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Root thread strength, landslide headscarp geometry, and observed root characteristics at the monitored CB1 landslide, Oregon, USA

April 12, 2022

This data release supports interpretations of field-observed root distributions within a shallow landslide headscarp (CB1) located below Mettman Ridge within the Oregon Coast Range, approximately 15 km northeast of Coos Bay, Oregon, USA. (Schmidt_2021_CB1_topo_far.png and Schmidt_2021_CB1_topo_close.png). Root species, diameter (greater than or equal to 1 mm), general orientation relative to the slide scarp, and depth below ground surface were characterized immediately following landsliding in response to large-magnitude precipitation in November 1996 which triggered thousands of landslides within the area (Montgomery and others, 2009). The enclosed data includes: (1) tests of root-thread failure as a function of root diameter and tensile load for different plant species applicable to the broader Oregon Coast Range and (2) tape and compass survey of the planform geometry of the CB1 landslide and the roots observed in the slide scarp. Root diameter and load measurements were principally collected in the general area of the CB1 slide for 12 species listed in: Schmidt_2021_OR_root_species_list.csv. Methodology of the failure tests included identifying roots of a given plant species, trimming root threads into 15-20 cm long segments, measuring diameters including bark (up to 6.5 mm) with a micrometer at multiple points along the segment to arrive at an average, clamping a segment end to a calibrated spring and loading roots until failure recording the maximum load. Files containing the tensile failure tests described in Schmidt and others (2001) include root diameter (mm), critical tensile load at failure (kg), root cross-sectional area (m^2), and tensile strength (MPa). Tensile strengths were calculated as: (critical tensile load at failure * gravitational acceleration)/root cross-sectional area. The files are labeled: Schmidt_2021_OR_root_AceCir.csv, Schmidt_2021_OR_root_AceMac.csv, Schmidt_2021_OR_root_AlnRub.csv, Schmidt_2021_OR_root_AnaMar.csv, Schmidt_2021_OR_root_DigPur.csv, Schmidt_2021_OR_root_MahNer.csv, Schmidt_2021_OR_root_PolMun.csv, Schmidt_2021_OR_root_PseMen_damaged.csv, Schmidt_2021_OR_root_PseMen_healthy.csv, Schmidt_2021_OR_root_RubDis.csv, Schmidt_2021_OR_root_RubPar.csv, Schmidt_2021_OR_root_SamCae.csv, and Schmidt_2021_OR_root_TsuHet.csv. File naming follows the convention of adopting the first three letters of the binomial system defining genus and species of their Latin names. Live and damaged roots were identified based on their color, texture, plasticity, adherence of bark to woody material, and compressibility. For example, healthy live Douglas-fir (Pseudotsuga menziesii) roots (Schmidt_2021_OR_root_PseMen_healthy.csv) have a crimson-colored inner bark, darkening to a brownish red in dead Douglas-fir roots. Both are distinctive colors. Live roots exhibited plastic responses to bending and strong adherence of bark, whereas dead roots displayed brittle behavior with bending and poor adherence of bark to the underlying woody material. Measured tensile strengths of damaged root threads with fungal infections following selective tree harvest using yarding operations that damaged bark of standing trees expressed significantly lower tensile strengths than their ultimate living tensile strengths (Schmidt_2021_OR_root_PseMen_damaged.csv). The CB1 site was clear cut logged in 1987 and replanted with Douglas fir saplings in 1989. Vegetation in the vicinity of the failure scarp is dominated by young Douglas fir saplings planted two years after the clear cut, blue elderberry (Sambucus caerulea), thimbleberry (Rubus parviflorus), foxglove (Digitalis purpurea), and Himalayan blackberry (Rubus discolor). The remaining seven species are provided for context of more regional studies. The CB1 site is a hillslope hollow that failed as a shallow landslide and mobilized as a debris flow during heavy rainfall in November 1996. Prior to debris flow mobilization, the ~5-m wide slide with a source area of roughly 860 m^2 and an average slope of 43? displaced and broke numerous roots. Following landsliding, field observations noted a preponderance of exposed, blunt broken root stubs within the scarp. Roots were not straight and smooth, but rather exhibited tortuous growth paths with firmly anchored, interlocking structures. The planform geometry represented by a tape and compass field survey is presented as starting and ending points of slide margin segments of roughly equal colluvial soil depths above saprolite or bedrock (Schmidt_2021_CB1_scarp_geometry.csv and Schmidt_2021_CB1_scarp_pts.shp). The graphic Schmidt_2021_CB1_scarp_pts_poly.png shows the horse-shoe shaped profile and its numbered scarp segments. Segment numbers enclosed within parentheses indicate segments where roots were not counted owing to occlusion by prior ground disturbance. The shapefile Schmidt_2021_CB1_scarp_poly.shp also represents the scarp line segments. The file Schmidt_2021_CB1_segment_info.csv presents the segment information as left and right cumulative lengths, averaged colluvium soils depths for each segment, and inclinations of the ground surface slope relative to horizontal along the perimeter (P) and the slide scarp face (F). Lastly, Schmidt_2021_CB1_rootdata_scarp.csv represents root diameter of individual threads measured by a micrometer, species, depth below ground surface, live vs. dead roots, general root orientation (parallel or perpendicular) relative to scarp perimeter, and cumulative perimeter distance within the scarp segments. At CB1 specifically and more generally across the Oregon Coast Range, root reinforcement occurs primarily by lateral reinforcement with typically much smaller basal reinforcements.