Biogeomorphic influence of soil depth to bedrock, volcanic ash soils, and surface tephra on silversword distribution, Haleakalā Crater (Maui, Hawai'i)

• Study site has many lava-flow deposits with shallow intervenient ash-filled troughs.
• Lava ridges have thin (≤ 17 cm) soils; deep (≤ 110 cm) Typic Haplustepts cover troughs.
• Silverswords grow densely on outcrops, shallow soils along ridges, and near stones.
• Outcrops and bedrock limit drainage, providing a perched watertable plants can tap.
• A sequence of soil development and landscape evolution since ~ 2700 B.P. is proposed.

This research examines the spatial distribution of Haleakalā silverswords (Argyroxiphium sandwicense DC subsp. macrocephalum) over basaltic 'a'ā lava flows at Silversword Loop, Haleakalā Crater (Maui, Hawai'i). The overall study goal is to situate an extensive field data set within an historical geomorphic framework to propose a conceptual, biogeomorphic model of landscape development at Silversword Loop. Landscape features were investigated along two belt transects (111–120 m long) crossing lava-flow deposits at ~ 2175 m; the lava flows, > 4.5 km long, originated ~ 2700 YBP at Pu’u O Māui, a large cinder cone. Transects consist of contiguous 3 × 3 m (9 m2) quadrats; plot data include topographic profiles, soil depth to bedrock, size of surface tephra – determined by photosieving – and plant density; 23 soil samples were analyzed within 10 profiles. Weathered lava outcrops, tephra blocks (≥ 10 cm), and cobbles (5–10 cm) cover lava-flow ridges; the gently sloping flanks of lava-flow deposits merge into intervenient concave troughs, filled with volcanic ash and small tephra. Soils along ridges are thin (≤ 17 cm), whereas profiles on troughs are deeper (≤ 110 cm). Soils are sandy volcanic Inceptisols (Lithic, Typic, and Aridic Lithic Haplustepts) averaging 21.6% gravel, 96.8% sand, and 3.2% fines (silt + clay); mean organic matter is just 0.92%. These soils provide low water retention (23.2% at field capacity) and rapid infiltration. Deeper trough soils are largely devoid of plants. In contrast, silverswords are virtually restricted to shallow soils on lava-flow crests, where they densely cluster about outcrops and blocks: 90% of plants grow on soils ≤ 40 cm deep, and 71.3% occupy plots with ≤ 40% ash cover; 84.4% of rosettes are rooted on outcrops or near stones; only 4.4% grow on soil; and 11.2% germinated on plant litter.Patterns of lava-flow weathering and soil depth to bedrock control silversword distribution at Silversword Loop. Shallow basalt bedrock and outcrops restrict soil drainage, providing a perched water table that silversword roots can exploit. Surface blocks also provide safe, stable microhabitats, thus rosettes grow preferentially near stones. Geomorphological, pedological, and ecological data are integrated into a biogeomorphic sequence of soil development and landscape evolution at Silversword Loop for the past three millennia, first through the eruptions and ash deposits of Pu'u O Māui, followed (~ 970–940 BP) by pyroclastic activity in Halāli'i and Ka lu'u o Ka 'O'o cinder cones.