Soil strata were determined in-situ at 265 locations by measuring the vertical thickness of three main strata (acrotelm, catotelm, and mineral soil) found in organic-rich or peat soils, typical of those in the region and the low Arctic.  At each site, an approximately 30×30 cm square section of soil was extracted using a serrated knife and then returned after description and subsampling by coring.  We used both visual and physical criteria to identify the contact between acrotelm and catotelm: visually, we identified the depth at which recently-dead roots, stems, and leaves no longer appeared in the soil.  Physically, we determined the depth below which the soil did not adhere to itself as one coherent mat.  We also used visual and physical criteria to differentiate between catotelm and mineral soil: visually, catotelm and mineral soil were often starkly different colors (catotelm being a dark brown or black, whereas mineral soils were a very light brown or grey), and physically, mineral soils were cohesive while catotelm peat was not.  Our description of acrotelm corresponds to fibric organic soils in soil taxonomy (or the Oi horizon), catotelm corresponds to hemic and sapric organic soils (or Oe and Oa horizons), and mineral soil corresponds to the A horizon.  Statistical tests were used to determine differences in soil stratification among the three soil strata.  Differences were considered ‘significant’ when the test p<0.05 and ‘substantial’ when the differences are obvious qualitatively, but were not statistically significant.

A hierarchical classification scheme that combined three easily-identified land surface properties - slope, dominant vegetation type, and microtopographic position (whether the site is at a local high or low point over decimeter scales) - was implemented.  Based on the classification used by Walker and Walker [1996], the surface slope was used to identify a landscape zone as either ‘Hillslope’ (steep) or ‘Riparian’ (relatively flat river valleys).  Riparian zones are flatter areas (<10% slope) that border surface streams, while Hillslope zones are steeper areas (>10% slope) and typically feed into Riparian zones.  These two zones comprise ~ 90% of the study site used by Walker and Walker [1996] as representative of the Alaskan Arctic Foothills.

The dominant vegetation type was identified within each landscape zone.  Substantial work has identified and classified vegetation types within the Toolik Lake Region [Walker and Walker, 1996; Walker et al., 2016; Walker et al., 2018], the North Slope [Payne, 2013], and in arctic continuous permafrost terrain in general [Stow et al., 2004].  Different vocabulary has been used across disciplines to describe different classifications and sub-classifications of this landscape.  Here we used four umbrella vegetation types based on common, simple criteria to simplify the various sub-classifications.  We used ‘Sedge’ to describe any wet to saturated, graminoid-dominated site; ‘Shrub’ to describe any site dominated by plants with woody stems (i.e., birch, willow, and alder); ‘Tussock’ to describe any site dominated by tussock forming sedges (e.g., “cottongrass”, genus Eriophorum), and ‘Sparse Vegetation’ to describe any plot with matted lichen vegetation or bare ground.  These four types include all the dominant land cover types in the Arctic Foothills. 

Next, the landscape zones and vegetation types were combined to produce five categories at the finest-scale classification: (1) hillslope shrub, (2) hillslope tussock, (3) hillslope sedge, and (4) riparian shrub and (5) riparian sedge (Fig. 2).  Hillslope sedge corresponds to so-called water tracks, which are zero-order linear drainage features that funnel substantial water flows from hillslopes, are spaced somewhat regularly in intervals of tens of meters, have narrow widths (1 to 3 m), and occur in subtle topographic lows within the landscape [McNamara et al., 1997].  

Finally, microtopographic position, whether it was high or low, was established visually by comparing the local elevation of the sample site to the surrounding elevation points.  However, microtopography was not discernible in all landscape zones and vegetation types; hence its addition as a criterion was not uniformly applied.  Overall, the hierarchical classification resulted in two landscape zones, four vegetation types, five categories that combine landscape zones and vegetation types, and two microtopographic groups.

References:

1.  McNamara, J. P., D. L. Kane, and L. D. Hinzman (1997), Hydrograph separations in an Arctic watershed using mixing model and graphical techniques, Water Resources Research, 33(7), 1707-1719, doi: 10.1029/97wr01033.

2.  Payne, J. (2013), NSSI Landcover Report: Landcover Mapping for North Slope of Alaska, edited, United States Bureau of Land Management.

3.  Stow, D. A., et al. (2004), Remote sensing of vegetation and land-cover change in Arctic Tundra Ecosystems, Remote Sensing of Environment, 89(3), 281-308.

4.  Walker, D. A., and M. D. Walker (1996), Terrain and vegetation of the Imnavait Creek watershed, in Landscape Function and Disturbance in Arctic Tundra, edited by J. F. R. J. D. Tenhunen, pp. 73-108, Springer.

5.  Walker, D. A., et al. (2016), Circumpolar Arctic vegetation: a hierarchic review and roadmap toward an internationally consistent approach to survey, archive and classify tundra plot data, Environmental Research Letters, 11(5).

6.  Walker, D. A., et al. (2018), Circumpolar Arctic Vegetation Classification, Phytocoenologia, 48(2), 181-201.