A warm, drying phase of the Pacific Decadal Oscillation, coinciding with a 10-20 fold increase in human population since 1950, has substantially changed landscapes in Southcentral Alaska. We investigated effects of both factors across the 300 km2 landscape (0—1500 m asl) that makes up Joint Base Elmendorf-Richardson (JBER) in Anchorage, Alaska. To measure landscape change we digitized polygons using (1) georeferenced scanned aerial images flown before 1997 (2) orthorectified digital images flown after 1997 (3) bare-earth and first-return LiDAR and (4) previously published land classifications (Jorgenson, et al 2002, Pullman et al 2003). The imagery covered over six decades (1947—2012) and the three years LiDAR (2002, 2009, 2010). We used 3-5 photosets covering six decades (1950—2010) to quantify invasion of black spruce (Picea glauca) across 25 km2 of a relatively undisturbed, lowland (< 100 m asl) glaciated landscape of drumlins, bogs, and drainageways. Similarly, we examined upland (100-200 m asl) spruce and mixed forests for canopy openings. Three photosets between 1974—2009 covered a 25 km2 alpine section (> 400 m asl) where we looked for signs of tree and shrub-line rise.
Results/Conclusions
We characterized landscape changes across JBER as follows. 1950—65: Humans built out base infrastructure. 1965—75: Black-spruce invaded herbaceous meadows over glacial drainageways while balsam-poplar (Populus balsamifera) invaded alpine tundra. 1975—2010: Black spruce invaded closed-basin drumlin bogs while shrubs invaded alpine. 1995—2010: Mixed-birch and white-spruce forest canopy opened following spruce-bark-beetle mortality. Overall, human-modified land area doubled from 3000 ha (1950) to 6000 ha (2010), an average rate of 1.1%/y, mostly along the southern and southeastern reaches of JBER with ~10% of this regrowing. Climatic change impacts were most dramatic as black spruce invasion into herbaceous wetlands, most notably along shallow-basin glacial drainageways and less so in steep-sided kettle bogs. Some ponds in the south and west dried out but deep lakes to the north and east retained their edge outlines across decades. In the relatively undisturbed northern lowlands, black spruce overgrew herbaceous meadow, which dropped from 13% of the area in 1950 to 3% in 2010, a reduction > 70%. The modal heights of maximum height-above-ground first returns within 3 x 3 m lidar pixels on spruce-invaded wetlands was 5 m in 2010, suggesting an average growth rate of ~10 cm/y since the mid-1950s.