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tundra to climate change. 2. Study design We measured variations in snow properties and vegetation across a landscape in arctic Alaska (69°06′N, 149°00′W) covered by three types of vegetation: 1) tussock tundra, 2) shrubby tussock tundra, and 3) riparian shrub ( McFadden et al. 1998 ). The site was near Happy Valley on the Dalton Highway, about half-way between Prudhoe Bay and the Brooks Range. Several shallow water tracks drained the gently sloping area. These were oriented perpendicular to the
tundra to climate change. 2. Study design We measured variations in snow properties and vegetation across a landscape in arctic Alaska (69°06′N, 149°00′W) covered by three types of vegetation: 1) tussock tundra, 2) shrubby tussock tundra, and 3) riparian shrub ( McFadden et al. 1998 ). The site was near Happy Valley on the Dalton Highway, about half-way between Prudhoe Bay and the Brooks Range. Several shallow water tracks drained the gently sloping area. These were oriented perpendicular to the
1. Introduction and historical perspective Although the surface energy balance is of fundamental importance to meteorological, hydrological, geomorphological, and ecological processes, the controls over the surface energy balance and its relationship to climate have not been well characterized for arctic and alpine tundra ecosystems. Greater attention has been paid to tropical environments (e.g., Henderson-Sellers and Gornitz 1984 ; Nobre et al. 1991 ) and boreal forest, particularly the
1. Introduction and historical perspective Although the surface energy balance is of fundamental importance to meteorological, hydrological, geomorphological, and ecological processes, the controls over the surface energy balance and its relationship to climate have not been well characterized for arctic and alpine tundra ecosystems. Greater attention has been paid to tropical environments (e.g., Henderson-Sellers and Gornitz 1984 ; Nobre et al. 1991 ) and boreal forest, particularly the
. Climate ). Reflected shortwave radiation is controlled by the surface albedo, which in the Arctic is mainly determined by the presence or absence of snow. In the spring, when much of the region is still snow covered, downward shortwave radiation increases rapidly. Much of this radiation is reflected due to the high albedo of the snow-covered land surface. RASM captures the difference in cold season albedos between the taiga and tundra, with typical values of 0.4 and 0.6 respectively ( Fig. 3
. Climate ). Reflected shortwave radiation is controlled by the surface albedo, which in the Arctic is mainly determined by the presence or absence of snow. In the spring, when much of the region is still snow covered, downward shortwave radiation increases rapidly. Much of this radiation is reflected due to the high albedo of the snow-covered land surface. RASM captures the difference in cold season albedos between the taiga and tundra, with typical values of 0.4 and 0.6 respectively ( Fig. 3
the Arctic frontal zone over North America for January, April, July, and October. Based on a trajectory analysis of air masses for July, Bryson (1966) demonstrated that the modal position of the summer Arctic frontal zone over North America coincides closely with Reed and Kunkel’s (1960) analysis as well as the position of the tree line. He postulated that the summer frontal position might be important in determining the distribution of forest versus tundra. However, Bryson also considered the
the Arctic frontal zone over North America for January, April, July, and October. Based on a trajectory analysis of air masses for July, Bryson (1966) demonstrated that the modal position of the summer Arctic frontal zone over North America coincides closely with Reed and Kunkel’s (1960) analysis as well as the position of the tree line. He postulated that the summer frontal position might be important in determining the distribution of forest versus tundra. However, Bryson also considered the
effect decreases evapotranspiration in both hemispheres, particularly in the tropical rain forests (not statistically significant), while the radiative effect increases evapotranspiration in the NH and decreases it in the SH. The radiative effect generally increases evapotranspiration in the Arctic ( p < 0.05), as vegetation expands into the tundra, and decreases it in the Tropics and SH subtropics ( p < 0.05 over the Amazon, South Africa, Australia), resulting from forest dieback. Fixed vegetation
effect decreases evapotranspiration in both hemispheres, particularly in the tropical rain forests (not statistically significant), while the radiative effect increases evapotranspiration in the NH and decreases it in the SH. The radiative effect generally increases evapotranspiration in the Arctic ( p < 0.05), as vegetation expands into the tundra, and decreases it in the Tropics and SH subtropics ( p < 0.05 over the Amazon, South Africa, Australia), resulting from forest dieback. Fixed vegetation
soil. Currently there is no explicit incorporation of moss, lichen, or peat layers in the National Center for Atmospheric Research Land Surface Model (NCAR LSM; Bonan 1996 ) that has been used to investigate land–atmosphere interactions in the Arctic ( Lynch et al. 1999a,b ; Eugster et al. 1997 ). Mosses are ubiquitous in boreal forest and tundra ecosystems, which occupy 14% of the total global land area. In boreal forests, feather mosses ( Hylocomium and Pleurozium spp.) dominate the
soil. Currently there is no explicit incorporation of moss, lichen, or peat layers in the National Center for Atmospheric Research Land Surface Model (NCAR LSM; Bonan 1996 ) that has been used to investigate land–atmosphere interactions in the Arctic ( Lynch et al. 1999a,b ; Eugster et al. 1997 ). Mosses are ubiquitous in boreal forest and tundra ecosystems, which occupy 14% of the total global land area. In boreal forests, feather mosses ( Hylocomium and Pleurozium spp.) dominate the
the increase in surface winter temperature in Europe and Asia (north of 40°N) ( Hurrell 1996 ; Hurrell and van Loon 1997 ; Thompson et al. 2000 ; Hurrell et al. 2003 ). A mode of climate variability with extensive effects in the Northern Hemisphere, is the northern annular mode (NAM) ( Thompson and Wallace 2001 ), which also goes by the name of the North Atlantic Oscillation (NAO) ( Hurrell 1995 ) or the Arctic Oscillation (AO) ( Thompson and Wallace 1998 ). Thompson and Wallace (2001
the increase in surface winter temperature in Europe and Asia (north of 40°N) ( Hurrell 1996 ; Hurrell and van Loon 1997 ; Thompson et al. 2000 ; Hurrell et al. 2003 ). A mode of climate variability with extensive effects in the Northern Hemisphere, is the northern annular mode (NAM) ( Thompson and Wallace 2001 ), which also goes by the name of the North Atlantic Oscillation (NAO) ( Hurrell 1995 ) or the Arctic Oscillation (AO) ( Thompson and Wallace 1998 ). Thompson and Wallace (2001
1. Introduction In the Arctic, significant increases of temperature and precipitation are projected as a consequence of increasing greenhouse gas concentrations ( Kattenberg et al. 1996 ). This warming could be amplified, if carbon dioxide (CO 2 ) is released from the large soil carbon pools in tundra or boreal forest ( Lashof 1989 ; Oechel et al. 1993 ) or if the tree line migrates northward and reduces regional albedo ( Bonan et al. 1992 ; Rowntree 1992 ; Foley et al. 1994 ). However
1. Introduction In the Arctic, significant increases of temperature and precipitation are projected as a consequence of increasing greenhouse gas concentrations ( Kattenberg et al. 1996 ). This warming could be amplified, if carbon dioxide (CO 2 ) is released from the large soil carbon pools in tundra or boreal forest ( Lashof 1989 ; Oechel et al. 1993 ) or if the tree line migrates northward and reduces regional albedo ( Bonan et al. 1992 ; Rowntree 1992 ; Foley et al. 1994 ). However
Arctic Ocean in summer, either alone or modified by orography ( Dzerdzeevskii 1945 ; Reed and Kunkel 1960 ; Serreze et al. 2001 ); or, in a reversal of Bryson's (1966) reasoning, from contrasts in surface heating between the tundra and boreal forest ( Hare and Ritchie 1972 ; Pielke and Vidale 1995 ). Typically, it is reasoned that climate is the “ultimate ecological control” ( Sorensen 1977 ). Ritchie and Hare (1971) noted that the present-day Arctic front is located at a median of 300 km
Arctic Ocean in summer, either alone or modified by orography ( Dzerdzeevskii 1945 ; Reed and Kunkel 1960 ; Serreze et al. 2001 ); or, in a reversal of Bryson's (1966) reasoning, from contrasts in surface heating between the tundra and boreal forest ( Hare and Ritchie 1972 ; Pielke and Vidale 1995 ). Typically, it is reasoned that climate is the “ultimate ecological control” ( Sorensen 1977 ). Ritchie and Hare (1971) noted that the present-day Arctic front is located at a median of 300 km
1. Introduction In response to global warming, vegetation covers are changing in the Arctic ( Hinzman et al. 2005 ; Post et al. 2009 ; Serreze et al. 2000 ). Shifts in species and abundance are being observed in many Arctic and subarctic regions, and the boundaries between the various vegetation communities are moving. Woody plants benefit the most from these changes, with the forest–tundra ecotone (also called tree line) moving northward ( Danby and Hik 2007 ; Harsch et al. 2009 ) and
1. Introduction In response to global warming, vegetation covers are changing in the Arctic ( Hinzman et al. 2005 ; Post et al. 2009 ; Serreze et al. 2000 ). Shifts in species and abundance are being observed in many Arctic and subarctic regions, and the boundaries between the various vegetation communities are moving. Woody plants benefit the most from these changes, with the forest–tundra ecotone (also called tree line) moving northward ( Danby and Hik 2007 ; Harsch et al. 2009 ) and
