title: Winter warming as an important co-driver for Betula nana growth in western Greenland during the past century.
abstract: Growing season conditions are widely recognized as the main driver for tundra shrub radial growth, but the effects of winter warming and snow remain an open question. Here, we present a more than 100 years long Betula nana ring-width chronology from Disko Island in western Greenland that demonstrates a highly significant and positive growth response to both summer and winter air temperatures during the past century. The importance of winter temperatures for Betula nana growth is especially pronounced during the periods from 1910-1930 to 1990-2011 that were dominated by significant winter warming. To explain the strong winter importance on growth, we assessed the importance of different environmental factors using site-specific measurements from 1991 to 2011 of soil temperatures, sea ice coverage, precipitation and snow depths. The results show a strong positive growth response to the amount of thawing and growing degree-days as well as to winter and spring soil temperatures. In addition to these direct effects, a strong negative growth response to sea ice extent was identified, indicating a possible link between local sea ice conditions, local climate variations and Betula nana growth rates. Data also reveal a clear shift within the last 20 years from a period with thick snow depths (1991-1996) and a positive effect on Betula nana radial growth, to a period (1997-2011) with generally very shallow snow depths and no significant growth response towards snow. During this period, winter and spring soil temperatures have increased significantly suggesting that the most recent increase in Betula nana radial growth is primarily triggered by warmer winter and spring air temperatures causing earlier snowmelt that allows the soils to drain and warm quicker. The presented results may help to explain the recently observed 'greening of the Arctic' which may further accelerate in future years due to both direct and indirect effects of winter warming.
authors: Center for Permafrost (CENPERM), Department of Geoscience and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, DK-1350, Copenhagen, Denmark; Department of Conservation and Natural Sciences, National Museum of Denmark, I.C. Modewegsvej, Brede, DK-2800, Lyngby, Denmark.
authors: Jørgen
authors: J
authors: Hollesen
authors: Agata
authors: A
authors: Buchwal
authors: Grzegorz
authors: G
authors: Rachlewicz
authors: Birger U
authors: BU
authors: Hansen
authors: Marc O
authors: MO
authors: Hansen
authors: Ole
authors: O
authors: Stecher
authors: Bo
authors: B
authors: Elberling
mesh_terms: D001110:Arctic Regions; D029662:Betula / Q000502:physiology*; D057231:Climate Change; D006116:Greenland; D046448:Ice Cover; D012621:Seasons; D012914:Snow; D012987:Soil; D013696:Temperature; D065929:Tundra
chemical_list: D012987:Soil
keywords: Arctic; Betula nana; Greenland; dendrochronology; shrub expansion; winter warming