Climate sensitivity of shrub growth across the tundra biome

Abstract

Rapid climate warming in the tundra biome has been linked to increasing shrub dominance 1-4 . Shrub expansion can modify climate by altering surface albedo, energy and water balance, and permafrost 2,5-8 , yet the drivers of shrub growth remain poorly understood. Dendroecological data consisting of multi-decadal time series of annual shrub growth provide an underused resource to explore climate-growth relationships. Here, we analyse circumpolar data from 37 Arctic and alpine sites in 9 countries, including 25 species, and ∼42,000 annual growth records from 1,821 individuals. Our analyses demonstrate that the sensitivity of shrub growth to climate was: (1) heterogeneous, with European sites showing greater summer temperature sensitivity than North American sites, and (2) higher at sites with greater soil moisture and for taller shrubs (for example, alders and willows) growing at their northern or upper elevational range edges. Across latitude, climate sensitivity of growth was greatest at the boundary between the Low and High Arctic, where permafrost is thawing 4 and most of the global permafrost soil carbon pool is stored 9 . The observed variation in climate-shrub growth relationships should be incorporated into Earth system models to improve future projections of climate change impacts across the tundra biome.The Arctic is warming more rapidly than lower latitudes owing to climate amplification involving temperature, water vapour, albedo and sea ice feedbacks 5,7 . Tundra ecosystems are thus predicted to respond more rapidly to climate change than other terrestrial ecosystems 4 . The tundra biome spans Arctic and alpine regions that have similar plant species pools and mean climates, yet vary in topography, seasonality, land cover and glaciation history. Concurrent with the recent high-latitude warming trend 7 , repeat photography and vegetation surveys have shown widespread expansion of shrubs 1-3 , characterized by increased canopy cover, height and abundance. However, climate warming 7 and shrub increase 2,10 have not occurred at all sites. Models predict that warming of 2-10 • C (ref. 11) could convert as much as half of current tundra to 'shrubland' by the end of the twenty-first century 8 , but the uniformity of the frequently cited relationship between climate

Vegetation composition and shrub extent on the Yukon coast, Canada, are strongly linked to ice-wedge polygon degradation

“…The Yukon Coastal Plain is part of a Low Arctic transition zone between low-shrub tundra and dwarf-shrub tundra, where the response of vegetation to warming is predicted to be fastest (Lantz et al 2010;Myers-Smith et al 2015). The study area lies within the region of continuous permafrost (Brown et al 1997).…”

“…An increase of shrubdominated tundra at the expense of graminoid dominated tundra could, however, considerably alter land surface properties. Snow retention potential could for instance be increased through the growth of shrubs, thereby altering albedo and ground thermal regime (Myers-Smith, Forbes et al 2011;Myers-Smith et al 2015). Melting of ice wedges could promote the interconnection of polygon troughs and promote water flow across the landscape (Liljedahl et al 2012;Godin et al 2014).…”

“…On the regional to global level, shrub growth is limited by summer air temperatures and the length of the growing season (Myers-Smith, Forbes et al 2011;Myers-Smith et al 2015), while locally other factors such as topography, hydrology and nutrient availability can become limiting (Shaver & Chapin 1980;Walker 2000;Naito & Cairns 2011;Ropars & Boudreau 2012), making the response of vegetation to climatic change more heterogeneous (Lantz et al 2010;.…”

Variability in above- and belowground carbon stocks in a Siberian larch watershed

“…We calculated the mass of woody debris according to Alexander et al (2012) using previously published multipliers for softwood boreal trees from the Northwest Territories of Canada for FWD (Nalder et al, 1997) and decay class and density values for softwood boreal tree species within Ontario, Canada, for CWD (Ter-Mikaelian et al, 2008). Mass values were converted to C pools based on average C concentration of L. cajanderi boles (47 %).…”

“…While vegetation stores a relatively small portion of the C pool in boreal forests (approximately 20 %; Pan et al, 2011), it plays a crucial role in local and global C cycling, and many future changes in C fluxes in this biome will likely occur as a result of changes in vegetation (Elmendorf et al, 2012;Euskirchen et al, 2009;Myers-Smith et al, 2015;Swann et al, 2010). With increased temperatures, boreal forests are susceptible to insect invasions (Berg et al, 2006;Kurz et al, 2008), moisture stress (Beck et al, 2011;Trahan and Schubert, 2016;Walker et al, 2015), tree line advance and retrogression (Lloyd, 2005;Pearson et al, 2013), and more frequent forest fires (Kasischke and Turetsky, 2006;Rogers et al, 2015;Soja et al, 2007), which all have the potential to alter C cycling significantly in the region.…”