We measured the fraction of soil volume containing fine and coarse roots using images from three soil pits in watershed N4D and three soil pits in watershed N1B. After opening each pit, the face was cleaned and the surface of the pit face was photographed. Each image was overlain with a 1x1 cm grid and analyzed using ImageJ software. The presence or absence of fine roots (diameter < 1 mm) and coarse roots (diameter > 1 mm) was noted for each grid cell. Only presence/absence and not count data were recorded. For cells containing both fine and coarse roots, the presence of only the coarse root(s) was recorded given their greater volume. The number of grid cells with roots were summed and then divided by the total number of cells evaluated to provide a root abundance fraction (fine, coarse, and total) for each 1-cm thick soil layer.

The face of each soil pit was cleaned and soil horizons were described using the Natural Resources Conservation Service Field Book for Describing and Sampling Soils (Schoeneberger et al., 2012).

Sampling protocol: Schoeneberger, P.J., Wysocki, D.A., Benham, E.C., 2012. Field book for describing and sampling soils. Version 3.0 Natural Resources Conversation Service. National Soil Survey Center, Lincoln, NE.