Introduction to Arctic Tundra and Polar Desert Ecosystems
This is Section 7.1 of the Arctic Climate Impact Assessment
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
The Arctic is generally recognized as a treeless wilderness with cold winters and cool summers. However, definitions of the southern boundary vary according to environmental, geographic, or political biases. This chapter focuses on biota (plants, animals, and microorganisms) and processes in the region north of the northern limit of the closed forest (the taiga), but also includes processes occurring south of this boundary that affect arctic ecosystems. Examples include animals that migrate south for the winter and the regulation of the latitudinal treeline. The geographic area defined in this chapter as the present-day Arctic is the area used for developing scenarios of future impacts: the geographic area of interest will not decrease under a scenario of replacement of current arctic tundra by boreal forests.
Characteristics of Arctic Tundra and Polar Desert Ecosystems (7.1.1)
The southern boundary of the circumpolar Arctic as defined in this chapter is the northern extent of the closed boreal forests (section 14.2.3).This is not a clear boundary but a transition from south to north consisting of the sequence: closed forest, forest with patches of tundra, tundra with patches of forest, and tundra.The transition zone is relatively narrow (30 to 150 km) when compared to the width of the forest and tundra zones in many, but not all areas. Superimposed on the latitudinal zonation of forest and tundra is an altitudinal zonation from forest to treeless areas to barren ground in some mountainous regions of the northern taiga.The transition zone from taiga to tundra stretches for more than 13,400 km around the lands of the Northern Hemisphere and is one of the most important environmental transition zones on Earth as it represents a strong temperature threshold close to an area of low temperatures. The transition zone has been called forest tundra, subarctic, and the tundra–taiga boundary or ecotone. The vegetation of the transition zone is characterized by an open landscape with patches of trees that have a low stature and dense thickets of shrubs that, together with the trees, totally cover the ground surface.
The environmental definition of the Arctic does not correspond with the geographic zone delimited by the Arctic Circle (66.5º N), nor with political definitions. Cold waters in ocean currents flowing southward from the Arctic depress the temperatures in Greenland and the eastern Canadian Arctic whereas the northward-flowing Gulf Stream warms the northern landmasses of Europe (Marine Arcticsection 2.3).Thus, at the extremes, polar bears and tundra are found at 51º N in eastern Canada whereas agriculture is practiced north of 69º N in Norway.
Arctic lands span some 20º of latitude, reaching 84º N in Greenland and locally, in eastern Canada, an extreme southern limit of 51º N.
The climate of the Arctic is largely determined by the relatively low solar angles with respect to the earth. Differences in photoperiod between summer and winter become more extreme toward the north. Beyond the Arctic Circle, the sun remains above the horizon at midnight on midsummer’s day and remains below the horizon at midday on midwinter’s day.
Climatically, the Arctic is often defined as the area where the average temperature for the warmest month is lower than 10 ºC , but mean annual air temperatures vary greatly according to location, even at the same latitude (see Chapter 2).They vary from -12.2 ºC at Point Barrow, Alaska (71.3º N) to -28.1 ºC at the summit of the Greenland Ice Sheet (about 71º N) and from 1.5 ºC at 52º N in subarctic Canada to 8.9 ºC at 52º N in temperate Europe.The summer period, or period of most biological activity, progressively decreases from about 3.5 to 1.5 months from the southern boundary of the Arctic to the north, and mean July temperature decreases from 10–12 ºC to 1.5 ºC. In general, annual precipitation in the Arctic is low, decreasing from about 250 mm in southern areas to as low as 45 mm in the northern polar deserts, with extreme precipitation amounts in subarctic maritime areas (e.g., 1100 mm at 68º N in Norway). However, owing to low rates of evaporation the Arctic cannot be considered arid: even in the polar deserts, air humidity is high and the soils are moist during the short growth period. In the Arctic context, “desert” refers to extreme poverty of life.
The Arctic is characterized by the presence of continuous permafrost (section 6.6.1), although there are exceptions such as the Kola Peninsula. Continuous and deep (>200 m) permafrost also exists south of the treeline in large areas of Siberia extending south to Mongolia. The depth of the active (seasonally frozen) layer of the soil during the growing season depends on summer temperatures and varies from about 80 cm near the treeline to about 40 cm in polar deserts. However, active-layer depth varies according to local conditions within landscapes according to topography: it can reach 120 cm on south-facing slopes and be as little as 30 cm in bogs even in the southern part of the tundra zone. In many areas of the Arctic, continuous permafrost occurs at greater depths beneath the soil surface and degrades into discontinuous permafrost in the southern part of the zone. Active-layer depth, the extent of discontinuous permafrost, and coastal permafrost are very likely to be particularly sensitive to climatic warming (section 6.6). Permafrost and active-layer dynamics lead to topographic patterns such as polygons in the landscape. Topography plays an important role in defining habitats in terms of moisture and temperature as well as active-layer dynamics, such that arctic landscapes are a mosaic of microenvironments. Topographic differences of even a few tens of centimeters (e.g., polygon rims and centers) are important for determining habitats, whereas larger-scale topographic differences (meters to tens of meters) determine wind exposure and snow accumulation that in turn affect plant communities and animal distribution.Topographic differences become more important as latitude increases.
Disturbances of ecosystems are characteristic of the Arctic. Mechanical disturbances include thermokarst induced by permafrost thaw (section 6.6.1); freeze–thaw processes; wind, sand, and ice blasts; seasonal ice oscillations; slope processes; snow load; flooding during thaw; changes in river volume; and coastal erosion and flooding. Biological disturbances include insect pest outbreaks, peaks of grazing animals that have cyclic populations, and fire.These disturbances operate at various spatial and temporal scales (Fig. 7.1) and affect the colonization and survival of organisms and thus ecosystem development.
Arctic lands are extensive beyond the northern limit of the tundra–taiga ecotone, encompassing an area of approximately 7,567,000 square-kilometers (km2), including about 2,560,000 km2 in the former Soviet Union and Scandinavia, 2480000 km2 in Canada, 2167000 km2 in Greenland and Iceland, and 360000 km2 in Alaska. Figure 7.2 shows the distribution of arctic and other vegetation types based on a classification by Walker and mapped by Kaplan et al..The distribution of arctic landmasses is often fragmented: seas separate large arctic islands (e.g., Svalbard, Novaya Zemlya, Severnaya Zemlya, New Siberian Islands, and Wrangel Island) and the landmasses of the Canadian Archipelago and Greenland. Similarly, the Bering Strait separates the arctic lands of Eurasia and North America. Large mountains such as the east–west running Brooks Range in Alaska and the Putorana Plateau in Siberia separate tundra and taiga. Such areas of relief contain outposts of boreal species on their southern major slopes that are likely to expand northward and higher-elevation areas that are likely to act as refuges for arctic-alpine species.The Taymir Peninsula is the only continuous landmass that stretches 900 km from the northern tundra limit to taiga without geographic barriers to the dispersal of animals and plants. The width of the tundra zone varies greatly in different parts of its circumpolar distribution. On average, it does not exceed 300 km, and in some regions (e.g., the lower reaches of the Kolyma River), the tundra zone extends only 60 km from the treeline to the coast. In such areas, the tundra zone is very likely to be highly vulnerable to climate warming.
The vegetation of the Arctic varies from forest tundra in the south, where plant communities have all the plant life forms known in the Arctic and have continuous canopies in several layers extending to more than 3 m high, to polar deserts in the north, where vegetation colonizes 5% or less of the ground surface, is less than 10 cm high, and is dominated by herbs, lichens (symbionts of algae and fungi), and mosses (Fig. 7.3). Species richness in the Arctic is low and decreases toward the north: there are about 1800 species of vascular plants, 4000 species of cryptogams, 75 species of terrestrial mammals, 240 species of terrestrial birds, 3000 species of fungi, 3300 species of insects, and thousands of prokaryotic species (bacteria and Archaea) whose diversity in the tundra has only recently started to be estimated. However, the Arctic is an important global pool of some groups such as mosses, lichens, springtails (and insect parasitoids) because their abundance in the Arctic is higher than in other biomes. Net primary production (NPP), net ecosystem production (NEP), and decomposition rates are low. Food chains are often short and typically there are few representatives at each level of the chain. Arctic soils are generally shallow and underdeveloped with low productivity and immature moor-type humus. Substantial heterogeneity of the soil cover, owing to numerous spatial gradients, has an important influence on the microtopographical distribution of the soil biota (invertebrates, fungi, and bacteria) that will possibly amplify any negative effects of climate change.
The Arctic has a long history of human settlement and exploitation, based initially on its rich aquatic biological resources and more recently on its minerals and fossil hydrocarbons. At the end of the last glacial stage, humans migrated from Eurasia to North America across the icefree Bering land bridge and along the southern coast of Beringia (ca. 14000–13500 years BP). As early as about 12200 years BP, areas north of the Fennoscandian Ice Sheet in northernmost Finnmark (Norway) had been settled. Even earlier Paleolithic settlements (ca. 40000 years BP) have been recorded in the eastern European Arctic. The impacts of these peoples on terrestrial ecosystems are difficult to assess but were probably small given their small populations and “hunter-gatherer” way of life.The prey species hunted by these peoples included the megafauna, such as the woolly mammoth, which became extinct.The extent to which hunting may have been principally responsible for these extinctions is a matter of continuing debate but this possibility cannot be excluded. It is also uncertain to what extent the extinction of the megafauna may have contributed to, or been at least partly a result of, the accelerated northward movement of trees and shrubs and consequent changes in vegetation structure (section 7.2). Although estimates of the population density of megafaunal species have large uncertainties, it seems unlikely that megafaunal populations were sufficient to constrain the spread of woody taxa in response to favorable environmental change.
During the last 1000 years, resources from terrestrial ecosystems have been central to the mixed economies of the Arctic: many inland indigenous communities still derive most of their protein from subsistence activities such as caribou/reindeer hunting. During this period, increasing trade between peoples of temperate latitudes and arctic indigenous peoples is likely to have affected a few target animal species, such as the reindeer that was domesticated in Fennoscandia and Russia, ermine hunted for fur, and birds of prey used for hunting as far away as the eastern Mediterranean. However, the most dramatic impacts occurred after World War II as a result of the exploitation of minerals and oil and fragmentation of the arctic landscape by infrastructure. Vlassova suggested that industrial activities and forestry have displaced the Russian forest tundra southward by deforesting 470000 to 500000 km2 of land that now superficially resembles tundra. Although this estimate has been challenged as greatly exaggerated (because northern taiga areas have been included in that estimate’s definition of forest tundra), such effects have occurred locally in the Yamal Peninsula and the estimate highlights a need for reappraisal. Knowledge of possible past interactions between humans and the environment that may have shaped present-day arctic ecosystems is limited, but shows that any future increases in population density and human activity are likely to modify the projected responses of arctic ecosystems to changes in climate and ultraviolet (UV) radiation levels.
Raison d’être for the chapter (7.1.2)
The Arctic is experiencing dramatic environmental changes that are likely to have profound impacts on arctic ecosystems.The Arctic is outstanding among global biomes in that climate change dominates the major factors affecting biodiversity. Present-day arctic biota are also relatively restricted in range and population size compared with their Quaternary situation. For example, when the treeline advanced northward during the early Holocene warming, a lowered sea level allowed a belt of tundra to persist around the Arctic Basin, whereas any future northward migration of the treeline is very likely to further restrict tundra areas because sea level is projected to rise. Arctic ecosystems are known to be vulnerable to disturbances and to have long recovery times: subarctic birch forest defoliated by insects can take 70 years to recover. Current and projected environmental changes are likely to create additional stresses and decrease the potential for ecosystem recovery from natural disturbances, while providing thresholds for shifts to new states (e.g., disturbance opening gaps for invasion of species new to the Arctic).
Changes in arctic ecosystems and their biota are important to arctic residents in terms of food, fuel, and culture (Chapter 12) and are likely to have global impacts because of the many linkages between the Arctic and more southerly regions. Several hundreds of millions of birds migrate to the Arctic each year and their success in the Arctic determines their success and impacts at lower latitudes (section 188.8.131.52). Physical and biogeochemical processes in the Arctic affect atmospheric circulation and the climate of regions outside of the Arctic (section 7.5). It is known that ecosystems have responded to past environmental changes (section 7.2) and that environmental changes are presently occurring in the Arctic.This understanding indicates that there are very likely to be responses of arctic ecosystems to projected future and ongoing climate change. It is also known that current levels of ultraviolet-B (UV-B) radiation, as well as higher levels, can affect subarctic plants. Arctic plants may be particularly sensitive to increases in UV-B irradiance because UV-B radiation damage is not dependent on temperature whereas enzyme-mediated repair of DNA damage could be constrained by low temperatures.
For all of these reasons, understanding the relationships between ecosystems and the arctic environment is important. Although many aspects of its environment are changing concurrently (e.g., climate, pollution, atmospheric nitrogen deposition, atmospheric concentrations of carbon dioxide (CO2), UV-B radiation levels, and land use), the specific mission of this chapter is to focus on the impacts of changes in climate and UV-B radiation levels on arctic terrestrial ecosystems and their species and processes.
Rationale for the structure of the chapter (7.1.3)
The effects of climate are specific to species, the age and developmental stages of individuals, and processes from metabolism to evolution (Fig. 7.1). Although there are many ways in which to organize an assessment of climate and UV-B radiation impacts, this chapter follows a logical hierarchy of increasing organizational biological complexity to assess impacts on species, the structure of ecosystems, the function of ecosystems, and landscape and regional processes. A basic understanding of biological processes related to climate and UV-B radiation is required before the impacts of changes in these factors on terrestrial ecosystems can be assessed. Consequently, this chapter progresses from a review of climate and UV radiation controls on biological processes to an assessment of the potential impacts of changes in climate and UV-B radiation levels on processes at the species and regional levels. Some effects of climate change on ecosystems may be beneficial to humans, while others may be harmful.
The changes in climate and UV-B radiation levels that are used in this chapter to assess biological impacts are of two types: those already documented (section 2.6) and those projected by scenarios of future change in UV-B radiation levels (section 5.7) and climate (section 4.4) derived from models. Mean annual and seasonal temperatures have varied considerably in the Arctic since 1965. Mean annual temperatures in western parts of North America and central Siberia have increased by about 1 ºC (up to 2 ºC in winter) per decade between 1966 and 1995 while temperatures in West Greenland and the eastern Canadian Arctic have decreased by 0.25 to 1 ºC per decade. Over a longer period, from 1954 to 2003, the annual increase and decrease in temperatures have been slightly less: about 2 to 3 ºC for the whole period (Chapter 1, Fig. 1.3). Temperature increases in Fennoscandia over the past century have been small, ranging from about 1 ºC in the west to near 0 ºC in the east.
Precipitation has also changed.The duration of the snow-free period at high northern latitudes increased by 5 to 6 days per decade and the week of the last observed snow cover in spring advanced by 3 to 5 days per decade between 1972 and 2000 . Stratospheric ozone has been depleted over recent decades (e.g., by a maximum of 45% below normal over the high Arctic in spring).This has probably led to an increase in surface UV-B radiation levels in the Arctic, although the measurement period is short (section 5.5). Scenarios of future change project that mean annual temperatures in the Arctic will increase by nearly 4 ºC by 2080 (section 4.4.2) and that spring (April) UV-B radiation levels will increase by 20 to 90% in much of the Arctic by 2010–2020.
The assessment of impacts on terrestrial ecosystems presented in this chapter is based on existing literature rather than new research or ACIA modeling activities. Existing long-term experimental manipulations of temperature and/or UV-B radiation relied on earlier scenarios of climate and UV-B radiation change. However, the most recent scenarios (Chapters 4, 5, and 6) are used to provide a context for the assessment in this chapter, and to modify projections of ecosystem responses based on earlier scenarios where appropriate. The ACIA climate scenarios (section 4.4) are also used directly to illustrate the responses of some species to projected climate changes.
Approaches used for the assessment: strengths, limitations, and uncertainties (7.1.4)
This chapter assesses information on interactions between climate, UV-B radiation levels, and ecosystems from a wide range of sources including experimental manipulations of ecosystems and environments in the field; laboratory experiments; monitoring and observation of biological processes in the field; conceptual modeling using past relationships between climate and biota (paleo-analogues) and current relationships between climate and biota in different geographic areas (geographic analogues) to infer future relationships; and process based mathematical modeling.Where possible, indigenous knowledge (limited to published sources) is included as an additional source of observational evidence. Relevant information from indigenous peoples on arctic tundra and polar desert ecosystems is given in Chapter 3.
Each method has uncertainties and strengths and these are discussed in section 7.7. By considering and comparing different types of information, it is hoped that a more robust assessment has been achieved. However, the only certainties in this assessment are that there are various levels of uncertainty in the projections and that even if an attempt is made to estimate the magnitude of these uncertainties, surprise responses of ecosystems and their species to changes in climate and UV-B radiation levels are certain to occur.
Chapter 7: Arctic tundra and polar desert ecosystems
7.2 Late-Quaternary changes in arctic terrestrial ecosystems, climate, and ultraviolet radiation levels
7.3 Species responses to changes in climate and ultraviolet-B radiation in the Arctic
7.3.1 Implications of current species distributions for future biotic change
7.3.2 General characteristics of arctic species and their adaptations in the context of changes in climate and ultraviolet-B radiation levels
7.3.3 Phenotypic responses of arctic species to changes in climate and ultraviolet-B radiation
7.3.4 Genetic responses of arctic species to changes in climate and ultraviolet-B radiation levels
7.3.5 Recent and projected changes in arctic species distributions and potential ranges
7.4 Effects of changes in climate and UV radiation levels on structure and function of arctic ecosystems in the short and long term
7.4.1 Ecosystem structure
7.4.2 Ecosystem function
7.5 Effects of climate change on landscape and regional processes and feedbacks to the climate system
7.6 Synthesis: Scenarios of projected changes in the four ACIA regions for 2020, 2050, and 2080
7.7 Uncertainties and recommendations
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