The role of tephra covers on soil moisture conservation at Haleakalaʹs crater (Maui, Hawaiʹi)

The influence of tephra covers on soil water was studied in Haleakala (Maui, Hawaiʹi) during two summers; eight sites with tephra layers and silverswords (Argyroxiphium sandwicense DC.) were sampled at 2415–2755 m. At each site, eight paired-sample sets were obtained in bare soils and under adjacent tephra, at three depths. Tephra were sharply separated from underlying soils and showed prominent vertical stratification. Tephra clast size-distribution was assessed by photosieving and on interstitial-gravel samples; stones included 45.6% cobbles, 29.4% pebbles, and 25% blocks. re content increased with depth in both positions, but soils below tephra had more water at all depths than exposed areas. Surface soils beneath tephra contained 83% more water than bare ground. Soils at 5–10 cm had ∼ 106% greater moisture under rocks, but only ∼ 70% at 10–15 cm. Differences between plots were statistically significant (∼ p < 0.001) for surface soils, but less pronounced for subsoils. Soils above 2650 m had greater water content than at lower elevations, and moisture disparity between sample pairs increased with altitude. ils were coarse, with ∼ 20% gravel and ∼ 94% sand; most fine material (≤ 0.063 mm) was silt, as clay content was negligible. Organic-matter percentage was low (1.65%). Bulk density and porosity were associated with moisture variation both in tephra-insulated and bare soils; 80% of field moisture was statistically (p < 0.001) accounted for by pore space. Air and soil temperatures were recorded at three sites during ∼ one-week periods prior to moisture sampling. Tephra substantially decreased soil maxima and daily thermal amplitude in underlying soils, but did not noticeably affect nightly minima. Thin (5–6 cm) tephra layers were nearly as effective as thicker (9–15 cm) deposits in depressing soil maxima. Possible water-conservation mechanisms under tephra include: decreased evaporation due to ground shielding and lower maxima; reduced capillary flow; greater infiltration depth; nocturnal dew condensation; and fog interception by blocks.