Introduction

 As the effects of climate change increase, warming temperatures have the potential to cause widespread thaw of permafrost dominating areas (Helbig et al., 2016). As a result, permafrost thaw in forested areas, such as boreal forests, has the ability to lead to the conversion of these boreal forests to wetland-dominated landscapes (Lara et al., 2015). Permafrost thaw-induced boreal forest loss, and potentially it’s disappearance, has received minimal attention as compared to the attention deforestation and wildfires have received (Helbig et al., 2016). One third of the world’s forested area is described as boreal forests, and these forests account for 66% of the world’s carbon store pools (Köster et al., 2018). Therefore, it is critical to further analyze permafrost thaw-induced changes on forest ecosystems as the release of stored carbon pools in boreal forests could result in alarming global consequences (Baltzer, Veness, Chasmer, Sniderhan, & Quinton, 2013).

Permafrost Definitions

Described in 1988 by the Permafrost Subcommittee of the National Research Council of Canada, permafrost or perennially cryotic ground, is defined as ground, including rock and soil, that remains at a maximum temperature of 0C for at least two years (Harris et al., 1988). Prior to this definition, in 1974, permafrost terminology by the Permafrost Subcommittee involved the use of the word “frozen” (Harris et al., 1988). This provoked dispute between researchers who believed that frozen should be used to refer to any earth matter below 0C whether or not water was present, while other researchers believed that earth material should not be considered as frozen unless it contains ice (Harris et al., 1988). Altogether, the use of the word frozen in the permafrost definition was removed in 1988 when it was decided that although all perennially frozen ground contains permafrost, not all permafrost is perennially frozen (Harris et al., 1988).

The Effects of Permafrost Thaw in Northern Forests

In order to ­­­­­further discuss permafrost, it is important to understand the effects of permafrost thaw on forests. For decades, it has been well understood that permafrost formed in peatlands often causes an elevation of 0.5-1.5m, referred to as ice plateaus, in the ground surface surrounding the wetlands (Payette, Samson & Lagarec, 1976) (Helbig et al., 2016). This phenomenon causes a hydraulic gradient between the elevated and well-drained forest area and the surrounding wetlands (Baltzer, Veness, Chasmer, Sniderhan, & Quinton, 2013). As permafrost warms and wears thin, it becomes susceptible to thaw and leads to thermokarst, which is described as the subsidence of the ground surface (Tanski et al., 2017).  As a result of the hydraulic gradient caused by ice plateaus, waterlogging of these forest margins is common (Baltzer, Veness, Chasmer, Sniderhan, & Quinton, 2013). However, in the boreal forests of Canada where Black Spruce is the dominant tree species, these coniferous trees are unable to tolerate waterlogging (Baltzer, Veness, Chasmer, Sniderhan, & Quinton, 2013). Waterlogging causes low oxygen concentrations in Black Spruce roots where the majority of metabolic processes, nutrient uptake and transpiration occurs (Baltzer, Veness, Chasmer, Sniderhan, & Quinton, 2013). Waterlogging contributes to reduced tree function and ultimately leads to tree mortality of the Black Spruce species (Baltzer, Veness, Chasmer, Sniderhan, & Quinton, 2013).

Forest Permafrost Thaw and Associated Forest Fragmentation

 In a study published in 2013, the effects of forest permafrost thaw and the resulting forest fragmentation and associated edge effects were thoroughly analyzed (Baltzer, Veness, Chasmer, Sniderhan, & Quinton, 2013). It was determined that as permafrost begins to thaw, a fragment-related positive feedback loop has the potential to develop due to microenvironmental changes extending inward from the forest edges (Baltzer, Veness, Chasmer, Sniderhan, & Quinton, 2013). As commonly discussed, forest edges often result in increased drought stress and tree mortality due to higher vapour pressure deficits, increased temperature and increased solar radiation (Baltzer, Veness, Chasmer, Sniderhan, & Quinton, 2013). As a result, tree mortality causes increased forest fragmentation in areas with discontinuous permafrost (Baltzer, Veness, Chasmer, Sniderhan, & Quinton, 2013). The pattern then continues to occur as edge effect in forests involves changes to microenvironment, biotic responses, increased disturbances and changes to their normal ecological function (Baltzer, Veness, Chasmer, Sniderhan, & Quinton, 2013). Furthermore, because the soil adjacent to the forests becomes uninsulated, this results in permafrost thaw-induced active layer thickening due to the contact between the warm bog soil (Baltzer, Veness, Chasmer, Sniderhan, & Quinton, 2013). To summarize, forest fragmentation leads to warmer outer edges, which waterlogs and kills trees, leading to inward extending edges which continues this process. Altogether, these processes result in a positive feedback loop that continues to move the system away from a state of equilibrium, and further enhances these changes (Schuur et al., 2015.

Forest Fires and Permafrost Thaw

 To examine what other factors affect permafrost thaw, changes to active layer thickness and seasonal thaw rates following a forest fire were first examined in a 1982 study where burned, mechanically removed and control plots were compared (Dryness, 1982). Firstly, it was determined that fire has the ability to increase the rates of permafrost seasonal thaw even 4 years following a forest fire (Dryness, 1982). Burned areas would thaw quicker in summer months than control plots, while mechanically removed plots would thaw at an even faster rate than both the burned areas and control areas (Dryness, 1982). Next, the researchers investigated the soil active layer thickness 4 years after the various treatments took place (Dryness, 1982). The only treatment to show a consistent, statistically significant effect on active layer thickness was the mechanical removal plot, while the data collected from the burned plots did not show a statistically significant effect (Dryness, 1982). Although the rate of increased active soil layer was consistently greater in the burned plots than the control plot, there was no statistical evidence that this rate was increasing (Dryness, 1982).

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A similar study published in 1991 analyzed seasonal thaw depths following a forest fire, as well as nutrient concentration in the soil (Heng, You-Wu & Jia-Cheng, 1991). As previously examined the 1982 study, a similar trend appeared in seasonal thaw depth, in that mechanically removed sites exhibited the greatest seasonal thaw depth, followed by the burned site, and the unburned control site showing the smallest seasonal thaw depth (Heng, You-Wu & Jia-Cheng, 1991). Overall, maximum seasonal thaw depths in burned plots were 40-50cm greater than comparable, unburned sites (Heng, You-Wu & Jia-Cheng, 1991). The knowledge that was gained since the original 1982 study, provides researchers with more thermal parameters and equations that can be used in the future to calculate seasonal thaw depths and make predictions (Heng, You-Wu & Jia-Cheng, 1991). A new aspect to this study was an analysis of new composition of vegetation growth after the burn. It was observed that burned areas grew back 15-20 days earlier than adjacent unburned areas and consisted of more nutrient rich soils and vegetation (Heng, You-Wu & Jia-Cheng, 1991). It was predicted that the rapid growth was due to nutrients being released from the soil due to the increased active layer depth in the ground (Heng, You-Wu & Jia-Cheng, 1991). Furthermore, rapid initial growth in burned areas came from fire causing increased decomposition of the litter layer, resulting in an increased release of nitrogen, phosphorous, potassium, etc. (Heng, You-Wu & Jia-Cheng, 1991).

Using a similar experimental design as these last 2 papers, a very recent 2018 study was conducted to understand the effects fire has on the degradation of permafrost and the associated changes in the dynamics of carbon dioxide and methane fluxes (Köster et al., 2018). The study observed that burned soil acted as a source of carbon dioxide and that as time since a fire increased, carbon dioxide emissions increased as well (Köster et al., 2018). The results also concluded that carbon dioxide flux lasts over 50 years, while the effect of methane flux due to fires was minimal in permafrost landscapes (Köster et al., 2018). Moreover, although the previous studies indicated that heat from a fire does not significantly affect the active layer, it was noted that this disturbance has the potential to remove the insolating organic layer, causing a decrease in surface albedo during the summer, thus allowing for long-lasting permafrost thaw (Köster et al., 2018). Altogether, this is important knowledge to have gained, because as there is increasing depth of seasonally thawed active layer, this increases soil temperature and therefore decomposition (Köster et al., 2018). As a result, as decomposition increases, it contributes to elevated emissions in the form of carbon dioxide and methane (Köster et al., 2018). With climate change becoming the focus of many ecological issues, this learned information can be leveraged in climate modelling and risk assessments.

Nutrient Concentration and Permafrost Forests

A 2013 study compared the effects of north-facing and south-facing slopes and the foliage nutrient concentration in larch trees in Siberian permafrost affected landscapes (Viers et al., 2013). As commonly researched, south-facing slopes tend to have higher insolation than north-facing slopes resulting in warmer soils, higher seasonal permafrost rates and a deeper soil active layer (Prokushin et al., 2018). Due to these aspects, the researchers recognized that in south-facing slopes, nutrients were more available for trees due to a deeper active layer soil and a higher decomposition rate which worked to release more nutrients into the environment (Viers et al., 2013). Likewise, in a similar 2018 study, the effects of permafrost on nutritional status of larch trees in Siberia were estimated in order to analyze responses of larch forests to warming conditions (Prokushin et al., 2018). In permafrost dominated regions, their warmer and deeper active layer soil contained 15-60% greater nutrient concentrations than permafrost free regions (Prokushin et al., 2018). Also, the larches found in permafrost areas had lower concentrations of 15N in its foliage (-2 to -6.9%) compared to the 15N concentrations found in the foliage of permafrost-free regions (1.4% to 2.4%) (Prokushin et al., 2018). This increased foliage 15N concentrations found in the permafrost-free regions was also linked with up to a 50-fold increase in biomass production (Prokushin et al., 2018). These findings suggest that with permafrost degradation there is accelerated nutrient cycling which ultimately contributes to increased productivity in Siberian larch forests (Prokushin et al., 2018).

Modelling Permafrost

 To make future predictions regarding permafrost thaw and to assess forest management techniques, precise models need to be initially created. One of the first maps to present such data on permafrost and ground ice in the Arctic in a consistent matter was published in 1997 (Brown, Farrians, Heginbottom & Melmikov, 1997). This map provided a generalized and small-scale introduction to permafrost and used a matrix of colour patterns to demonstrate areas of most intense permafrost development (Brown, Farrians, Heginbottom & Melmikov, 1997). However, this coursed-scale resolution map failed to account for local factors that have since been proven to have an effect on permafrost distribution in forested areas. Following this study, not much had been done to quantify critical permafrost properties across large areas in a precise way (Pastick et al., 2015). Until however, a 2015 study described how it created climatic models to map permafrost location and active-layer depths in mainland Alaska (Pastick et al., 2015). To create such a model, complex interactions among surface and subsurface conditions were used to map permafrost in small-scale settings, that was then statistically and spatially extended using climatic data, subsurface biophysical characteristics and other thematic maps (Pastick et al., 2015). This fusion of a variety of data sets generated medium-resolution maps describing permafrost, active layer thickness estimates and areas of uncertainty that required further analysis (Pastick et al., 2015). These models were then used in conjunction with varying future climatic scenarios to assess the changes that will occur while assuming there would be no other changes in biophysical factors (Pastick et al., 2015). Following the introduction of these types of models, many scientists have since been working to increase their precision (Endalamaw et al., 2017). 

Future Research

 Based on how research focus has already changed over the past 30 years, it appears a large amount of future research will focus on creating more precise climate modelling techniques. As such, when new aspects are learned to have an effect on permafrost thaw, they must be introduced into the current models.  As discussed by a 2016 study, there is a need to understand the trends of air temperature and atmospheric moisture in boreal forests in order to better evaluate the performance of current land surface schemes and terrestrial biosphere models (Helbig et al., 2016). This study focused on a need to develop these models, acknowledging the unclear vegetation shifts between wetlands and boreal forests and their effect on turbulent energy fluxes (Helbig et al., 2016). Another study recognized further areas of development and a path for potential future research (Endalamaw et al., 2017). Researchers described that small-scale complexities in discontinuous permafrost distribution, make it challenging to create large-scale hydrological process simulations (Endalamaw et al., 2017). The variety in small areas included differences between north and south facing slopes, contrasting watersheds, varying vegetation cover, soil hydraulics and thermal properties. (Endalamaw et al., 2017). Because these aspects change so quickly in the landscape, it remains difficult to map out and form simulations for the future (Endalamaw et al., 2017). Only when these small-scale properties are better understood and represented, can large-scale simulations and modelling be perfected. Therefore, it is heavily believed that the future of research in permafrost thaw affected forests remains in performing more detailed studies to further exhibit these small-scale changes.

Conclusion

 As analyzed for decades, there is a close relationship between permafrost aggradation and forest succession (Payette, Samson & Lagarec, 1976). Currently, it is estimated that 87% of Canada’s 421 500km2 of peatlands that reside above permafrost will be severely impacted by warming induced changes (Baltzer, Veness, Chasmer, Sniderhan, & Quinton, 2013). Since these boreal peatlands are only ranked second to the tropical forests with the amount of carbon they store, the melting of this permafrost will have dramatic and negative effects on our environment (Baltzer, Veness, Chasmer, Sniderhan, & Quinton, 2013). Overall, understanding these complicated but important mechanisms is crucial to predicting the future interactions between climate systems and land surface processes in northern forested areas (Baltzer, Veness, Chasmer, Sniderhan, & Quinton, 2013). Also, in order to proceed effectively with future permafrost and forest research, it is important to reflect on past research to determine gaps in our current knowledge. Altogether, by leveraging past research to progress current and future development, we can hopefully determine strategies to suppress warming-induced changes in permafrost, in order to maintain these rich northern forested areas.

References

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