H.E. Epstein1, U.S. Bhatt2, D.A. Walker3, M.K. Raynolds3, P.A. Bieniek4, J. Comiso5, J. Pinzon6,
C.J. Tucker6, I.V. Polyakov4,7, G.J. Jia8, H. Zeng8, B.C. Forbes9, M. Macias-Fauria10, L. Xu11,
G.V. Frost1, G.R. Shaver13, M.S. Bret-Harte3, M.C. Mack14, A.V. Rocha15
1Department of Environmental Sciences, University of Virginia, Charlottesville, VA, USA
2Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA
3Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, AK, USA
4International Arctic Research Center, University of Alaska Fairbanks, AK, USA
5Cryospheric Sciences Branch, NASA Goddard Space Flight Center, Greenbelt, MD, USA
6Biospheric Science Branch, NASA Goddard Space Flight Center, Greenbelt, MD, USA
7Department of Atmospheric Sciences, University of Alaska Fairbanks, Fairbanks, AK, USA
8Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China
9Arctic Centre, University of Lapland, Rovaniemi, Finland
10Department of Zoology, University of Oxford, Oxford, UK
11Institute of the Environment and Sustainability, University of California Los Angeles, Los Angeles, C.A., USA
12Department of Earth and Environment, Boston University, Boston, MA, USA
13The Ecosystems Center, Marine Biological Laboratory, Woods Hole, MA, USA
14Department of Biology, University of Florida, Gainesville, FL, USA
15Department of Biological Sciences, University of Notre Dame, Notre Dame, IN, USA
December 3, 2013
- Since observations began in 1982, Arctic tundra vegetation productivity (greenness) has increased, monotonically in Eurasia and at an accelerated rate since 2005-09 in North America.
- The length of the growing season (photosynthetically active period) has increased by 9 days per decade since 1982.
- Tall shrubs and trees have expanded their range across the forest-tundra ecotone in Siberia, with areal expansion rates of up to 25% since the mid- to late-1960s.
- The number and severity of tundra wildfires on the North Slope of Alaska has increased dramatically during the last decade. Since the 2007 Anaktuvuk River fire, above-ground net primary productivity has been greater on moderately burned sites than sites that did not burn.
Tundra Vegetation Productivity
Vegetation productivity (greenness) trends for the arctic tundra have been updated for the period 1982-2012 using the Global Inventory Modeling and Mapping Studies (GIMMS) Maximum Normalized Difference Vegetation Index (MaxNDVI, no units) from the Advanced Very High Resolution Radiometer (AVHRR). MaxNDVI (NDVI at the peak of the growing season) trends from 1982 to 2012 are broadly positive, with exceptions in northwestern Siberia, eastern Russia and western Alaska (Fig. 34). To better understand the trends, break points in the time series were quantified using the Breakfit algorithm (Mudelsee 2009). MaxNDVI has increased monotonically in Eurasia since 1984, and at accelerated rates since 2005 and 2009 in western North America and eastern North America, respectively (Fig. 35). Using the latest GIMMS dataset, Xu et al. (2013) reported that 39% of Arctic vegetation is now significantly greener (p < 0.1) than in 1982, with 4% more brown (a significant decrease) and 57% showing no significant change. Consistent with Bhatt et al. (2013), the greatest increases were found in the North American High Arctic and adjacent to the Beaufort Sea, and in northwestern Siberia.
NDVI is often positively related to summer temperature, as indicated in a recent synthesis by Post et al. (2013). However, factors other than warming affect NDVI and plant growth (Pouliot et al. 2009, Forbes et al. 2010, Macias-Fauria et al. 2012). For example, large-scale atmospheric circulation is likely a key contributor to lower temperatures and more consistent greening over Eurasia through increased summer cloud cover, compared to the accelerated greening in North America under more cloud-free skies (Bhatt et al. 2013).
The length of the growing season (photosynthetically active period, PAP) in the Arctic has increased by 9 days per decade since 1982 (Xu et al. 2013) (Fig. 36). Snow cover variability is considered to be an important cause of such phenological change in Arctic tundra. For example, for the period 2000-2010 on the Yamal Peninsula (western Arctic Russia), Zeng and Jia (2013) found greening onset was related to final snowmelt date (r2 range 0.78-0.93) and the end of the growing season was strongly related to the date of first snow cover (r2 range 0.86-0.94), with the exception of the initiation of growth for prostrate-shrub tundra. (See the essay on Snow for more information about its extent, duration, depth and water equivalence). Post et al. (2013) also demonstrated that the mid-point of the plant-growing season at an inland Greenland site was positively related to June sea-ice extent, i.e., sea-ice reduction has led to an earlier growing season. (See the essay on Sea Ice for more information about its extent, age and thickness).
Tall Shrub and Tree Expansion at the Forest-Tundra Ecotone in Siberia
Numerous studies over the past decade have indicated the expansion of shrubs throughout the Arctic (Isla Myers-Smith 2011) and experiments continue to show that warming increases vegetation productivity and the dominance of woody plants (Sistla et al. 2013). Few observations of woody plant expansion had been made in Siberia until Frost and Epstein (2013) quantified changes in tall shrub and tree canopy cover in eleven, widely-distributed Siberian forest-tundra ecotone landscapes. This study compared very-high-resolution photography from the Cold War-era "Gambit" and "Corona" satellite surveillance systems (1965-1969) with modern imagery. The total cover of tall shrubs increased by 5.3-25.9% in nine of ten ecotones (Fig. 37a), and tree cover increased by 3.0-18.2% in four of five ecotones (Fig. 37b). Shrub and tree canopy cover expansion rates were better correlated with mean annual precipitation than with mean summer temperature.
Frost et al. (2013) sampled two tall shrub ecotonal landscapes near the Polar Urals of northwestern Siberia and found that establishment of tall alder was strongly facilitated by small, widely-distributed disturbances associated with patterned-ground landscapes resulting from frost-heave (Fig. 38). Within expanding and newly-established shrub stands, almost all new shrubs occurred on bare, circular microsites disturbed by seasonal frost-heave, a widespread phenomenon that maintains mosaics of mineral seedbeds with warm soils and few competitors. The bare circles of mineral soil are immediately available to shrubs during favorable climatic periods. Alder abundance and extent have likely increased rapidly in the northwest Siberian Low Arctic since at least the mid-20th century, and this region has high potential for continued expansion of tall shrubs due to the broad distribution of patterned-ground landscapes.
Vegetation Response Following Tundra Fire
Historically, wildfires have been rare in the Arctic, although they are a dominant feature of the boreal forests south of the Arctic tundra. In the last decade, however, the number and severity of tundra wildfires on the North Slope of Alaska has increased dramatically, including more than half of all wildfires reported since 1950, and greatly increasing the area known to have burned.
The recent, 2007, Anaktuvuk River wildfire, burned >1000 km2 of tundra about 40 km north of Toolik Lake on the North Slope of Alaska, and alone accounts for over half of the area burned on the North Slope since 1950. One result of the fire was a huge emission of soil carbon, approximately 2.1 Tg, effectively equivalent to the present-day annual uptake (sink) of the entire global arctic tundra biome (Mack et al. 2011). All of the above-ground vegetation was burned, and there were major increases in energy inputs (radiative forcing) to the system and other changes such as large increases in depth of soil thaw.
Recovery of the vegetation canopy and concomitant surface energy exchange has been quite rapid in the region of the Anaktuvuk River fire, as indicated by a soil and plant biomass harvest conducted in 2011 at the burn site, which included areas that had not been burned, or were moderately or severely burned (Rocha et al. 2012). The results suggest that the vegetation, particularly the graminoids (dominated by the tussock-forming sedge Eriophorum vaginatum), was able to regrow relatively quickly from below-ground rhizomes, with above-ground net primary productivity of the moderately burned tundra being slightly greater than in tundra that had not been burned (Fig. 39) (Bret-Harte et al. in press). However, lichens and mosses are showing little sign of recovery, and shrub wood was lost in the fire. Therefore, the total biomass of the vegetation is substantially lower in the burned areas than those that did not burn.
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