On the influence of shrub height and expansion on northern high latitude climate

There is a growing body of empirical evidence documenting the expansion of shrub vegetation in the circumpolar Arctic in response to climate change. Here, we conduct a series of idealized experiments with the Community Climate System Model to analyze the potential impact on boreal climate of a large-scale tundra-to-shrub conversion. The model responds to an increase in shrub abundance with substantial atmospheric heating arising from two seasonal land?atmosphere feedbacks: a decrease in surface albedo and an evapotranspiration-induced increase in atmospheric moisture content. We demonstrate that the strength and timing of these feedbacks are sensitive to shrub height and the time at which branches and leaves protrude above the snow. Taller and aerodynamically rougher shrubs lower the albedo earlier in the spring and transpire more efficiently than shorter shrubs. These mechanisms increase, in turn, the strength of the indirect sea-ice albedo and ocean evaporation feedbacks contributing to additional regional warming. Finally, we find that an invasion of tall shrubs tends to systematically warm the soil, deepen the active layer, and destabilize the permafrost (with increased formation of taliks under a future scenario) more substantially than an invasion of short shrubs.

[1]  Qin Yu,et al.  Simulating Future Changes in Arctic and Subarctic Vegetation , 2007, Computing in Science & Engineering.

[2]  Compton J. Tucker,et al.  Seasonality and trends of snow‐cover, vegetation index, and temperature in northern Eurasia , 2002 .

[3]  J. Foley.  Tipping Points in the Tundra , 2005, Science.

[4]  C J Tucker,et al.  Drier summers cancel out the CO2 uptake enhancement induced by warmer springs. , 2005, Proceedings of the National Academy of Sciences of the United States of America.

[5]  C. Symon,et al.  Arctic climate impact assessment , 2005 .

[6]  David M. Lawrence,et al.  Sensitivity of a model projection of near‐surface permafrost degradation to soil column depth and representation of soil organic matter , 2008 .

[7]  J. Kutzbach,et al.  Feedbacks between climate and boreal forests during the Holocene epoch , 1994, Nature.

[8]  M. A. Arain,et al.  Impacts of peat and vegetation on permafrost degradation under climate warming , 2007 .

[9]  Michael F. Wehner,et al.  Attribution of polar warming to human influence , 2008 .

[10]  William J. Collins,et al.  Possible role of wetlands, permafrost, and methane hydrates in the methane cycle under future climate change: A review , 2010 .

[11]  Christopher B. Field,et al.  Changing feedbacks in the climate–biosphere system , 2008 .

[12]  Marika M. Holland,et al.  Accelerated Arctic land warming and permafrost degradation during rapid sea ice loss , 2008 .

[13]  A. McGuire,et al.  The Effect of Moisture Content on the Thermal Conductivity of Moss and Organic Soil Horizons From Black Spruce Ecosystems in Interior Alaska , 2009 .

[14]  M. Torre Jorgenson,et al.  Abrupt increase in permafrost degradation in Arctic Alaska , 2006 .

[15]  S. Goetz,et al.  Tundra vegetation effects on pan-Arctic albedo , 2011 .

[16]  D. Lawrence,et al.  Simulation of Present-Day and Future Permafrost and Seasonally Frozen Ground Conditions in CCSM4 , 2012 .

[17]  G. Bonan,et al.  Effects of boreal forest vegetation on global climate , 1992, Nature.

[18]  M. Sturm,et al.  The evidence for shrub expansion in Northern Alaska and the Pan‐Arctic , 2006 .

[19]  Francis W. Zwiers,et al.  Human influence on Arctic sea ice detectable from early 1990s onwards , 2008 .

[20]  M. Sturm,et al.  Climate change: Increasing shrub abundance in the Arctic , 2001, Nature.

[21]  Konrad A Hughen,et al.  Arctic Environmental Change of the Last Four Centuries , 1997 .

[22]  B. Forbes,et al.  Russian Arctic warming and ‘greening’ are closely tracked by tundra shrub willows , 2010 .

[23]  S. Goetz,et al.  Shrub Cover on the North Slope of Alaska: a circa 2000 Baseline Map , 2011 .

[24]  Michael G. Ryan,et al.  Stomatal conductance and photosynthesis vary linearly with plant hydraulic conductance in ponderosa pine , 2001 .

[25]  R. Neale,et al.  The Mean Climate of the Community Atmosphere Model (CAM4) in Forced SST and Fully Coupled Experiments , 2013 .

[26]  David Pollard,et al.  Potential high‐latitude vegetation feedbacks on CO2‐induced climate change , 1999 .

[27]  Jed O. Kaplan,et al.  Arctic climate change with a 2 ∘C global warming: Timing, climate patterns and vegetation change , 2006 .

[28]  X. Zeng,et al.  Improving the treatment of the vertical snow burial fraction over short vegetation in the NCAR CLM3 , 2009 .

[29]  D. Lawrence,et al.  Parameterization improvements and functional and structural advances in Version 4 of the Community Land Model , 2011 .

[30]  Jon Holmgren,et al.  Snow-Shrub Interactions in Arctic Tundra: A Hypothesis with Climatic Implications , 2001 .

[31]  F. Chapin,et al.  Role of Land-Surface Changes in Arctic Summer Warming , 2005, Science.

[32]  David M. Lawrence,et al.  Permafrost response to increasing Arctic shrub abundance depends on the relative influence of shrubs on local soil cooling versus large-scale climate warming , 2011 .

[33]  J. Welker,et al.  Winter Biological Processes Could Help Convert Arctic Tundra to Shrubland , 2005 .

[34]  F. Stuart Chapin,et al.  Primary and secondary stem growth in arctic shrubs: implications for community response to environmental change , 2002 .

[35]  G. Schaepman‐Strub,et al.  Shrub expansion may reduce summer permafrost thaw in Siberian tundra , 2010 .

[36]  G. Danabasoglu,et al.  The Community Climate System Model Version 4 , 2011 .

[37]  W. Oechel,et al.  Observational Evidence of Recent Change in the Northern High-Latitude Environment , 2000 .

[38]  F. Chapin,et al.  Evidence and Implications of Recent Climate Change in Northern Alaska and Other Arctic Regions , 2004 .

[39]  G. Bonan,et al.  Changes in Arctic vegetation amplify high-latitude warming through the greenhouse effect , 2009, Proceedings of the National Academy of Sciences.

[40]  Michael F. Wehner,et al.  Identification of human-induced changes in atmospheric moisture content , 2007, Proceedings of the National Academy of Sciences.

[41]  S. Goetz,et al.  Satellite-observed photosynthetic trends across boreal North America associated with climate and fire disturbance. , 2005, Proceedings of the National Academy of Sciences of the United States of America.

[42]  Donald J. Cavalieri,et al.  Arctic sea ice extents, areas, and trends, 1978-1996 , 1999 .

[43]  David M. Lawrence,et al.  Improved modeling of permafrost dynamics in a GCM land‐surface scheme , 2007 .

[44]  P. Grogan,et al.  Birch shrub growth in the low Arctic: the relative importance of experimental warming, enhanced nutrient availability, snow depth and caribou exclusion , 2012 .

[45]  Martin Hallinger,et al.  Establishing a missing link: warm summers and winter snow cover promote shrub expansion into alpine tundra in Scandinavia. , 2010, The New phytologist.

[46]  G. Liston,et al.  Changing snow and shrub conditions affect albedo with global implications , 2005 .

[47]  S. Goetz,et al.  Tundra vegetation effects on pan-Arctic albedo Tundra vegetation effects on pan-Arctic albedo , 2011 .

[48]  Steven F. Oberbauer,et al.  Plant community responses to experimental warming across the tundra biome , 2006, Proceedings of the National Academy of Sciences of the United States of America.

[49]  W. Collins,et al.  The Community Climate System Model: CCSM3 , 2004 .

[50]  Scott J. Goetz,et al.  Trends in Satellite-Observed Circumpolar Photosynthetic Activity from 1982 to 2003: The Influence of Seasonality, Cover Type, and Vegetation Density , 2006 .

[51]  R. Wein,et al.  BETULA NANA L. AND BETULA GLANDULOSA MICHX. , 1997 .