Tundra to Boreal forest
The main driver behind the shift from tundra to boreal forest is the increasingly warm climate due to high concentrations of carbon in the atmosphere, allowing pioneer shrubs associated with the boreal forest regime to increase significantly. The actual shift to boreal forest with spruce and pine as the dominant species is unlikely to occur this century due to time lags involved with species migration. Shrub expansion in the Arctic tundra is the first phase of this regime shift, which is reinforced by carbon release due to permafrost degradation, which in turn increases climate warming and microbial activity enhancing shrub growth. Sufficient numbers of herbivores can limit shrub expansion and potentially maintain the shrub state on a long term basis.
Key direct drivers
- Global climate change
- Extensive livestock production (rangelands)
Key Ecosystem Processes
- Primary production
- Nutrient cycling
- Water cycling
- Wild animal and plant products
- Climate regulation
- Aesthetic values
- Knowledge and educational values
- Spiritual and religious
- Food and nutrition
- Livelihoods and economic activity
- Cultural, aesthetic and recreational values
Typical spatial scale
Typical time scale
- Irreversible (on 100 year time scale)
- Contemporary observations
Confidence: Existence of RS
- Speculative – Regime shift has been proposed, but little evidence as yet
Confidence: Mechanism underlying RS
- Well established – Wide agreement on the underlying mechanism
Links to other regime shifts
This regime shift entails extensive change in biodiversity and soil structure in the Arctic tundra due to increased climate warming. To date shrub invasion has been the main indicator of a potential regime shift.
Arctic tundra regime
This regime is characterized by low atmospheric temperatures that enable the formation of a layer of permanently frozen subsoil (permafrost) consisting mostly of gravel and finer material. The ecosystem has a low rate of primary production due to impeded mineral nutrient cycling. The short growing season in summer influences the vegetation and its root systems by limiting vertical expansion. Only plant species (lichens, liverworts, mosses) that are adapted to sweeping winds and soil disturbance can survive in these conditions. The atmospheric cold and existing permafrost restrains the plant growth ensuring that their roots are shallow and surface parts are low growing due to the frozen ground underneath the permafrost. Snow does not accumulate keeping the soil temperature low thus not allowing for permafrost to thaw and release carbon into atmosphere keeping the atmosphere cold.
Boreal forest regime
This regime is characterized by flora that mostly consists of cold-tolerant evergreen conifers, such as the evergreen spruce (Picea), fir (Abies), and pine (Pinus), and the deciduous larch or tamarack (Larix). Before these species are established in a region, there is an increased canopy of early-successional species such as aspen and paper birch (Frelich et al. 1995). Dwarf shrubs (e.g., Vaccinium uliginosum, Ledum decumbens, Betula nana and Cassiope tetragona) and low shrubs (e.g., Betula glandulosa, Salix glauca) act as pioneers that colonize the tundra before trees are able to establish (CAVM Team 2003). The shift to boreal forest can occur over a long time period varying from several decades to centuries depending on the migration time lag for each species and the occurrence of favorable conditions for seedling establishment.
Drivers and causes of the regime shift
The main external driver of this regime shift is CO2 emissions that increase climate warming (IPCC 2007), leading to changes in the composition and abundance of arctic vegetation, animals and soil structure (Olofsson et al. 2009). Anthropogenic activities that elevate atmospheric greenhouse gas concentrations are generally considered to be the primary cause for this driver. Carbon release from anthropogenic sources is projected to continue and increase during the coming decades (IPCC 2007). This global driver is well established and could be looked as irreversible in the scale of next hundred years.
How the regime shift works
The tundra regime is maintained by the low atmospheric temperatures that ensure low soil temperature and the existence of permafrost. This determines the lack of microbial activity and scarce nutrient availability therefore ensuring species composition that are only able to exist in such circumstances.
A shift from the tundra regime to boreal forest is initiated by the atmospheric warming that is caused by continuing increase of CO2 emissions. The atmospheric temperature increases the period of time when microbial activity and thus nutrient availability is sufficient enough for shrubs and boreal forest pioneer species (birch etc.) to withstand the winter conditions.
As a result the increasing number of shrubs ensure that snow drifting creates deep drifts that surround and extend downwind from shrub patches. These drifts increase the insulation of the soil below. This insulation elevates the soil temperature thus further allowing microbial activity to continue during the frigid arctic winter. The elevated soil temperatures under the snow lead to permafrost degradation that release carbon trapped in tundra soils, thereby further contributing to climate warming.
Impacts on ecosystem services and human well-being
Shift from Tundra to Boreal Forest
Tundra provides biodiversity for different animal and plant species that characterize tundra ecosystem. The shift from tundra to boreal forest is projected to occur over large geographic areas throughout the tundra zone, with substantial impacts on the landscape and biodiversity (Bonan 1992). Changes in vegetation are also likely to affect composition of foraging mammals and birds (Hinzman et al. 2005). In addition to modifying wildlife habitat, increased woody shrub abundance will make traversing tundra more difficult for caribou, subsistence hunters and communities that rely on caribou for food, as well as hikers. Herbivores such as reindeers and microtine rodents can also be influenced by the shift towards woody vegetation as they prefer lichens, dwarf shrubs graminoids and deciduous shrubs over tall woody shrubs (Sturm et al. 2005). Increased expansion of woody vegetation may therefore result in tundra species extinctions or radical decrease in numbers. The loss of the climate regulating ecosystem service is a major concern as permafrost thawing is reducing the carbon sink service.
For local populations the tundra provides traditional recreation opportunities and contains their knowledge and educational as well as spiritual and religious values, which would be lost with the regime shift.
Shift from Boreal Forest to Tundra
The change in biodiversity could provide new ecosystem services such as timber production and different wild animals and plant foods. This would benefit loggers as they would increase their income from tree cutting and local communities from goods that come from forest.
In order to prevent or reverse this regime shift it is essential that the input of carbon to the atmosphere is reduced in order to prevent further climate warming. The suggestion of a planetary boundary of 350 ppm for CO2 levels is a first step in this direction (Rockstrom et al. 2009). Increased understanding of the influence of climate change on the complexity of arctic systems is essential for informing the adaptation of social, economic, and cultural systems to the changes taking place in the Arctic (Hinzman et al. 2005). Technology transfer could be a good initiative from developed countries as they can provide more advanced technological solutions to developing countries to help accelerate the learning curve on GHG emissions. A global response particularly from developed nations that are using the majority of the world's resources on a per capita basis should be in place to deal with such complex systems. Nevertheless, discussions about desirable management strategies and outcomes will be essential as there will be stakeholders who stand to lose from this regime shift (e.g. caribou herders) while others may see the change as desirable and beneficial (e.g. timber production, game hunting).
Bonan GB, Pollard D, and Thompson SL. 1992. Effects of Boreal Forest Vegetation on Global Climate. Nature 359,716-18
CAVM Team: Circumpolar Arctic Vegetation. Arctic Portal. http://web.arcticportal.org/uploads/OY/RT/OYRTN1ieqI5IVfPL2U3SqA/side1_031016.pdf. Last visited: 1 Apr. 2010
Chapin III FS. et al. 2005. Role of land-surface changes in Arctic summer warming. Science 310,657-660.
Frelich LE, and Reich PB. 1995. Spatial patterns and succession in a Minnesota southern boreal forest. Ecological Monographs 65(3),325-346.
Hinzman LD, et al. 2005. Evidence and Implications of Recent Climate Change in Northern Alaska and Other Arctic Regions. Climatic Change 72(3).
IPCC 2007. Climate Change 2007: The Physical Science Basis. Cambridge University Press, New York.
Lloyd AH. 2003. Effects of Permafrost Degradation on Woody Vegetation at Arctic Treeline on the Seward Peninsula, Alaska. Permafrost Periglac. Process. 14,93–101.
Myers-Smith I. 2007. Shrub line advance in alpine tundra of the Kluane Region: mechanisms of expansion and ecosystem impacts. Arctic 60(4),447-451.
Olofsson J, Oksanen L, Callaghan T, Hulme PE, Oksanen T, Suominen O. 2009. Herbivores inhibit climate-driven shrub expansion on the tundra. Global Change Biology 15(11),2681 – 2693.
Rockström J. et al. 2009. Planetary Boundaries: Exploring the Safe Operating Space for Humanity. Ecology and Society 14(2),32.
Sturm M, Schimel J, Michaelson G, Welker JM, Oberbauer SF, Liston GE, Fahnestock J, and Romanovsky VE. 2005. Winter biological processes could help convert arctic tundra to shrubland. Bioscience 55(1),17-26.
UCMP: The tundra biome. University of California Museum of Paleontology. http://www.ucmp.berkeley.edu/exhibits/biomes/tundra.php Last visited: 26 Jan. 2010.
Welker JM, Fahnestock JT, Jones MH. 2000. Annual CO, flux from dry and moist arctic tundra: Field responses to increases in summer temperature and winter snow depth. Climatic Change 44(1-2),139-150.
Zimov SA, Schuur EAG, Chapin III SF. 2006. Permafrost and the Global Carbon Budget. Science 312(16),1612-1613.