Ninth International Conference on Permafrost ... - IARC Research

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The Influence of Shrubs on Soil Temperatures in Alpine TundraIsla H. Myers-SmithUniversity of Alberta, Edmonton, Alberta, CanadaDavid S. HikUniversity of Alberta, Edmonton, Alberta, CanadaIntroductionWith a warming climate, northern ecosystems will facesignificant ecological changes such as permafrost thaw,increased forest fire frequency, and shifting ecosystemboundaries, including the spread of tall shrubs into tundra. Innorthern mountain ranges such as those in the southwesternYukon, the shrub line will likely advance up mountain slopeswith climate warming. In the last 50 years, rapid shrubexpansion has been documented in arctic Alaska (Sturm et al.2001a, Tape et al. 2006) and the Northern Yukon and NWT(Trevor Lantz, pers. com.) using repeat aerial photography.Paleoecological evidence suggests that tall shrubs lastinvaded tundra ecosystems in Alaska and northwesternCanada between 7,000 and 12,000 years ago, during thewarm post-glacial period (Ritchie 1984). Growing seasontemperatures are again warming in Alaska and westernCanada (Stafford et al. 2000), and concurrent with this trend,satellite imagery shows a greening of the Arctic (Stow etal. 2004). The correlation between warming and greeninghas been used to link climate change with shrub expansion(Sturm et al. 2001a); however, the exact mechanisms drivingshrub increase are probably more complex. A combination ofchanges in nutrient mineralization, snow depth, microclimate,(Sturm et al. 2001b) disturbance (Trevor Lantz, pers. com.),and species interactions are most likely all contributingfactors to shrub expansion patterns on the landscapeInfluence of shrubs on soil temperaturesIncreased shrubs in arctic and alpine tundra alter thepartitioning of solar energy during the growing season,the distribution and physical characteristics of snow in thewinter (Liston et al. 2002), and soil thermal dynamics yearround(Sturm et al. 2001b, Sturm et al. 2005a). In the winter,snow trapping by shrubs can insulate soils (by trapping heat)and has been proposed as a positive feedback mechanismpromoting the expansion of shrubs in the Arctic (Sturm etal. 2001b, Sturm et al. 2005a). During spring, dark-coloredshrubs that extend above the snow alter the albedo andaccelerate local snowmelt (Sturm et al. 2005b, Pomeroyet al. 2006). In summer, shading by shrubs decreases soiltemperatures under shrub canopies (Pomeroy et al. 2006).Though complex, the interactions between shrubs, snow,and soil warming may act as a positive feedback to shrubexpansion (Fig. 4, Chapin et al. 2005). In addition, wintersoil warming may enhance nutrient cycling and reduce soilcarbon stores (Mack et al. 2004).Study SiteThe field site is located in the Ruby Range Mountains(61°20′N, 139°17′W, Fig. 1) adjacent to Kluane Lake. Thisarea of the southwest Yukon is located at the convergenceof the coastal and arctic air masses, and as a result, climatechange could lead to increased variability in wintertemperatures and precipitation. The study area varies inelevation, aspect, and proximity to glaciers, making it anideal location to test a shrub expansion hypotheses.MethodsTo measure the influence of snow-capture by shrubson soil warming, we manipulated willow (Salix spp.)cover to compare soil temperatures beneath plots with (a)intact shrubs, (b) shrubs removed, (c) artificial vegetationcanopies, and (d) adjacent, shrub-free tundra. In September2007, 6 artificial shrub and tundra plots were constructed bycutting down shrubs and affixing them to stakes in the soil inadjacent shrub-free tundra (Fig. 2).The 6 manipulation plots and paired control monitoringplots are instrumented with snow stakes with iButtonThermochron temperature loggers (Dallas SemiconductorCorporation, Dallas, Texas, USA) at 2, 5, 25, 50, 100, and150 cm along their length, and with Hobo micro station 12-bit temperature sensors (HOBO, Onset Computer Corp.,Massachusetts, USA), installed at 2 and 5 cm below the soilsurface. This experiment will test whether shrubs trap moresnow than the adjacent tundra, whether this snow melts outearlier in the spring season, and how much the trapped snowinsulates the soil.Initial ResultsFigure 1. Map of the study region and the Ruby Range Mountainfield site.Over the growing season of 2007, shrub plots weresignificantly cooler than tundra plots (for DOY 160 to 244,at 2 cm depth: 1.5°C cooler under shrubs, F 1,172= 33.9, p< 0.001, at 5 cm depth: 1.3°C cooler under shrubs, F 1,172=25.7, p < 0.001, Fig. 3).219
Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s tFigure 2. Manipulative experiment.Figure 3. Mean daily soil temperatures (±SE) at 2 cm and 5 cmdepth under shrub and tundra plots (n = 12).Since the manipulation was conducted at the end of the2007 field season, we were only able to collect one week ofexperimental data. During the last week of September, boththe shrub and artificial shrub plots were cooler than the tundraand artificial tundra plots; however, these comparisons arenot statistically significant. Data from the 2007–08 winterseason will indicate whether shrub canopies trap more snowand if insulation results in soil warming.DiscussionIncreasing shrubs in arctic and alpine tundra will altermicroclimates and soil thermal dynamics (Sturm et al.2001b). Cooling of summer soil temperatures may reducedecomposition and resulting carbon loss. Warming of wintersoils may, however, increase decomposition, acceleratenutrient cycling and promote the further expansion of shrubs(Sturm et al. 2001b, Sturm et al. 2005a). This study attemptsto quantify the impact of shrubs on soil thermal dynamics.Shrubs cool soils during the growing season (Pomeroy etal. 2006 this study); however, the contribution of shrubs towinter soil thermal dynamics has been little explored in theliterature. By manipulating shrub cover (removing aboveground woody biomass and erecting artificial shrubs), theinsulation provided by a shrub canopy and trapped snow canbe isolated from the influence of understory vegetation andsoils. If increasing shrubiness is resulting in warmer wintersoils, this may lead to enhanced nutrient cycling and reducedsoil carbon stores; however, the 2°C summer cooling mayoffset a winter warming response. By the summer of 2008,we will have collected data for a full winter season, andwill be able to determine snow trapping and soil warmingpotential of willow shrubs in the alpine tundra of the KluaneRegion.AcknowledgmentsWe would like to thank Jodie Pongracz, Catherine Henry,Eric Vezeau, and Mark Wong for field assistance. Funding wasprovided by NSERC, NSTP, C/BAR, Yukon College NRI, theAlberta Ingenuity Fund, and ANIA’s Grant-in-Aid Program.ReferencesChapin, F.S. III, Sturm, M., Serreze, M.C. et al. 2005. Roleof land-surface changes in Arctic summer warming.Science 310: 657-660.Liston, G., McFadden, J.P., Sturm, M. & Pielke, R.A. 2002.Modelled changes in arctic tundra snow, energyand moisture fluxes due to increased shrubs. GlobalChange Biology 8: 17-32.Mack, M.C., Schuur, E.A.G., Bret-Harte, M.S. et al. 2004.Ecosystem carbon storage in arctic tundra reduced bylong-term nutrient fertilization. Nature 431: 440.Pomeroy, J.W., Bewley, D., Essery, R. et al. 2006. Shrubtundra snowmelt. Hydrological Processes 20: 923-941.Ritchie, J.C. 1984. Past and present vegetation of the farNorthwest of Canada. University of Toronto Press:Toronto, Canada.Stafford, J.M., Wendler, G. & Curtis, J. 2000. Temperatureand precipitation of Alaska: 50 year trend analysis.Theoretical and Applied Climatology 67: 33-44.Stow, D.A., Hope, A., McGuire, D. et al. 2004 Remotesensing of vegetation and land-cover change in ArcticTundra Ecosystems. Remote Sensing of Environment89: 281-308.Sturm, M., Racine, C. & Tape K. 2001a. Increasing shrubabundance in the Arctic. Nature 411: 546-547.Sturm, M., McFadden, J.P., Liston G.E. et al. 2001b. Snowshrubinteractions in Arctic tundra: A hypothesis withclimatic implications. Journal of Climatology 14:336-344.Sturm, M., Schimel, J., Michaelson G. et al. 2005a. Winterbiological processes could help convert arctic tundrato shrubland. BioScience 55: 17-26.Sturm, M., Douglas, T. Racine, C. & Liston, G.E. 2005b.Changing snow and shrub conditions affect albedowith global implications. Journal of GeophysicalResearch 110: doi:10.1029/2005JG000013.Tape, K., Sturm M. & Racine C. 2006. The evidence forshrub expansion in Northern Alaska and the Pan-Arctic. Global Change Biology 12: 686-702.220
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ContentsPreface ...................
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Mapping and Modeling the Distributi
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Satellite Observations of Frozen Gr
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xiiIce Wedge Thermal Regime in Nort
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xivPermafrost Response to Dynamics
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NICOP SponsorsUniversitiesUniversit
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Deep Permafrost Studies at the Lupi
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Effect of Fire on Pond Dynamics in
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Cryological Status of Russian Soils
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Acoustical Surveys of Methane Plume
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Permafrost Delineation Near Fairban
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Preparatory Work for a Permanent Ge
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A Provisional Soil Map of the Trans
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Martian Permafrost Depths from Orbi
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Time Series Analyses of Active Micr
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Impact of Permafrost Degradation on
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DC Resistivity Soundings Across a P
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Modeling Thermal and Moisture Regim
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A Provisional Permafrost Map of the
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Alpine Permafrost Distribution at M
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Cryogenic Formations of the Caucasu
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Modeling Potential Climatic Change
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A Hypothesis: A Condition of Growth
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Modeled Continual Surface Water Sto
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Freeze/Thaw Properties of Tundra So
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Discontinuous Permafrost Distributi
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Thermal Regime Within an Arctic Was
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Seasonal and Interannual Variabilit
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Twelve-Year Thaw Progression Data f
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Continued Permafrost Warming in Nor
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Landsliding Following Forest Fire o
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A Permafrost Model Incorporating Dy
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Seasonal Sources of Soil Respiratio
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Greenland Permafrost Temperature Si
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The Importance of Snow Cover Evolut
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The Account of Long-Term Air Temper
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The Combined Isotopic Analysis of L
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Adaptating and Managing Nunavik’s
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Human Experience of Cryospheric Cha
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HiRISE Observations of Fractured Mo
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A Soil Freeze-Thaw Model Through th
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Mapping and Modeling the Distributi
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First Results of Ground Surface Tem
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Historical Changes in the Seasonall
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Rock Glaciers in the Kåfjord Area,
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Snowpack Evolution on Permafrost, N
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Climate Change in Permafrost Region
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Maximizing Construction Season in a
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Pleistocene Sand-Wedge, Composite-W
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