H.E. Epstein1, D.A. Walker2, U.S. Bhatt3, P. Bieniek3, J. Comiso4, J. Pinzon5, M.K. Raynolds2, C.J. Tucker5,
G.J. Jia6, H. Zeng6, I.H. Myers-Smith7, B.C. Forbes8, D. Blok9, M.M. Loranty10, P.S.A. Beck11, S.J. Goetz11,
T.V. Callaghan12, G.H.R. Henry7, C.E. Tweedie13, P.J. Webber14, A.V. Rocha15, G.R. Shaver16, J.M. Welker17
1Department of Environmental Sciences, University of Virginia, Charlottesville, VA, USA
2Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK, USA
3Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA
4Cryospheric Sciences Branch, NASA Goddard Space Flight Center, Greenbelt, MD, USA
5Biospheric Science Branch, NASA Goddard Space Flight Center, Greenbelt, MD, USA
6Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
7Department of Geography, University of British Columbia, Vancouver, BC, Canada
8Arctic Centre, University of Lapland, Rovaniemi, Finland
9Center for Permafrost, University of Copenhagen, Copenhagen, Denmark
10Department of Geography, Colgate University, Hamilton, NY, USA
11Woods Hole Research Center, Falmouth, MA, USA
12Department of Animal and Plant Sciences, University of Sheffield, Sheffield, UK
13Department of Biology, University of Texas - El Paso, El Paso, TX, USA
14Department of Plant Biology, Michigan State University, East Lancing, MI, USA
15Department of Biological Sciences, University of Notre Dame, Notre Dame, IN, USA
16The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA, USA
17Department of Biological Sciences, University of Alaska Anchorage, Anchorage, AK, USA
December 4, 2012
- Over the past 30 years (1982-2011), the Normalized Difference Vegetation Index (NDVI), an index of green vegetation, has increased 15.5% in the North American Arctic and 8.2% in the Eurasian Arctic. In the more southern regions of Arctic tundra, the estimated aboveground plant biomass has increased 20-26%.
- Increasing shrub growth and range extension throughout the Low Arctic are related to winter and early growing season temperature increases. Growth of other tundra plant types, including graminoids and forbs, is increasing, while growth of mosses and lichens is decreasing.
- Increases in vegetation (including shrub tundra expansion) and thunderstorm activity, each a result of Arctic warming, have created conditions that favor a more active Arctic fire regime.
Eighty percent of the non-alpine tundra biome in the Arctic lies within 100 km of the Arctic Ocean and adjacent seas, and its distribution is largely controlled by cold summer air masses associated with the pack ice. It is expected that if sea ice decline continues at recently observed rates (see the Sea Ice essay), the adjacent tundra areas will continue to become warmer during the summer (Lawrence et al., 2008), and the higher temperatures will increase tundra primary productivity and biomass, and alter species composition (Bhatt et al., 2010; Elmendorf et al., 2012b; Callaghan et al., 2011; Epstein et al., 2012), with consequences for other tundra ecosystem components and processes. This essay reports on changes in tundra biomass and greenness, phenology, shrubs, non-native species and wildfires.
Long-term circumpolar change in tundra biomass and greenness
A very strong correlation (r2 = 0.94, p< 0.001) has been established between above-ground plant biomass at the peak of the growing season and the NDVI (Normalized Difference Vegetation Index), an index of vegetation greenness, derived from Advanced Very High Resolution Radiometer (AVHRR) data, along Arctic transects in both North America and Eurasia (Raynolds et al., 2012). Using this relationship, Epstein et al. (2012) determined that above-ground tundra biomass at representative sites increased by 19.8% during the period of the NDVI record (1982-2010), with the greatest increases (20-26%) evident in the mid- to southern (Low Arctic) tundra. This has major implications for tundra ecosystem properties, including active layer depth, permafrost temperature and distribution, hydrology, wildlife and human use of Arctic landscapes. The active layer and permafrost temperature are described in the Permafrost essay.
Over the thirty years of AVHRR observations (1982-2011), area-averaged maximum (peak growing season) NDVI (Fig. 4.1, right side) has increased 15.5% in the North American Arctic, with a particularly sharp increase since 2005, and 8.2% in the Eurasian Arctic, although values there have been nearly constant since 2001 (Bhatt et al., 2010 updated to 2011). Summer land temperature trends (based on AVHRR radiative surface temperatures) in the tundra regions also show different geographic patterns (1982-2011), with the area-averaged Summer Warmth Index (SWI, the sum of mean monthly surface temperatures >0°C) increasing 10.1% for North America and decreasing 2.6% for Eurasia (Fig. 4.1, left side). Also see Figs. 1.2 and 1.3 in the essay on Air temperature, Atmospheric Circulation and Clouds for information on change in air temperature over land.
During the period 2000-2010, based on Moderate Resolution Imaging Spectroradiometer (MODIS) NDVI, the growing season (SOS) began earlier while the end of the growing season (EOS) was delayed; consequently, the length of the growing season (LOS) increased over much of the high-latitude region (Zeng et al., 2011). Spring green-up in Eurasia occurred on average 15.2 days earlier than green-up in North America, and senescence in Eurasia occurred on average 13.6 days later than in North America. The date of peak NDVI has become earlier in Eurasia since 2000, with no substantial shifts in North America. The correlations between NDVI phenology parameters and monthly temperatures were generally strong, with higher temperatures in May and September likely contributing to the earlier SOS and delayed EOS, respectively.
Observations of shrub expansion
There are a growing number of observations of increasing shrubs at sites around the Arctic (Fig. 4.2, e.g. Naito and Cairns, 2011; Myers-Smith et al., 2011a). Circumpolar plot-based monitoring of vegetation changes indicates that shrub species have increased in canopy height and abundance at sites that have experienced recent summer warming, particularly in wetter versus drier conditions (Elmendorf et al., 2012b). Various studies have used measurements of radial and lateral stem growth to examine the factors controlling the expansion of tundra shrub species. Areas in Arctic Russia, sub-Arctic Sweden, western Canada and Alaska, and other sites in the High Arctic, show strong correlations between early growing season temperatures and the growth of shrub species (Forbes et al., 2010; Hallinger et al., 2010; Weijers et al., 2010; Rayback et al.. 2011; Blok et al., 2011a; Myers-Smith et al., 2011b). Some studies indicate that almost half of the variation in growth of certain shrub species can be explained by early summer temperatures alone (Forbes et al., 2010; Macias-Fauria et al., 2012). In addition, winter temperatures and snow were also found to correlate with shrub growth at some sites (Hallinger et al., 2010; Schmidt et al., 2010). Long-term snow addition and removal studies as part of the International Tundra Experiment (ITEX) found that deeper snow and the resultant winter soil warming led to increases in shrub growth (Rogers et al., 2011). See the Snow essay for more information about its changing distribution and characteristics.
A recent study in the northwest Eurasian Arctic (Macias-Fauria et al., 2012) showed that during the past half century, alder (Alnus) and willow (Salix) shrubs have responded over a >100,000 km2 area to a rise in summer temperature of approximately 2°C. In the same region, there has been an increase in permafrost temperatures - see the Permafrost essay. Alder and willow are two of the three most abundant erect shrub genera (the other is Betula) north of the continental treeline. Analysis of annual growth in Salix lanata L. revealed remarkably high correlations with summer temperature across the tundra and taiga zones of west Siberia and Eastern Europe (Forbes et al., 2010). The data indicate that low shrub thickets can transform into taller shrublands, which suggests that rather than a coniferous treeline migrating northward, Low Arctic vegetation change might include structurally novel deciduous woodland (tall shrubland) ecosystems (Macias-Fauria et al., 2012).
Shrub expansion is, however, not uniform, and in Alaska it is becoming increasing clear from repeat photography that some shrub communities are expanding, while others are stagnant (Tape et al., 2012). Expanding patches of Alnus viridis ssp. fruticosa (Siberian alder) contained shrub stems with thicker growth rings than in stable patches. Also, growth in expanding patches showed strong correlation with spring and summer warming, whereas alder growth in stable patches showed little correlation with temperature. Shrub expansion also appeared to be related to the landscape position and soil conditions, often associated with floodplains, stream corridors, and rock outcrops. This indicates that existing soil properties predispose certain parts of the landscape to rapid vegetation responses to climate change.
The role of herbivores in affecting vegetation changes is becoming increasingly evident (Johnson et al., 2011; Cahoon et al., 2012). There is evidence from Fennoscandia that heavy grazing by reindeer may significantly check deciduous shrub growth (Kitti et al., 2009; Olofsson et al., 2009), and prevent the disappearance of shorter-statured tundra (Yu et al., 2011). In cases where erect shrubs are already above the reindeer browse line, ~1.8 m, their transformation into tree-size individuals is likely to correlate with warming temperatures rather than grazing intensity (Forbes et al., 2010). In ice-free regions of central west Greenland, the exclusion of caribou and muskoxen led to dramatic increases in shrub cover, leaf area, photosynthesis, and a nearly threefold increase in net carbon uptake by tundra ecosystems (for a more detailed description of potential changes in soil carbon uptake/release, see the Carbon Dioxide and Methane essay). These responses were accentuated by warming, but only in the absence of the herbivores (Cahoon et al., 2012). See the Rangifer essay for information about the status of caribou and reindeer in the Arctic.
Evidence for general tundra greening in addition to shrub expansion
Interestingly, the greening of Arctic tundra in response to warming does not appear to be confined to shrub vegetation. Beck and Goetz (2011, 2012) found increases in summer NDVI across the North Slope of Alaska since the mid-1990s regardless of shrub cover. This supports field-based observational and experimental results that other tundra plant types are responding to multi-year warming with increased growth (e.g. Walker et al. 2006, Callaghan et al. 2011, Elmendorf et al., 2012a, 2012b). Elmendorf et al. (2012b) analyzed repeated surveys of tundra vegetation plot data between 1980 and 2010 at 46 locations and found biome-level trends of increased plant canopy height for most vascular plant growth forms. Increased abundances of shrub, forbs and rushes were related to summer warming. In a meta-analysis of 61 tundra warming experiments, most of which were part of the ITEX network (Elmendorf et al., 2012a), warming yielded significant increases in the heights of shrubs, graminoids and forbs, significant increases in shrub abundance, and significant declines in lichens and mosses. In a synthesis of studies that were part of the Back to the Future (BTF) project, Callaghan et al. (2011) presented several observations of tundra vegetation increases including, but not limited to, shrubs, as well as several plant community changes (e.g. Myers-Smith et al., 2011a; Hudson and Henry, 2009; Hill and Henry, 2011; Daniëls and de Molenaar, 2011; Madsen et al., 2011; Villarreal et al., 2012; Lin et al., 2012).
Non-native plant species are a minor vegetation component throughout the Arctic, composing less than 10% of the flora and almost entirely restricted to disturbed areas near human habitation (Hultén, 1968; Elven & Elvebakk, 1996; Panarctic Flora Project, 2000; Aiken et al., 2007; AKEPIC, 2012). The majority of these species are widespread ruderal (weedy) plants that do not reach high densities and are not considered to be a major threat to the ecology of the biome (Carlson et al., 2008; AKEPIC 2012). Greater diversity of non-native species is associated with sites with long histories of transportation of goods and materials (e.g., Hudson Bay Company posts in Nunavut), areas with more extensive use of livestock (e.g., Barentsburg, Spitsbergen) and, more recently, sites of deliberate introductions for gardens and landscaping (Elven and Elvebakk, 1996; Panarctic Flora Project, 2000; Aiken et al., 2007). Short growing seasons and low temperatures in the Arctic appear to restrict establishment, growth and reproduction of many non-native species. Sites with warm microclimates (e.g., Pilgrim Hot Springs in northwestern Alaska) often harbor many more non-native species than surrounding areas. More broadly across Alaska, the diversity of non-native species is strongly correlated with growing degree days (Fig. 4.3) in settlements of fewer than 10,000 people (AKEPIC, 2012). While non-native species are currently a small component of the diversity and biomass of the region, increasing propagule pressure, rising temperatures and high levels of disturbance, including the increase in natural resource development activities, suggest that the Arctic will likely face growing rates of non-native plant establishment, as we are already seeing spread throughout the sub-Arctic (Carlson and Shephard, 2007; Conn et al., 2008).
New evidence suggests that Arctic warming may be influencing fire regimes both directly and indirectly. In the past, Arctic fires were rare and limited by low temperatures, fuel loads and ignition sources, resulting in fire return intervals in the hundreds to thousands of years (Wein, 1977). However, increases in vegetation (including shrub tundra expansion) and thunderstorm activity, each a result of Arctic warming, have created conditions that favor a more active Arctic fire regime (Higuera et al., 2008; Hu et al., 2010). Conditions over the last decade have been particularly favorable for Arctic fires in Alaska, with 37 fires burning ~400 km2 in the Noatak region in 2010, and the single largest fire (the Anaktuvuk River fire) burning 1000 km2 in 2007 (Higuera et al., 2011). These events have profound effects on land-atmosphere exchanges of carbon and energy. For example, the 2007 Anaktuvuk River fire resulted in a loss of 2.1 Tg C to the atmosphere (for a more detailed description of carbon in the atmosphere and land to atmosphere carbon flux, see the Carbon Dioxide and Methane essay), enough to offset the annual net carbon uptake of the entire biome (Mack et al. 2011); increased burn-severity led to reduced post-fire carbon sequestration, as well as increases in soil temperature and active layer thaw depth (Rocha and Shaver, 2011a, 2011b). For a circum-Arctic perspective on the active layer and permafrost temperature, see the Permafrost essay.
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