HR: 0830h
AN: C21B-0814    [PDF]
TI: Effects of Permafrost on the Rate of Spruce Migration During Climate Warming
AU: * Lloyd, A H
EM: lloyd@middlebury.edu
AF: Middlebury College, Department of Biology Bicentennial Hall, Middlebury, VT 05753 United States
AU: Hinzman, L
EM: ffldh@uaf.edu
AF: Water and Environmental Research Center, Institute of Northern Engineering University of Alaska Fairbanks, Fairbanks, AK 99775 United States
AU: Fastie, C L
EM: cfastie@middlebury.edu
AF: Middlebury College, Department of Biology Bicentennial Hall, Middlebury, VT 05753 United States
AU: Yoshikawa, K
EM: ffky@uaf.edu
AF: Water and Environmental Research Center, Institute of Northern Engineering University of Alaska Fairbanks, Fairbanks, AK 99775 United States
AU: Rupp, S
EM: scott.rupp@uaf.edu
AF: Forest Sciences Department, University of Alaska Fairbanks, Fairbanks, AK 99775 United States
AB: Although white spruce has advanced into tundra ecosystems throughout Alaska since the late 1800s, presumably in response to a regional increase in temperature, there is high variance in the magnitude of change in treeline position among arctic treeline sites. We investigated the role of permafrost as a modulator of vegetation response to climate by estimating the migration rate of spruce advancing into lowland sites with very poorly drained soils and upland sites with more well-drained soils. Migration rates were estimated from published reconstructions of spruce population dynamics at 2 lowland and 4 upland sites on the Seward Peninsula, Alaska. Migration rate was calculated by dividing the distance between two locations along an advancing front by the difference in stand age at the two locations. We estimated the hypothetical effect of variation in migration rate on the areal extent of land cover change over a 100 year period by multiplying the length of the forest-tundra boundary on the Seward Peninsula by the estimated migration rates. Migration rates in upland sites (129 m/yr) were greater than those in lowland sites (0.2 m/yr). Differences in estimated spruce migration rates had a large effect on the land area that could hypothetically be affected by spruce expansion. Migration rates equivalent to those found in upland sites would result in spruce colonizing 33% of the treeless area of the Seward Peninsula over a 100 year period, while rates equivalent to those found in lowland sites would result in 0.05% of the treeless area being colonized by spruce. Three hypotheses may explain the variation in migration rates in the two types of sites. First, differences in migration rate may reflect different climatic histories. The upland and lowland sites are separated by $<$10 km, however, so it is unlikely that the rate of warming differed significantly between them. Second, the density of spruce behind the advancing front may limit seed availability and thus reduce migration rate. Spruce density behind the advancing front at lowland sites was an order of magnitude greater than in upland sites, however, so source population density is not a likely explanation for the slow migration rate from lowland sites. Finally, migration into lowland tussock tundra may be limited by the availability of suitable microsites. Two lines of evidence support this hypothesis. First, lowland sites have extremely high subsurface soil moisture, which may inhibit spruce colonization. Second, prior research has shown that spruce preferentially colonize lowland tundra in thermokarst-affected sites with improved drainage. Our results thus suggest that permafrost, by impeding soil drainage, reduces the rate of the transition from tundra to forest vegetation, and therefore acts to dampen the response rate of forest vegetation to climate.
DE: 1600 GLOBAL CHANGE (New category)
DE: 1800 HYDROLOGY
DE: 1823 Frozen ground
DE: 1851 Plant ecology
DE: 1866 Soil moisture
SC: Cryosphere [C]
MN: 2003 Fall Meeting