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Arctic Tundra and Polar Desert Ecosystems

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Arctic Tundra and Polar Desert Ecosystems Chapter 7 Arctic Tundra and Polar Desert Ecosystems Lead Author Terry V.Callaghan Contributing Authors Lars Olof Björn, F. Stuart Chapin III,Yuri Chernov,Torben R. Christensen, Brian Huntley, Rolf Ims, Margareta Johansson, Dyanna Jolly Riedlinger, Sven Jonasson, Nadya Matveyeva,Walter Oechel, Nicolai Panikov, Gus Shaver Consulting Authors Josef Elster, Heikki Henttonen, Ingibjörg S. Jónsdóttir, Kari Laine, Sibyll Schaphoff, Stephen Sitch, Erja Taulavuori, Kari Taulavuori, Christoph Zöckler Contents Summary . .244 7.4. Effects of changes in climate and UV radiation levels on 7.1. Introduction . .244 structure and function of arctic ecosystems in the short 7.1.1. Characteristics of arctic tundra and polar desert ecosystems . .244 and long term . .292 7.1.2. Raison d’être for the chapter . .247 7.4.1. Ecosystem structure . .292 7.1.3. Rationale for the structure of the chapter . .248 7.4.1.1. Local and latitudinal variation . .292 7.1.4.Approaches used for the assessment: strengths, limitations, 7.4.1.2. Response to experimental manipulations . .295 and uncertainties . .248 7.4.1.3. Recent decadal changes within permanent plots . .298 7.2. Late-Quaternary changes in arctic terrestrial ecosystems, 7.4.1.4.Trophic interactions . .298 climate, and ultraviolet radiation levels . .249 7.4.1.5. Summary . .303 7.2.1. Environmental history . .249 7.4.2. Ecosystem function . .305 7.2.2. History of arctic biota . .250 7.4.2.1. Biogeochemical cycling: dynamics of carbon and 7.2.3. Ecological history . .252 nutrients . .305 7.2.4. Human history related to ecosystems . .252 7.4.2.2. Soil processes and controls over trace-gas exchanges . .311 7.2.5. Future change in the context of late-Quaternary changes . .253 7.4.2.3.Water and energy balance . .314 7.2.6. Summary . .254 7.4.2.4. Summary . .314 7.3. Species responses to changes in climate and ultraviolet-B 7.5. Effects of climate change on landscape and regional radiation in the Arctic . .254 processes and feedbacks to the climate system . .315 7.3.1. Implications of current species distributions for future biotic 7.5.1. Impacts of recent and current climate on carbon flux . .315 change . .255 7.5.1.1. Recent changes in carbon dioxide fluxes . .316 7.3.1.1. Plants . .255 7.5.1.2. Current circumpolar methane fluxes . .317 7.3.1.2.Animals . .259 7.5.1.3. Relative contributions of methane and carbon dioxide 7.3.1.3. Microorganisms . .260 to the carbon budget . .317 7.3.1.4. Summary . .262 7.5.2. Current circumpolar water and energy balances . .318 7.3.2. General characteristics of arctic species and their adaptations 7.5.3. Large-scale processes affecting future balances of carbon, in the context of changes in climate and ultraviolet-B radiation water, and energy . .319 levels . .263 7.5.3.1. Permafrost degradation . .319 7.3.2.1. Plants . .263 7.5.3.2. Changes in circumpolar vegetation zones . .319 7.3.2.2.Animals . .264 7.5.4. Projections of future balances of carbon, water, and energy 7.3.2.3. Microorganisms . .269 exchange . .321 7.3.3. Phenotypic responses of arctic species to changes in climate 7.5.4.1. Carbon balance . .322 and ultraviolet-B radiation . .271 7.5.4.2. Energy and water exchange . .324 7.3.3.1. Plants . .271 7.5.5. Summary . .324 7.3.3.2.Animals . .279 7.6. Synthesis: Scenarios of projected changes in the four ACIA 7.3.3.3. Microorganisms . .282 regions for 2020, 2050, and 2080 . .327 7.3.4. Genetic responses of species to changes in climate and 7.6.1. Environmental characteristics . .327 ultraviolet-B radiation levels . .284 7.6.2.Vegetation zones and carbon balance . .327 7.3.4.1. Plants . .284 7.6.3. Biodiversity . .328 7.3.4.2.Animals . .285 7.7. Uncertainties and recommendations . .329 7.3.4.3. Microorganisms . .286 7.7.1. Uncertainties . .329 7.3.4.4. Summary . .286 7.7.1.1. Uncertainties due to methodologies and conceptual 7.3.5. Recent and projected changes in species distributions and frameworks . .329 potential ranges . .287 7.7.1.2. Uncertainties due to surprises . .331 7.3.5.1. Recent changes . .287 7.7.1.3. Model-related uncertainties . .331 7.3.5.2. Projected future changes in species distributions . .287 7.7.2. Recommendations to reduce uncertainties . .332 7.3.5.3. Summary . .291 7.7.2.1.Thematic recommendations and justification . .332 7.7.2.2. Recommendations for future approaches to research and monitoring . .333 7.7.2.3. Funding requirements . .334 Acknowledgements . .335 Personal communications and unpublished data . .335 References . .335 244 Arctic Climate Impact Assessment Summary environmental, geographic, or political biases.This chapter focuses on biota (plants, animals, and micro- The dominant response of current arctic species to cli- organisms) and processes in the region north of the mate change, as in the past, is very likely to be relocation northern limit of the closed forest (the taiga), but also rather than adaptation. Relocation possibilities vary includes processes occurring south of this boundary that according to region and geographic barriers. Some affect arctic ecosystems. Examples include animals that changes are occurring now. migrate south for the winter and the regulation of the latitudinal treeline.The geographic area defined in this Some groups such as mosses, lichens, and some herbivores chapter as the present-day Arctic is the area used for and their predators are at risk in some areas, but produc- developing scenarios of future impacts: the geographic tivity and number of species is very likely to increase. area of interest will not decrease under a scenario of Biodiversity is more at risk in some ACIA regions than in replacement of current arctic tundra by boreal forests. others: Beringia (Region 3) has a higher number of threat- ened plant and animal species than any other ACIA region. 7.1.1. Characteristics of arctic tundra and polar desert ecosystems Changes in populations are triggered by trends and extreme events, particularly winter processes. The southern boundary of the circumpolar Arctic as defined in this chapter is the northern extent of the closed Forest is very likely to replace a significant proportion of boreal forests (section 14.2.3).This is not a clear bound- the tundra and this will have a great effect on the compo- ary but a transition from south to north consisting of the sition of species. However, there are environmental and sequence: closed forest, forest with patches of tundra, sociological processes that are very likely to prevent for- tundra with patches of forest, and tundra.The transition est from advancing in some locations. zone is relatively narrow (30 to 150 km) when compared to the width of the forest and tundra zones in many, but Displacement of tundra by forest will lead to a decrease in not all areas. Superimposed on the latitudinal zonation of albedo, which will increase the positive feedback to the forest and tundra is an altitudinal zonation from forest to climate system.This positive feedback is likely to dominate treeless areas to barren ground in some mountainous over the negative feedback of increased carbon sequestra- regions of the northern taiga.The transition zone from tion. Forest development is very likely to also ameliorate taiga to tundra stretches for more than 13400 km around local climate, for example, by increasing temperature. the lands of the Northern Hemisphere and is one of the most important environmental transition zones on Earth Warming and drying of tundra soils in parts of Alaska have (Callaghan et al., 2002a,b) as it represents a strong tem- already changed the carbon status of this area from sink to perature threshold close to an area of low temperatures. source.Although other areas still maintain their sink sta- The transition zone has been called forest tundra, sub- tus, the number of source areas currently exceeds the arctic, and the tundra–taiga boundary or ecotone. number of sink areas. However, geographic representation The vegetation of the transition zone is characterized by of research sites is currently small. Future warming of an open landscape with patches of trees that have a low tundra soils is likely to lead to a pulse of trace gases into stature and dense thickets of shrubs that, together with the atmosphere, particularly from disturbed areas and the trees, totally cover the ground surface. areas that are drying. It is not known if the circumpolar tundra will be a carbon source or sink in the long term, The environmental definition of the Arctic does not but current models suggest that the tundra is likely to correspond with the geographic zone delimited by the become a weak sink for carbon because of the northward Arctic Circle (66.5º N), nor with political definitions. movement of vegetation zones that are more productive Cold waters in ocean currents flowing southward from than those they displace. Uncertainties are high. the Arctic depress the temperatures in Greenland and the eastern Canadian Arctic whereas the northward-flowing Rapid climate change that exceeds the ability of species Gulf Stream warms the northern landmasses of Europe to relocate is very likely to lead to increased incidence of (section 2.3).Thus, at the extremes, polar bears and fires, disease, and pest outbreaks. tundra are found at 51º N in eastern Canada whereas agriculture is practiced north of 69º N in Norway. Enhanced carbon dioxide concentrations and ultraviolet-B Arctic lands span some 20º of latitude, reaching 84º N in radiation levels affect plant tissue chemistry and thereby Greenland and locally, in eastern Canada, an extreme have subtle but long-term impacts on ecosystem processes southern limit of 51º N.
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