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Effects of Global Warming
Average global temperature change per decade over the past 50 years has increased 0.13oC. Arctic regions have increased even greater with an average rate of 0.40oC per decade.[15] Some areas of the tundra have experienced greater than 10oC changes compared to the temperature 50 years ago.[15] Temperatures in the Arctic have reached their highest state in 2000 years, and are not slowing down.[15][16]

Even though plants and animals can tolerate extreme climate conditions found in the tundra, the smallest disruption in equilibrium can greatly change the fragile environment. Stresses caused by human impacts are enough to heavily alter the ecosystems found in the biome. Temperature increases have a direct effect on daily freeze-thaw processes, partial melting, permafrost thawing, and carbon dioxide, methane and nitrogen levels.[17][18][19][20][21] Increases in these processes lead to a change in plant vitality and vegetation communities.

Physical Changes
Warmer air increases the canopy temperature leading to the melting of snow and ice found under the canopy.[17][22][23][24][25] The cryosphere decreased by ~25% in area and ~20% in volume in tundra environments over the past 50 years.[26] Soil temperature is projected to increase, melting the ground cover and producing wet and humid, marsh-like summers.[21][22][23] Greater presence of liquid water increases evaporation putting moisture back into the atmosphere.[26] Precipitation has increased 10% over the past 50 years in the tundra and will continue to increase.[26]

Melting the top layer of protective snow and permafrost exposes vegetation, releases trapped CO2 and methane, and allows nutrients and minerals that were once immobile in the solid ice lattice to flow. [17][22][23][24][25] Vascular plants and non-vascular plants will change drastically from global warming in both quantity and distribution.[17][18][19][20][21][23][25][26][27]

Vascular Plants
Vascular plants contain a xylem and phloem and can take advantage of mineralization. More minerals in the soil increase growth and sexual reproduction. [27] Compared to non-vascular plants they break dormancy slower so they will not die from short winter warming events.

Shrubs
Shrubs have a positive response to warming because they are resilient.[19][20][21][23][26][27] Different species of shrubs will respond individualistically to climate change, favoring some species over others. [23][27] Higher temperatures lead to wider shrubs that consequently can collect more snow. [15][19] A thicker snowpack increases the temperature in shrub patches, having a positive feedback on the overall shrub community.[19]

Future projections predict taller shrubs favored over dwarf shrubs because they can grow upward absorbing more sunlight.[23][27] Once mineralization occurs deciduous shrubs are favored over evergreen shrubs because of their ability to utilize minerals more efficiently and enter dormancy. [23][26][27]

Short periods of extreme warming led to a loss in snow cover initiating spring-like responses in evergreens.[25] Once the temperature dropped after the warming period the ground was less insulated, damaging roots, shoots, and buds.[19][24][27] Deciduous shrubs responded slower to melting, so they did not suffer as much damage as evergreens.[25]

Non-Vascular Plants
Increases in moisture will cause lichen and bryophytes population to decrease because of enhanced plant growth for shrubs and grasses. [17][18][19][20][21][23][25][26][27] Non-vascular plants cannot utilize minerals like vascular plants, so they will be outcompeted. An increase in freeze thaw cycles, lichens and bryophytes will be reactivated from dormancy too early leading to extensive plant damage and even death.[17][18][23][24][25]

Lichens
Lichens are found throughout the tundra and are important for animals and nitrogen inputs. Lichens can tolerate extreme dry cold temperatures and can carry out photosynthesis at -20oC.[18] They are made up of two parts: a fungus (mycobiont) and a photosynthetic partner (photobiont).[18] However, even though lichens are thought to be temperature resistant, moisture plays a crucial role in vitality and growth.[18][26] Moisture promoted extracellular freezing and anaerobic respiration in multiple lichen studies, resulting in death.[18][26] Anaerobic respiration occurred producing CO2, ethanol, and lactic acid. Accumulation of these by-products in the cells killed the lichens.[18]

Bryophytes
Mosses, liverworts, and hornworts make up the classes of plants found in the Bryophyta phylum.[17] Bryophytes influence the ecosystem through the carbon, nitrogen, and water cycle.[17] They thrive in humid climates, but are negatively affected by rising temperature.[17][21][23][27] Bryophytes have shown expedited growth in areas that were warm and wet, compared to those that were warm and dry.[21][27] However, when comparing hot to cold areas bryophytes flourished in the colder regions.[21] Short intense winter warming cycles melted the insulated snow layer, heavily damaging the plants.[17][21][23][27]  Even though they respond positively to an increase in humidity, experiments indicate in a natural environment they will be outcompeted by shrubs and vascular plants, decreasing as warming occurs.[17][21][23][27]

Table 1: Summary of how shrubs, lichens and bryophytes responded/will respond to changes in the biome. [15][17][18][19][20][21][23][24][25][26][27][28]

Future for Tundra
Global warming will affect the tundra biome first, making the tundra a bellwether of climate change.[16][22][23][24] Predictions for the tundra include a shift towards vascular plants and decline of non-vascular species. There will be exceptions toward the general trend since climates within the tundra biome vary slightly and species respond individually to changes in their environment. Ultimately plants and vegetations with the highest resistance and ability to adapt will survive and outcompete others. Species richness throughout the tundra will decrease because of bottle-necking and competition.[27] Changes in temperature will not just affect the plants and landscape, but the herbivores and people living in the area too.[18][23][25] Multiple ecosystems will undergo change in the upcoming centuries and even decades, including the tundra. Studying the tundra vegetation now, and its changes over the next few decades, will help humans understand the long-term effects of global warming for life on earth. [21][26]

References:

15. ^ Pieper S, Loewen V, Gill M, Johnstone J.BioOne. Plant Responses to Natural and Experimental Variations in Temperature in Alpine Tundra, Southern Yukon, Canada. Arctic, Antarctic and Alpine Research, 43(3):442-456. 2011

16. ^ University of California Museum of Paleontogloy. The tundra biome. February 10, 2012. Available from: http://www.ucmp.berkeley.edu/exhibits/biomes/tundra.php

17. ^ Bjerke J, Bokhorst S, Zielke M, Callaghan T, Bowles F, Phoenix G. Contrasting sensitivity to extreme winter warming events of dominant sub-Artic heathland bryophyte and lichen species. Journal of Ecology, 99: 1481-1488. 2011.

18. ^ Bjerke, J. Winter climate change: Ice encapsulation at mild subfreezing temperatures kills freeze-tolerance lichens. Environmental and Experimental Botany, 72:404-408. 2011.

19. ^ Blok D, Sass-Klaassen U, Schaepman-Strub G, Heijmans M.M.P.D., Sauren P, Berendse F. What are the main climate drivers for shrub growth in Northeastern Siberian tundra? Biogeosciences, 8: 1169-1179. 2011.

20. ^ Nybakken L, Sandvik SM, Klanderud K. Environmental warming had little effect on carbon-based secondary compounds, carbon and nitrogen in selected alpine plants and lichens. Environmental and Experimental Botany, 72: 368-376. 2011.

21. ^ Hudson J, Henry G. High Arctic plant community resists 15 years of experimental warming. Journal of Ecology, 98: 1035-1041. 2010.

22. ^ Whitney S. Tundra. February 10, 2012. Available from: http://www.blueplanetbiomes.org/tundra.htm

23. ^ Elmendorf S, Henry G, Hollister R, Bjork R, Bjorkman A, Callaghan T, Collier L, Cooper E, Cornelissen J, Day T. Global assessment of experimental climate warming on tundra vegetation: heterogeneity over space and time. Ecology Letters, 15: 164-175. 2012.

24. ^ Permentier F. J. W, Van der Molen M. K., Van Huissteden J, Karsanaev S. A., Kononov A. V., Suzdalov D. A., Maximov T.C., Dolman A.J. Longer growing season do not increase net carbon uptake in northeastern Siberian tundra. Journal of Geophysical Research, 116: 1-11. 2011.

25. ^ Bokhorst S, Bjerke J. W., Street L. E., Callaghan T. V., Phoenix G. K.. Impacts of multiple extreme winter warming events on sub-Arctic heathland: phenology, reproduction, growth and CO2 flux responses. Global Change Biology 17: 2817-2830. 2011.

26. ^ Callaghan T, Tweedie C, Akerman, J. Multi-Decadal Changes in Tundra Environments and Ecosystems: Synthesis of the International Polar Year-Back to the Furture Project (IPY-BTF). Ambio: A Journal of the Human Environment, 40(6):555-557. 2011.

27. ^ Moulton C, Gough L. Effects of Soil Nutrient Availability on the Role of Sexual Reproduction in an Alaskan Tundra Plant Community. Arctic, Antarctic and Alpine Research, 43 (4): 612-620. 2011. ''