- The shrub-microbial activity mechanism (regional; well established): This mechanism is perceived to be essential to manage in order to avoid the regime shift to boreal forest. The low temperatures in Arctic regions are maintaining low soil temperatures that results in low microbial activity throughout the seasons. As a result there are few nutrients available in the soil thus affecting shrub growth. Drifting snow is common in the tundra, but without shrub expansion there are few deep drifts and thus avoiding insulation of the soil below. The lack of soil insulation further ensures low soil temperature therefore locking up the feedback mechanism. This mechanism creates a reinforcing feedback that promotes tundra establishment, maintaining the structure and the function of the tundra ecosystems.
- The shrub-surface albedo mechanism (local/regional; well established ): Shrub patches are expanding throughout the Arctic tundra as a result of Arctic warming and the influence of the altered shrub-microbial activity mechanism that is causing an increase in the length of the growing season. The establishment of shrubs and trees have a significant impact on seasonal and annual land surface energy exchange. Their primary effect is to mask the high albedo of snow, but they also partition net radiation into sensible and latent heat in summer months, thus warming the climate even further (Bonan et al. 1995).
- The shrub-microbial activity mechanism (regional; well established): Changes in this mechanism are initially behind the introduction in other feedback mechanism that maintain the new boreal forest regime. The increasing shrub patches in tundra as a result of increasing atmospheric temperatures are causing changes in other processes in this mechanism. Drifting snow is common in the tundra, and deep drifts often surround and extend downwind from shrub patches. The shrub patches therefore help trap and hold the snow, increasing the insulation of the soil below (Sturm et al. 2005). This insulation elevates the soil temperature allowing microbial activity to continue during the frigid arctic winter, producing enough critical nutrients – particularly nitrogen that stimulates shrub growth - to utilize the following summer and further increase shrub abundance (Chapin III 2005). This mechanism creates a reinforcing feedback that promotes shrub establishment, altering both the structure and the function of the tundra ecosystems (Myers-Smith 2007).
- The shrub-permafrost mechanism (regional; well established): This mechanism is closely related to shrub-microbial activity mechanism and is well established. The warming atmospheric temperatures and increasing soil temperatures and shrub expansion are also part of this mechanism. Elevated soil temperatures under the snow lead to permafrost degradation. Thawing permafrost can release carbon trapped in tundra soils, thereby contributing to climate warming which further accelerates the rate of carbon release (Welker et al. 2000).
- The soil drainage-woody vegetation mechanism (regional; contested): Increasing abundance in shrubs and resulting increase in soil temperatures also create areas of improved subsurface drainage due to improved vertical flow of water through the soil. This significantly enhances the establishment of tall woody vegetation in level terrain underlain by permafrost as they require on well-drained soils (Loyd 2003). Reindeer can preserve open heathlands by inhibiting the expansion of shrubs and trees through grazing (Olofsson et al. 2009). Nevertheless the lichen and other tundra plants favored by caribou are gradually being replaced by woody shrubs and trees that are not consumed by caribou. Given sufficient abundance of reindeer and other large herbivores, this suggests the possibility for the transitional shrub state to persist without shifting to successional boreal forest species such as birch and aspen. This mechanism is still contested and must be further studied in order to understand the affect of woody vegetation on the diet of caribou and other factors that affect caribou population.
Tundra to Boreal regime
Important shocks (eg droughts, floods) that contribute to the regime shift include:
- Droughts (local/regional, contested): Dry conditions in Arctic is directly having an effect on this regime as drying of tundra soils in parts of Alaska have already changed the carbon status of this area from sink to source (Callaghan et al. 2005).
The main external direct drivers that contribute to the shift include:
- Greenhouse gas emissions (global, well established): This driver is well established and initially affects the shrub-microbial activity mechanism by increasing the soil temperature as the time period of warmer atmospheric temperatures increase, thus enhancing microbial activity. Carbon release from both anthropogenic and natural sources (due to permafrost degradation) is projected to continue and increase (IPCC 2007), which will lead to further climate warming and more rapid expansion of woody vegetation.
Slow internal system changes that contribute to the regime shift include:
- Atmospheric temperatures (global; well established): This variable mainly affects ice volume variations throughout the year and in case of continuous depletion of Arctic sea ice it triggers various mechanisms that maintain the new regime of Arctic without summer sea ice.
- Carbon accumulation
Summary of Drivers
|#||Driver (Name)||Type (Direct, Indirect, Internal, Shock)||Scale (local, regional, global)||Uncertainty (speculative, proposed, well-established)|
|1||Green house gases||Direct||Global||Well-established|
Tundra to Boreal forest regime
- Atmospheric temperature: threshold at which thermal balance is established for promoting permafrost depleting conditions, high microbial activity and nutrient availability in soil throughout the seasons.
- Soil temperature: threshold at which the microbial activity and nutrient availability in soil is sufficient for the invasion of shrubs and other pioneer species of boreal forest.
Boreal forest to Tundra regime
- Atmospheric temperature (global, well established): threshold at which thermal balance is established for promoting permafrost enhancing conditions and low microbial activity and nutrient availability in soil throughout the seasons.
- Soil temperature (regional, well established): threshold at which the microbial activity and nutrient availability in soil is insufficient for the invasion of shrubs and other pioneer species of boreal forest.
- Atmospheric CO2: Both atmospheric and soil temperature could be manage by reducing CO2 emissions. It is crucial to manage increasing atmospheric temperatures as this event is altering the other processes that occur in this regime shift. Soil temperatures on the other hand result in changes in soil structure and permafrost degradation.
- Low albedo (local/regional, well established) - increases absorption of solar radiation thus adding to the temperature increase. Thus increasing albedo by using bright materials that reflect solar radiation and covering rooftops and dark surfaces (cliffs etc.) could increase albedo reflecting more solar radiation therefore cooling the surface temperature.
Ecosystem service impacts
Ecosystem services associated with the tundra regime are biodiversity and climate regulation. Tundra is also ensuring the provision of typical wild animal and plant foods. Tundra can contribute to knowledge and educational as well as aesthetic values. It also embeds spiritual and religious values.
Summary of Ecosystem Service impacts on different User Groups
||References (if available)|
|Feed, Fuel and Fibre Crops|
|Wild Food & Products|
|Air Quality Regulation|
|Soil Erosion Regulation|
|Pest & Disease Regulation|
|Protection against Natural Hazards|
|Cognitive & Educational|
|Spiritual & Inspirational|