Ninth International Conference on Permafrost ... - IARC Research

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