Climate change impacts on wildfires in boreal and tundra forest areas in the 21st Century

Boreal Forests

As the world’s largest land biome, boreal forests are an increasingly important ecosystem to study and understand, especially under the impacts of climate change. Boreal forests are dominantly composed of black spruce and related evergreen trees, such as white spruce. Tundra biomes lack forests due to consistent permafrost, but they are composed of vegetative species in moist and wet shrub, tussock, and non-acidic ecosystem types. One of the largest influences of climate change on boreal forests is increased wildfire frequency.

Boreal forests store approximately one-third of global terrestrial carbon on Earth. Wildfires release this carbon into the atmosphere and impact the nutrients, thaw depth and active layer, and change the plant functional types, thus producing both positive and negative feedbacks that take place immediately and last several decades after the fire.

Some effects are relatively permanent, such as thaw depth. The thaw depth is the instantaneous level down to which the soil has warmed to 0 degrees C. The active layer thickness is the maximum thaw depth over a period of two years (or sometimes just seasonally). The layer of soil over the thaw depth is called the active layer, while the soil below is called the permafrost.

Wildfires in the Tundra?

Wildfires in arctic tundra areas have historically been overlooked as an important feature in fire assessments and simulations. However, arctic wildfire burned area has increased over the past 70 years, which has lasting impacts on albedo and carbon storage in permafrost.

Wildfires are projected to increase by 2100 due to climate change and human influence. Climate change includes factors such as soil moisture, soil and air temperature, and number of lightning flashes. Wildfires have a wide range of impacts on the boreal and tundra forest environments and on the future climate.

Boreal forests contain significant amounts of stored carbon with 50% of carbon stored in soils and 27% stored in the forest trees. After a fire event, the combustion of soil organic material is the largest source of carbon emissions. Boreal forests are projected to experience an extension of the fire season and an increase in fire size and intensity.

Fire: Good and Bad

Fires are both beneficial and detrimental to the environment, various ecosystems, and human health. Fire management is an essential tool for land management, but even controlled fires can cause substantial damage to infrastructure and resources.

Fires are often necessary for some ecosystems to persist, yet they can destroy whole ecosystems and contribute to climate change. Thus, there is a duality to the human-environment interaction with fire. Due to the vast influence of fire on humans and the environment, it is essential to better understand the effects, patterns, and drivers of fire.

Subsurface Permafrost and the Thaw Depth

Tundra regions are categorized by a vast extent of surface and subsurface permafrost, and this permafrost layer may store more than 1,700 billions tons of carbon, which is larger than atmospheric carbon amounts. Boreal forests also contain a permafrost layer beneath the active layer.

Consequently, understanding the impacts on permafrost from tundra wildfires and the northern migration of boreal forests is crucial to mapping future projections of carbon within the next century.

The thaw depth, or the level down to which soil has warmed to 0°C, and the active layer thickness, which is the maximum seasonal thaw depth, are both heavily influenced by wildfires. Therefore, the relationship between the active layer thickness, the organic layer, and the thaw depth, and their influence on boreal forest/tundra permafrost is an important feature to understand.

Where is the CO2 in the ice from?

The source of carbon stored in permafrost is the frozen remains of plants animals over thousands of years. Due to this buildup of carbon, there is about twice as much carbon in permafrost as in the atmosphere. Therefore, there could be significant greenhouse effects if the permafrost melted. The current projections of climate change rates by Earth system models needs to be further updated to include the scale of positive feedbacks from carbon emissions in northern regions due to permafrost melting.

Mineralization of permafrost soils by microbes occurs over years to decades and releases CO₂ and CH₄, significantly impacting near-term climate warming. The surface permafrost carbon pool, which extends from 0 to 3 meters deep, is 1,035 ± 150 Pg carbon according to the northern permafrost zone carbon inventory.

In comparison, all other biomes on earth, excluding Arctic and boreal regions, are estimated to collectively hold 2,050 Pg carbon from 0-3 meters of soil. Carbon pools deeper than 3 meters likely contain several hundred more picograms of carbon in northern regions, bringing the estimation of the terrestrial permafrost carbon pool to 1,330 ± 1,580 Pg carbon. This does not include carbon sources in permafrost below the Arctic Ocean continental shelves.

Impacts of Climate Change

Climate change, and the resulting rising temperatures, are causing many arctic and boreal regions to have annual temperatures near the freezing point. As a result, permafrost in these regions is very vulnerable to thawing.

As permafrost thaws, it exposes deeper soil deposits of carbon to temperatures above freezing. This initiates decomposition and ultimately the release of soil organic carbon, usually as methane or carbon dioxide, and this carbon enters and adds to the atmospheric carbon stores. This, of course, has significant impacts on the greenhouse effect, contributing to a positive climate warming feedback system.

Projections into the 21^st century suggest a decrease in the organic layer thickness, an increase in the active layer thickness, and a decrease in stored organic carbon. The effects are more pronounced with climate and an increase in fire events acting on regions. This is particularly due to soil carbon being more easily exposed to decomposition following fire events than from just climate warming.

Combining climate and fire effects, organic layer thickness is projected to have decreased 6.5 centimeters by 2100 from levels in 1900, and carbon stocks stored in the soil are projected to lose 9,817 gC m^-2 by 2100.

What happens to the thaw depth and the active layer thickness?

Future climate warming and increased fire activity will cause the soil active layer to deepen and soil temperatures to increase. A deeper active layer increases the amount of short-term nutrients available for vegetation, thereby producing conditions favorable for broadleaf deciduous trees. Thaw depths are increased long-term after area fires. Thaw depths are the same for recently burned areas and for areas burned over 10 years prior, suggesting a long-term or permanent thaw depth reduction.

The active layer is projected to be further deepened by increases in heat advection and in precipitation. The surface organic layer of soil insulates the soil underneath from atmospheric temperature variations, and this mechanism has a large impact of the stability of the permafrost layer. The thickness and moisture content of the organic layer influences how efficient the organic layer is at insulating the soil.

The active layer thickness experiences a greater change in flat uplands and slopes than in flat lowlands due to how well-drained flat uplands and slopes are compared to flat lowlands. The deepest median removal of surface material after a fire occurs in flat lowlands (0.18 meters). Flat lowlands are additionally found not suffer the greatest elevation losses at 0.21 meters.

The removal of the surface organic layer has more impact on permafrost vulnerability and the post-fire ecosystem of boreal forest than the canopy volume loss does. Permafrost melting may further promote long-term transitions in boreal forests from spruce-dominated to deciduous-dominated for tree species composition.

What happens to the forests?

Following a fire event there will be increased competition both for nutrients during the early succession period and for light in the later succession phase . This competition will lead to a shift in northern boreal forests in plant function types, such as evergreen trees, graminoids, deciduous trees, and non-vascular plants.

After a fire, the exposed soil absorbs a higher amount of net energy, which warms the top 0 – 30 centimeters of the soil by about 4.5°C, and this effect lasts for several years. Warmer soil temperatures allow for rapid nitrogen uptake and CO₂ fixation, which is preferable for deciduous trees in predominantly evergreen boreal forests. The expansion of deciduous trees into boreal forests will have ramifications for surface energy fluxes and for the carbon cycle.

Climate change and wildfires will likely promote the northern migration of deciduous trees into traditionally evergreen-dominated boreal forests . Along with this northern migration are various impacts on the surrounding environment and global system, such as soil composition n change, shifting albedo, wildfire fuel availability, carbon sequestration and storage, and alterations in forest canopy effects.

Vegetation recovery after boreal forest wildfires is largely influenced by seed availability, climate, wildfire severity, and the spatial variability of the wildfire. In tundra regions, carbon and nutrients stored in the roots of vegetation below ground are slightly protected during wildfires, possibly contributing to the quick recovery of vegetation. The mechanisms for boreal forest/tundra landcover change and vegetation recovery after wildfires are therefore significant.

…And, what happens to fire regimes?

Land use change, climate change, and direct human interactions, like suppression and ignition, all interact and contribute to current changes in fire regimes. Fires are both beneficial and detrimental to the environment, ecosystems, and human health.

Fire emergence and behavior is difficult to model and predict due to complex shifting variables in fire drivers, such as weather features, potential sources of ignition, and the conditions of potential fuel. Ignition sources are commonly human-caused and initiated naturally by lightning.

Climate and weather influence both the moisture and dryness of fuel and the possible spread areas for fires. Approximately 3% of Earth’s surface experiences fires each year.

Anthropogenic impacts on fire regimes include vegetation disruptions, land use and land use changes, increases in human population, and contributions to climate change. Extreme weather and natural influences on climate change also contribute to the changing fire regimes.

Shifting Forests

Most tundra fires occur in area not burned recently, suggesting that fire regions are migrating further north as conditions for fire progressively become more favorable under climate change. Projections show deciduous trees becoming dominant by 2058 in boreal forests. As graminoid species and deciduous trees replace evergreens, like black spruce, in tundra forests, this change in species composition will produce a larger fuel load. Combined with higher temperatures and increasing dryness, this will favor fire conditions.

Tundra and boreal forest compositions will shift under climate change-induced stressors, such as increased wildfire frequency. In boreal forests there is a projected change from evergreen conifers to deciduous broadleaf trees.

After a tundra fire, the enhanced vegetation index (EVI) dips to its lowest point immediately after the fire but is restored within a decade to pre-fire levels. Within 10-40 years after the fire, the area’s EVI will rise slightly above pre-fire values. Albedo also rapidly recovers after a fire event. Carbon and nutrients stored in roots of vegetation belowground are slightly protected during forest fires, possibly contributing to the quick recovery vegetation after fires.

After severe fires that have spatially consistent intensity, seed sources are generally found further away from the burned area. This has major impacts on post-fire vegetation succession and the overall poster-fire recovery of the ecosystem. Following a fire event, soil moisture is decreased and soil temperature is increased, producing soil conditions that are favorable for deciduous seedlings to take root.

Black spruce trees tend to dominate in boreal forests in areas with flat lowlands and flat uplands. Topography and landscape are influential for canopy loses after a fire event.

Pre-fire species composition is also strongly linked to surface loses and changes in canopy. Pre-fire canopy volume is not strongly indicative of changes in species composition or canopy after a fire event. Instead, the species composition and placement of trees on the landscape are strongly linked to canopy volume in both magnitude and variability.

Following a forest fire, the largest canopy losses occur for black spruce in flat lowlands, which can be expected since black spruce dominate flat lowland areas. The canopy losses and surface layer removal are higher for areas with homogeneous black spruce forest compared to mixed species forests, pointing to a possible negative feedback system once a boreal forest is dominated by deciduous broadleaf trees .

What are the feedback systems?

There are numerous positive and negative feedback systems associated with boreal forest and tundra wildfires. Some of the feedback systems include elements such as albedo, thaw layer depth, permafrost, northern migration of forests, and greater abundance of deciduous trees in boreal forests.

Positive Feedbacks

A positive feedback system occurs as albedo is reduced by charred ground from fire events, which releases carbon stored in permafrost and then releases CO₂ stored in soil stocks.

Fire events increase soil temperatures, increase thaw depth, and increase nutrient availability, which in turn produces a more favorable environment for vegetation, thus encouraging growth. A larger proportion of deciduous trees in a boreal forest would create an additional positive feedback for microbial decomposition and nutrient cycling by producing more leaf litter, which reduces carbon-to-nitrogen ratios.

Negative Feedbacks

A couple of negative feedback systems might occur with the replacement of evergreen by deciduous as the dominant plant functional types in boreal forests.

Deciduous trees increase transpiration more than evergreens, so there would be a significant influence on water vapor and therefore longwave radiative forcing over deciduous-dominated areas. Albedo would also be affected, resulting in a cooling effect from a higher albedo.

Additionally, deciduous trees are less flammable than evergreen trees. Due to this, regions habitually experiencing fire events may actually see a decrease in total burned area, thereby acting to suppress fire under a warmer climate. The strengths of these feedback systems are still not well-known and will depend on future carbon dynamics and fire-vegetation-climate interactions.

Um, can I get a summary of that?

Together climate and wildfire will contribute to providing more favorable conditions for future fires. The impacts of increased frequency and burned area of boreal and tundra fires is evidenced in the northern migration of deciduous trees and replacement of evergreen trees as the dominant species in boreal forests by the mid-21^st century.

Further impacts are seen in the increase in soil temperatures, the deepening of the active layer, the decrease in the organic layer, and the resulting accelerated melting of permafrost. The deepening of the active layer and the melting of permafrost releases substantial amounts of organic carbon into the atmosphere, thus perpetuating the positive feedback system associated with climate warming.

There may be negative feedback systems linked to deciduous-dominated boreal forests, but it is unknown whether these would adequately compensate for the positive feedback systems. A more robust fire model could account for some of the variability in these characteristics.

These factors interact in complicated ways, but it is projected that climate warming and fire intensity will increase by the end of the 21^st century, making it essential to explore and further understand fire dynamics in boreal and tundra regions.

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