D. A. Walker1, U. S. Bhatt2, J. C. Comiso3, H. E. Epstein4, W. A. Gould5, G. H. R. Henry6, G. J. Jia7, S. V. Kokelj8, T. C. Lantz9, J. A. Mercado-Díaz5, J. E. Pinzon3, M. K. Raynolds1, G. R. Shaver10, C. J. Tucker3, C. E. Tweedie11, and P. J. Webber12
1 Institute of Arctic Biology, University of Alaska Fairbanks, Fairbanks, Alaska
The greatest percentage greening changes are occurring the High Arctic of Canada and West Greenland and Northern Alaska, where increases of up to 15% have been observed from 1982 to 2008. A few ground-based studies indicate that the observations of greening trends detected from space are consistent with changes detected in tundra biomass. At present it is not clear if the greening associated with disturbances (e.g. landslides, fires, thawing permafrost) has increased or if the disturbance are sufficiently large in area to contribute to the global changes.
The vegetation report in the 2009 update of the Arctic Report Card documented recent trends in tundra summer greenness and correlations with coastal sea-ice and summer land temperatures, as observed from earth-orbiting satellites. These observations of changes in vegetation were interpreted from variations in the annual maximum Normalized Difference Vegetation Index (MaxNDVI), which is an indicator of the photosynthetic capacity of the vegetation. Increases in the NDVI generally correspond to positive changes in the tundra biomass, whereas negative trends indicate a loss of green plant biomass. During the studied period of satellite observations (1982-2008) Arctic sea ice within 50-km of the coast retreated by a circumpolar average of over 25%, tundra land temperatures within the 50-km coastal strip increased an average of 5˚C (24% increase), and the MaxNDVI increased 5%. Absolute MaxNDVI changes were by far the greatest in the northern Alaska/Beaufort Sea area (0.09 AVHRR NDVI units or a 14% increase) (Fig. V1, left), whereas the percentage changes have been highest in the Baffin Bay, Beaufort Sea, Canadian Archipelago and Davis Strait areas (10-15% changes) (Fig. V1, right) (Bhatt et al., 2010).
The greening trends observed in the satellite data are now supported by quantitative, long-term in situ vegetation measurements from the International Tundra Experiment (ITEX) and the Back to the Future (BTF) projects. As in the satellite measurements, the most evident changes appear to be occurring first in the sparsely vegetated areas of the far North. A study of plots at Alexandra Fiord, Ellesmere Island is the first to demonstrate significant changes in above and below ground biomass over the last 25-30 years (Hill and Henry, 2010, accepted; Hudson and Henry, 2009) (Fig. V2). In addition, there has been a change in the relative abundance of species with an increase in the dominant species over this same time period. The changes in the tundra plant communities are most likely in response to the increase in air temperature over the past 35 years of between 0.6-1.0°C/decade, with the strongest increases seen in the winter temperatures. The increases in biomass also correspond with longer growing seasons, with extensions into the late summer, and with deeper active layers (depth of summer soil thawing). In another far-north Canada study, repeat photographs of permanent vegetation study plots 46 years after their initial installation near the Lewis Glacier, Baffin Island, document rapid vegetation changes along the margins of large retreating glaciers (Johnson et al., 2009; Webber and Tweedie, personal communication, 2009).
Further south, in the more lush tundra near Toolik Lake, a detailed analysis of a 22-year record (1989-2008) of tundra vegetation structure and composition from a set of 156 permanent monitoring plots indicates a general increase in above ground biomass (Gould and Mercado-Diaz, 2008). Over the last two decades the relative abundance of vascular vegetation increased by 16%, while the relative abundance of nonvascular vegetation decreased by 18%. The canopy height, as well as the extent and complexity of the canopy have been increasing over time with the amount of horizontal surface having multiple strata increasing from about 60% to 80%.
The frequencies of landslides, thermokarst features (irregular land surfaces formed in permafrost regions by melting ground ice), and fires have been noted in several areas of the Arctic (Jones et al., 2009; Kokelj et al., 2009; Lantz, 2008; Lantz and Kokelj, 2008; Walker et al., 2009; Leibman and Kizyakov, 2007; Ukraientseva, 2008). Warmer soil temperatures, thawing permafrost, more abundant water, and increased nutrients on these disturbed features result in pronounced greening.
Tundra fires have been predicted to increase with warming summer temperatures. In late summer 2007, the Anaktuvuk River fire near the University of Alaska's Toolik Lake Field Station burned almost 1000 km2. It is the largest known fire to occur in northern Alaska and offered an opportunity for detailed analysis of the changes to the tundra energy and nutrient balance (Liljedahl et al., 2007) and spectral properties (Rocha and Shaver, 2009) . The burning itself released ~1.9M tonnes of carbon to the atmosphere, which was about 30% of the carbon stock within the vegetation and active layer of this area (M.C. Mack unpublished data).
Bhatt, U. S., Walker, D. A., Raynolds, M. K., Comiso, J. C., Epstein, H. E., Jia, G. J., Gens, R., Pinzon, J. E., Tucker, C. J., Tweedie, C. E., and Webber, P. J. (2010), Circumpolar Arctic tundra vegetation change is linked to sea-ice decline: Earth Interactions, 14, doi: 10.1175/2010EI315.1.
Gould, W. A., and, J. A. Mercado-Díaz (2008), Twenty year record of vegetation change from long-term plots in Alaskan tundra, Eos Trans. AGU, 89(53), Fall Meet. Suppl., Abstract C11C-0524.
Hill, G. B., and G. H. R. Henry (2010), Responses of High Arctic wet sedge tundra to climate warming since 1980, Global Change Biology. Published online 14 Jun 2010. DOI: 10.1111/j.1365-2486.2010.02244.x.
Hudson, J. M. G., and G. H. R. Henry (2009), Increased plant biomass in a High Arctic heath community from 1981 to 2008, Ecology, 90, 2657-2663.
Johnson, D. R., S. Villarreal, M. Lara, P. J. Webber, T. Callaghan, D. Hik, and C. E. Tweedie (2009), IPY-Back to the Future: Determining decadal time scale change in ecosystem structure and function in high latitude and high altitude tundra ecosystems, Eos, Trans/ AGU 90(52), Fall Meet. Suppl., Abstract B33A-0372.
Jones, B. M., C. A. Kolden, R. Jandt, J. T. Abatzoglou, F. Urban, and C. D. Arp (2009), Fire behavior, weather, and burn severity of the 2007 anaktuvuk River tundra fire, North Slope, Alaska, Arctic Antarctic and Alpine Research, 41, 309-316.
Kokelj, S. V., T. C. Lantz, J. Kanigan, S. L. Smith, and R. Coutts (2009), Origin and polycyclic behavior of tundra thaw slumps, Mackenzie delta region, Northwest Territories, Canada, Permafrost Periglac. Proc., 20, 173-184.
Lantz (2008), Relative influence of temperature and disturbance on vegetation dynamics in the Low Arctic: an investigation at multiple scales, 153 pp, University of British Columbia, Vancouver.
Lantz, T. C., and S. V. Kokelj (2008), Increasing rates of retrogressive thaw slump activity in the Mackenzie delta region, N.W.T. Canada., Geophysical Research Letters, 35, L06502, doi:06510.01029/02007GL032433.
Lantz, T. C., S. V. Kokelj, S. E. Gergel, and G. H. R. Henry (2009), Relative impacts of disturbance and temperature: persistent changes in microenvironment and vegetation in retrogressive thaw slumps, Global Change Biology, 15, 1664-1675.
Leibman, M. O., and A. I. Kizyakov (2007), Cryogenic Landslides of the Yamal and Yugorsky Peninsulas (in Russian), 206 pp., Earth Cryosphere Institute SB RAS, Moscow.
Liljedahl, A., L. Hinzman, R. Busey, and K. Yoshikawa (2007), Physical short-term changes after a tussock tundra fire, Seward Peninsula, Alaska, JGR, 112, doi:10:1029/2006JF000554.
Rocha, A. V., and G. R. Shaver (2009), Advantages of a two band EVI calculated from solar and photosynthetically active radiation fluxes, Agricultural and Forest Meteorology, 149, 1560-1563.
Ukraintseva, N. G. (2008), Vegetation response to landslide spreading and climate change in the West Siberian Tundra, in Ninth International Conference on Permafrost, edited by D. I. Kane and K. M. Hinkel, pp. 1793-1798, Institute of Northern Engineering, University of Alaska Fairbanks, Fairbanks.
Walker, D. A., et al. (2009), Spatial and temporal patterns of greenness on the Yamal Peninsula, Russia: interactions of ecological and social factors affecting the Arctic normalized vegetation index, Environmental Research Letters, 4, doi:10.1088/1748-9326/1084/1084/045004.