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.

Water flow in the active layer along an arctic slope – An investigation based on a field campaign and model simulations

“…The permafrost distribution on Disko Island is characterized as being continuous (Humlum, 1998) while recent temperature changes suggest more discontinuous permafrost (Hollesen et al, 2015). This is in line with the general trend of the climate model predictions of the future climate of Greenland where permafrost temperatures are expected to increase which result in 5 an associated increase in the thickness of the active layer (Daanen et al, 2011).…”

“…The climate is classified as maritime low arctic with an annual average air temperature of -3.3°C for the 25 period 1991-2004 and with maximum in July (mean monthly average 7.1°C) and minimum in February-March (mean monthly average -16°C) (Hansen et al, 2006b). For the observation period 1991-2011, the mean annual air temperature has increased by 0.2°C per year (Hollesen et al, 2015) now reaching near zero as the annual mean air temperature. Positive daily average air temperatures are typically between May and September (Hansen et al, 2006b).…”

The role of summer precipitation and summer temperature in establishment and growth of dwarf shrub Betula nana in northeast Siberian tundra

“…Similar to other woody species in the Arctic tundra, B. nana forms exceptionally narrow annual tree rings (Groot et al 1997;Meinardus et al 2011;Hollesen et al 2015). The average ring width of B. nana collected by Miller (1975) was merely 130 lm.…”

“…Dendrochronological analyses of such material require application of serial sectioning, i.e., studying multiple samples along the main stem to account for frequently occurring missing rings, specifically in outer stem parts (Wilmking et al 2012). Despite being a common circum-Arctic species that seems responsive to climate warming (Shaver et al 2001;Wahren et al 2005;Hollesen et al 2015), B. nana has only recently been used for dendrochronological studies (Miller 1975;Blok et al 2011a;Meinardus et al 2011).…”

A warmer and greener cold world: summer warming increases shrub growth in the alpine and high Arctic tundra

“…As B. nana (MeinarduS et al 2011;HolleSen et al 2015) and E. hermaphroditum (Bär et al 2006;2007) exhibit high intra-plant growth variability, serial sectioning (KoliSHCHuK 1990) was applied on the shrub samples from these species. Thin crosssections of ~5-10 µm were cut at regular intervals along the main stem(s), starting at the base, (Bär et al 2006;2007;BuCHWal et al 2013) with a sledge microtome (GSL-1; gärtner et al 2014), and stained with a mixture of Safranin and Astrablue.…”