Shrub Line Advance in Alpine Tundra of the Kluane Region: Mechanisms of Expansion and Ecosystem Impacts
Dates
Year
2007
Citation
Myers-Smith, Isla, 2007, Shrub Line Advance in Alpine Tundra of the Kluane Region: Mechanisms of Expansion and Ecosystem Impacts: Arctic, v. 60, no. 4, p. 447-451.
Summary
In the last 50 years, repeat aerial photography has documented rapid shrub expansion in Arctic Alaska ([Sturm] et al., 2001a, Tape et al., 2006) and the northern Yukon and Northwest Territories (T. Lantz, University of British Columbia, pers. comm. 2007). Paleoecological evidence suggests that tall shrubs last invaded tundra ecosystems in Alaska and northwestern Canada during the warm postglacial period, between 7000 and 12 000 years ago (Ritchie, 1984). Growing-season temperatures are again warming in Alaska and western Canada (ACIA, 2004; Chapin et al., 2005), and satellite imagery shows a concurrent greening of the Arctic tundra (Jia et al., 2003; Stow et al., 2004). The correlation between warming and greening has been used to [...]
Summary
In the last 50 years, repeat aerial photography has documented rapid shrub expansion in Arctic Alaska ([Sturm] et al., 2001a, Tape et al., 2006) and the northern Yukon and Northwest Territories (T. Lantz, University of British Columbia, pers. comm. 2007). Paleoecological evidence suggests that tall shrubs last invaded tundra ecosystems in Alaska and northwestern Canada during the warm postglacial period, between 7000 and 12 000 years ago (Ritchie, 1984). Growing-season temperatures are again warming in Alaska and western Canada (ACIA, 2004; Chapin et al., 2005), and satellite imagery shows a concurrent greening of the Arctic tundra (Jia et al., 2003; Stow et al., 2004). The correlation between warming and greening has been used to link climate change with shrub expansion (Sturm et al., 2001a; Epstein et al., 2003); however, the mechanisms driving shrub increase are likely more complex. A combination of changes in nutrient mineralization, snow depth, microclimate (Sturm et al., 2001b; Grogan and Jonasson, 2006), disturbance (Forbes et al., 2001; Racine et al., 2004; T. Lantz, University of British Columbia, pers. comm. 2007), and species interactions are all contributing to the landscape patterns of shrub expansion. To quantify the influence of shrub expansion on nutrient cycling and ecosystem function, I am investigating nutrient cycling. I have installed anion and cation exchange resin probes (Plant Root Simulator(TM) probes from Western Ag Innovation Inc. Saskatoon, Saskatchewan, Canada) to measure ammonium and nitrate bioavailability, and litter bags to quantify the rate of decomposition under shrubs and in adjacent shrub-free tundra. Tying together flux rates, decomposition, and nutrient pools will help to elucidate the impact of shrubs on carbon storage. I hypothesized that rates of CO2 efflux would be higher under the shrub canopy during the growing season due to enhanced decomposition and higher autotrophic respiration. To test this hypothesis, I conducted CO2 flux measurements using a Li6400 infrared gas analyzer (LI-COR Environmental Lincoln, Nebraska USA) from May to September 2007, in plots under the shrub canopy and in adjacent shrub-free tundra. Preliminary data do not show a significant relationship between shrub cover and CO2 efflux however. Winter soil warming has also been attributed to enhanced nutrient cycling and reduced soil carbon stores ([Mack] et al., 2004). During summer, conversely, shading by shrubs decreases soil temperatures under shrub canopies ([Pomeroy] et al., 2006). Though complex, the interactions between shrubs, snow, and soil warming may act as a positive feedback to shrub expansion (Fig. 4, Chapin et al., 2005). To measure the influence of snow-capture by shrubs on soil warming, I have manipulated shrub cover to compare soil temperatures beneath plots with (a) intact shrubs, (b) shrubs removed, (c) artificial vegetation canopies, and (d) adjacent, shrub-free tundra. In September 2007, six artificial shrub and tundra plots were constructed by cutting down shrubs and affixing them to stakes in the soil in adjacent shrub-free tundra (Fig. 5). Six manipulation plots and paired control monitoring plots are instrumented with snow stakes that have iButton Thermochron temperature loggers (Dallas Semiconductor Corporation, Dallas, Texas, USA) at 2, 5, 25, 50, 100, and 150 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 soil surface. The experiment will test whether shrubs trap more snow than the adjacent tundra, whether this snow melts out earlier in the spring season, and how much this snow insulates the soil.