrient stored in the permafrost, there are also reserves of nitrogen, phosphorus, mercury, and salts. Carbon sequestration is a valuable service provided by these forest, however, due to the low photosynthetic and transpiration rates of the vegetation the carbon being sucked down and sequestered is low. It is estimated that only about 20% of the total carbon sink of the world’s forests is sequestered in the boreal region [2].
As the world’s climate changes and the temperature increases, the boreal forests will be the most affected [2][4][6][8]. The problem of permafrost thaw is the biggest issue, and will cause many other problems and ecosystem changes for years to come if current climate projection models are correct. While the earth warms, the arctic and subarctic areas will be amplified due to the rapid increase in productivity and solar radiation absorption. The average permafrost temperatures have risen 4 degrees celsius since the last cooling period causing a thawing event to occur [3]. It is predicted that in 2100, 30% of the modern permafrost extent will be degraded When the permafrost thaws, the large reservoirs of carbon locked up in frozen soil will start to be re entered into the global carbon cycle. This increase in available carbon is estimated to accelerate the process of global warming [3]. The quickening of temperature change will create a positive feedback loop forcing further thawing and more carbon to be released from the soil furthering climate change. Areas of the region have actually been observed to change from carbon sinks to carbon sources due to the thawing of frozen soils [6]. The warming and thawing of permafrost has caused degradation to the environment and shifts in ecosystem structure [3]. Jorgenson et al. has determined 16 different forms of surface degradation that is caused by permafrost thaw which change the shape and integrity of the sensitive boreal ecosystem as well as creating differential warming of climate across the region. Some of these landscape changes are drastic and causes lasting damage to the area and shifts in ecological processes all together [3]. When rapid thawing occurs under stands of black spruce, the trees can wilt or tilt giving them a “drunken” look. This happens because the black spruce is adapted to live on permafrost having short roots that do not penetrate deep into the soil. When the soil loosens from thawing the trees become uprooted and loose the ability to uptake nutrients and eventually die. This is an example of how the thawing of permafrost can change landscapes and even vegetation distributions.
Within frozen ground, microbial activity is arrested and therefore the process of decomposition is slowed in the layers of soil with permafrost. This mechanism of lowered microbial activity has been a key player in the capacity of boreal soils to store carbon. When predicted thawing occurs the microbial activity and respirations rates will increase in-acting  a rapid release of CO2 into the environment [4]. Along with the release of CO2, nitrogen release from mineralization will occur increasing the available nitrogen to plants and increasing productivity above ground [4]. An increase in above ground biomass could possibly throw off the water energy balance in areas leading to further loss of soil carbon content to the atmosphere. Microbial activity in thawed permafrost of bogs and wetlands has been found to generate Methane, a much more potent greenhouse gas, from decomposition of peat and other highly available plant matter.
In a study done by Reyes and Lougheed [5] it was found that its not only temperature that plays a role in nutrients dumping from permafrost thaw but it actually depends a lot on the type of microbes in the soil as well as the chemical environment, or redox environment, of the soils. The dumping of these nutrients and the leaking of sediments from thaw can have a negative effect on the aquatic ecosystems of the sub-arctic regions [14]. Microbes that become active after permafrost thaw have been found to make nitrogen and phosphorus more readily available for transport from soils to aquatic bodies. Sediment is one of the biggest pollutants of aquatic systems and permafrost thaw can erode the ground causing particulate matter to get into nearby bodies of water. When this erosion brings in new sediments to aquatic systems, it also brings along “particulate organic carbon” increasing free floating and suspended nutrients in the water which could increase the formation of algal colonies [14]. The thawing can have a physical effect on hydrology as well changing pathways of flow.
Due to warm climates and the positive feedback from carbon released from permafrost thawing has shown an increase in harmful insect populations. The insects are a natural disturbance however they have become more frequent and having a broader range due to the fact that areas of the forest are accessible and livable with less frost days to kill them off. Insects like the Mountain pine beetle and forest tent caterpillar have been increasing and widening their field of destruction due to their increased survivability in the boreal ecosystem from milder winters and warmer summers. The decimation of tree populations could be detrimental to the lumber industry as well as lower the ability of the forest to sequester carbon. There has been an increase in mosquito populations and as well as a range expansion. The bogs and wetlands that are created in the summers from permafrost thawing can be breeding grounds for mosquitoes. An increased range for mosquitoes also means the range for disease spreads.
Wild and controlled fires have an effect on the amount of permafrost degradation that occurs [15]. Wildfires occur naturally in the boreal forest and are an important part in ecological succession and vegetation population dynamics. Fires also affect the properties of the frozen soils directly and indirectly. The burning of the organic layer can increase the soil thermal conductivity, or ability of the ground to conduct and absorb more heat [15]. This can cause soil temperatures to rise and thaw permafrost layers. It will also affect the ability of the soil to refreeze after a thawing event [15]. The impact immediately after a fire is minuscule compared to the long term effects it has. Forest fires are an important disturbance for the boreal forest ecosystem, however the increase in fire occurrences due to climate change poses as a big threat to permafrost integrity.
The boreal forest biome is expected to see major temperature increases and it is expected that large carbon storages will be released from permafrost deposits and will further global climate change [2][3][4][5][6][7][8]. The solution to this problem lies in the solutions to climate change. The root of this hastening ecosystem shift is the continuing increase in temperature from carbon emissions mainly from anthropogenic sources. The boreal forests are an invaluable carbon sink, house most of the freshwater on earth, and provide countless capital and ecosystem services to the world's populations. It is necessary that preservation of this ecosystem is seen through.
A possible solution is practicing sustainable forest management techniques to lessen the human impact. This could involve reforesting areas to maintain or increase carbon sequestration in the areas affected heavily from the lumber industry [2]. There was a large abandonment of 45 million hectares of agricultural land in the boreal region of Russia which can now undergo succession and natural recovery [2]. There are not many policies that promote the preservation of boreal ecosystem.
Increased monitoring and control of natural disturbances would help to ease the pressures on current carbon stocks and permafrost extent. Areas that are considered ecologically sensitive to climate change like permafrost deposits, wetlands, and peat-bogs should be paid close attention to and studied to further our knowledge of the complex processes that cause warming of the boreal forests. Action needs to be taken up now in order to combat the growing pressures of climate change and permafrost thaw on these priceless high latitude forests.

Citations and References

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