Widespread increases in shrub cover across a successional gradient in south-central Alaska

Background/Question/Methods Rates of community change generally decrease over the course of succession, with rates of species gain declining through time.  In south-central Alaska, accelerated rates of glacial recession have resulted in an increase in early-seral communities on the landscape, particularly in coastal areas.  In order to quantify rates of vegetation change in a section of coastline affected by recent glacial retreat, we used a combination of (1) historic air photos (1951-1993) from Kenai Fjords National Park, orthorectified to IKONOS base imagery (2005), and (2) the re-measurement of twenty-two 50-m² inventory plots (1993-2008) to document changes in vegetation across the successional (age) gradient.  At both the landscape and community scale, we expected to find the greatest changes in species composition at the early seral sites.

Results/Conclusions At the landscape scale, the time-series of historic air photos and satellite imagery, interpreted for 4-km² areas of interest (AOIs) within the larger study area, showed the greatest changes occurring in recently deglaciated areas, as expected.  The conversion from ice-covered or barren areas to shrubland over the 50-year period ranged from 0% to 43% of a given AOI, across the age gradient.  Conversion from shrub to tree cover ranged from 0% to 3%.  Shrub cover remained relatively stable in most AOIs, with >90% of shrub cover unchanged.  At the community scale, ordination using nonmetric multidimensional scaling (NMDS) indicated distinct groupings by life form, and vectors showed a trend toward later successional species in early to mid-seral groups.  Mean species richness increased 50% in the the non-forested groups (early to mid-seral tall shrub, coastal meadow, and dwarf shrub-forb).  Although the direction of change (toward increased shrub cover and species richness) was more consistent on the recently deglaciated sites, we nevertheless found evidence of similar changes occurring on the older landscapes.